Olivier Brette
email: olivier.brette@insa-lyon.fr

Thomas Buhler
email: thomas.buhler@univ-fcomte.fr

Nathalie Lazaric
email: lazaric@gredeg.cnrs.fr

Kevin Marechal
email: kevin.marechal@ulb.ac.be

Reconsidering the nature and effects of habits in urban transportation behaviour

Keywords: JEL classification: B52, D01, D11, D12, R40, R49 Habit, Behaviour, Urban transportation, Mode choice, Synchronic, Diachronic

This paper adds to the growing empirical evidence on the importance of habits in governing human behaviour, and sheds new light on individual inertia in relation to transportation behaviour. An enriched perspective rooted in Veblenian evolutionary economics (VEE) is used to construct a theoretical framework in order to analyse the processes at play in the formation and reinforcement of habits. The empirical study explores more specifically the synchronic processes strengthening the carusing habit. In addition to underlining the shortcomings of a "decision theory" perspective to address urban transportation behaviours, we find that synchronic habits can have a significant effect on behavioural inertia. Our results suggest the existence of positive feedback between the development of synchronic habits, qualitative perceptions of driving times and reinforcement of the car-using habit.

The paper points out also that the diachronic dimension of habits would constitute another promising domain for further research on behavioural inertia in transportation.

Introduction

Starting with a study by [START_REF] Verplanken | Attitude versus general habit: Antecedents of travel mode choice[END_REF], growing empirical evidence in social psychology supports the idea that habits play a major role in transportation behaviour (see, for instance, [START_REF] Aarts | Predicting behaviour from actions in the past: repeated decision making or a matter of habit?[END_REF][START_REF] Gärling | Empirical tests of a model of determinants of script-based driving choice[END_REF][START_REF] Klöckner | How habits interfere with norm-directed behaviour: A normative decision-making models for travel choice[END_REF][START_REF] Gardner | Modelling motivation and habit in stable travel mode context[END_REF]. The rationale is that transportation behaviour (especially commuting trips) is characterized by a stable context and repeated behaviours.

Several studies acknowledging the power of habit to lead human behaviour have suggested policies and schemes to break patterns in transportation and other activities characterized by inertia [START_REF] Fujii | Development of script-based travel mode choice after forced change[END_REF][START_REF] Fujii | What does a one-month free bus ticket do to habitual drivers? An experimental analysis of habit and attitude change[END_REF][START_REF] Matthies | Applying a modified moral decision making model to change habitual car use -How can commitment be effective?[END_REF][START_REF] Verplanken | Interventions to break and create consumer habits[END_REF][START_REF] Bamberg | Using a residential relocation as starting point for breaking car use habits[END_REF][START_REF] Eriksson | Interrupting habitual car use: The importance of car habit strength and moral motivation for personal car use reduction[END_REF][START_REF] Schäfer | Breaking habits: linking sustainable consumption campaigns to sensitive events[END_REF][START_REF] Thøgersen | Breaking car use habits: the effectiveness of a free one-month travelcard[END_REF][START_REF] Verplanken | Context change and travel mode choice: Combing the habit discontinuity and self-activation hypotheses[END_REF][START_REF] Middleton | I"m on autopilot, I just follow the route": exploring the habits, routines, and decision-making practices of everyday urban mobilities[END_REF]. However, in a review of measures aimed at reducing car use, Graham-Rowe (2011: 415) finds that "the reductions generated by effective, rigorously-evaluated interventions seem to have been relatively modest, and approximately half the interventions evaluated using strong designs were found to be ineffective". The picture becomes even less rosy when long-term effects are assessed.

This paper aims to provide an enriched perspective on habits using a Veblenian evolutionary economics (VEE) framework to shed new light on individual inertia in transportation behaviour. The rationale for this study is that, to be more effective, policies to change transportation behaviour require a detailed understanding of the habituation processes. The approach to habits proposed by Thorstein Veblen (1857Veblen ( -1929) ) based on the Pragmatist philosophy and psychology, provides a useful framework to analyse both the diachronic and synchronic dimensions of habits, and their roles in behavioural inertia. The diachronic perspective focuses on the factors that govern the processes of formation, reinforcement and change of a given habit; the synchronic dimension deals with the relations such a habit can develop with other habits.

In line with the well-known work by [START_REF] Ouellette | Habit and intention in everyday life: The multiple processes by which past behavior predicts future behavior[END_REF], several scholars confirm that a stable context and repeated behaviour provide a breeding ground for the development of habits. 1 However, this is only one aspect of the diachronic dimension of habits, and refers only to the conditions surrounding the emergence of habit. This leaves crucial questions regarding the nature of habit and the key characteristics of its dynamics. For example, what is the precise link between habit and behaviour? What determines the specific content of a given habit? What are the different modalities of reinforcement of this habit? These and other questions are referred to in presenting our theoretical framework. The empirical part of the paper mainly provides a thorough account of the synchronic dimension of habit, which, to our knowledge, has been mostly overlooked in the literature on transportation. 2 It is obvious that recurrent performance of an action that has become habit-based can trigger the concomitant performance of related habits (Middletown, 2011). For instance, the habit of commuting by car is linked trivially to the habit of driving, but may be associated less trivially to other habits, such as listening to music or eating breakfast. Our study assesses the influence of such associated habits on the habit of driving and particularly their role in reinforcing the car-use habit. We questioned a random sample of 124 car users in the metropolitan area of Lyon about several dimensions of their transportation behaviourespecially the activities they perform when driving a carin order to highlight their habitbased features. We distinguished two groups of car-users based on the frequency of their use of public transport: the first group includes people whose transportation behaviours are 1 See [START_REF] Danner | Habits vs. intention in the prediction of future behaviour: the role of frequency, context stability and mental accessibility of past behaviour[END_REF] for recent findings, notably based on a study on bicycling behaviour. 2 Two noteworthy exceptions are the work of Patricia Mokhtarian and colleagues (e.g. [START_REF] Mokhtarian | How derived is the demand for travel? Some conceptual and measurement considerations[END_REF][START_REF] Handy | Driving by choice or necessity?[END_REF] which suggests that the act of commuting can be enjoyable, and studies by the Centre for Transport and Society, University of the West of England, on travel time use [START_REF] Lyons | Travel time use in the Information Age[END_REF][START_REF] Lyons | The use of travel time by rail passengers in Great Britain[END_REF][START_REF] Jain | The classy coach commute[END_REF]. Among the factors that can make commuting enjoyable are aspects that we refer to as associated practices (e.g. conversing on the phone, listening to music or audio books, looking at the landscape, etc.). Although insightful, these researches depart from our paper in that they are not aimed at assessing specifically the role of those activities in strengthening the habit of driving with which they are associated. based primarily on using a car; the second group includes people with more diverse transportation behaviour. The results show that the VEE framework offers original and fruitful insights on transportation behaviour and highlight the significance of the synchronic dimension on habits for understanding inertia in transportation behaviour.

The paper is organized as follows. Section 2 describes the theoretical, VEE based framework used to analyse habit-based behaviour. Section 3 presents the data and data collection methods. Sections 4 and 5 respectively present and discuss our results and Section 6 concludes by pointing to some major issues that need to be addressed to extend the research agenda initiated in this paper.

Analysing behavioural inertia: the relevance of the Veblenian approach to habits

Spurred by the need to provide a better explanation of behavioural inertia in relation to transportation modes, we propose an interpretative framework that departs from previous theoretical work based on "methodological collectivism" or "decision theory". For several decades these two approaches have dominated the social science literature on human behaviour in general and behavioural inertia in particular. Both approaches have generated major insights, but considering behavioural inertia as the sole expression of the higher forces ("cultural", "social" or "technological") that are supposed to determine individual actions and thoughts (i.e. "methodological collectivism"), is no more satisfactory than considering that it is a simple problem of choice (i.e. "decision theory"). 3Some authors try to go beyond this dualism. However, in our view, these attempts do not develop a framework that is sufficiently comprehensive to contemplate a third way of analysing behavioural inertia. For instance, [START_REF] Kaufmann | Re-Thinking Mobility[END_REF] and [START_REF] Petit | Cinq logiques de mobilité et leurs conséquences sur la planification des déplacements urbains[END_REF] adopt a Weberian perspective and consider habits as the expression of a certain logic of action (among others), that underpins the meaning the actor assigns them. In this view, an actor"s transportation habits are justified and reinforced by other dimensions of his/her way of life and social integration. Our VEE theoretical framework clarifies this synchronic dimension of habits, highlighting its foundations, and integrating it with a general approach to human behaviour.

The VEE framework is rooted in a set of theories that explain behavioural inertia from a perspective on the individual that gives analytical and ontological primacy to habit over choice and reason. This perspective was foundational for the evolutionary and institutional economics of Thorstein Veblen, which owes much to the Pragmatist philosophy and psychology of Charles Sanders Peirce , William James (1842James ( -1910) ) and John Dewey (1859Dewey ( -1952)). It is noteworthy that these theses, developed more than a century ago, have experienced a certain revival in social psychology [START_REF] Ouellette | Habit and intention in everyday life: The multiple processes by which past behavior predicts future behavior[END_REF][START_REF] Verplanken | Habit versus planned behaviour: A field experiment[END_REF][START_REF] Aarts | Predicting behaviour from actions in the past: repeated decision making or a matter of habit?[END_REF][START_REF] Verplanken | Habit, attitude and planned behaviour: is habit an empty construct or an interesting case of goal-directed automaticity?[END_REF][START_REF] Wood | A new look at habits and the habit-goal interface[END_REF], and economics [START_REF] Hodgson | The ubiquity of habits and rules[END_REF](Hodgson, , 2004a[START_REF] Hodgson | Reclaiming habit for Institutional Economics[END_REF][START_REF] Hodgson | Institutions and individuals: interaction and evolution[END_REF][START_REF] Hodgson | Choice, habit and evolution[END_REF][START_REF] Brette | Un réexamen de l"économie "évolutionniste" de Thorstein Veblen. Théorie de la connaissance, comportements humains et dynamique des institutions[END_REF][START_REF] Brette | Expanding the dialogue between institutional economics and contemporary evolutionary economics: Veblen"s methodology as a framework[END_REF][START_REF] Maréchal | An evolutionary perspective on the economics of energy consumption: The crucial role of habits[END_REF][START_REF] Maréchal | Not irrational but habitual: the importance of "behavioural lock-in" in energy consumption[END_REF]. As discussed in depth in [START_REF] Maréchal | An evolutionary perspective on the economics of energy consumption: The crucial role of habits[END_REF], there is obvious common ground underlying the notion of habit developed in these different literatures.

In line with this legacy, an important aspect of our VEE framework is that it considers habit not as behaviour, but as "a propensity to behave i.e. to implement specific patterns of action and/or thought in a particular way in a particular class of situations" (Hodgson, 2010: 4original emphasis). Along similar lines, Wood and Neal (2007: 843) assert that "habits are learned dispositions to repeat past responses". [START_REF] Verplanken | Beyond frequency: Habits as mental construct[END_REF] emphasizes that such a perspective on habits differs from the behaviourist tradition of equating habit with frequent behaviour. 4 This confusion between the propensity to behave and the behaviour per se, can be compared to the conflation in biology of genotype and phenotype. Avoidance of this confusion is crucial. First, it distinguishes the process of habituation, which generally depends on behaviour repetition, from its outcomes, namely the habit, which may be more or less frequently actualized. Second, defining habits as behaviours makes it difficult to explain how they may be transmitted among individuals, except through strict face-to-face mimicking.

Defining habit as a propensity to behave allows consideration of a wide range of mechanisms for its transmission, which may establish institutions, namely "systems of established and embedded social rules that structure social interactions" (Hodgson, 2007: 96). In other words, such an approach provides a rationale for how social structures emerge from the interactions of individuals and how these social structures in turn may constrain, enable or shape individual behaviours. This perspective, which is based on the idea of mutual constitution in a recursive fashion, has proved insightful for explaining the puzzling efficiency gap in energy consumption [START_REF] Maréchal | Not irrational but habitual: the importance of "behavioural lock-in" in energy consumption[END_REF]. We claim that it could prove fruitful in the context of transportation issues which are infused with social and structural dimensions.

Third, considering habit as propensity keeps open the possibility that the same habit may result in a diversity of actualizations. Rather than determining a specific reaction to a given stimulus, habit may grant the individual a more or less significant degree of freedom to define the content of a response. Although a certain degree of automaticity is regarded as one of the main features of habit [START_REF] Verplanken | Beyond frequency: Habits as mental construct[END_REF][START_REF] Maréchal | Not irrational but habitual: the importance of "behavioural lock-in" in energy consumption[END_REF], "[h]abit is not mere automatic behaviour …. Even the most ingrained habits are the objects of recurring mental activity and evaluation" (Hodgson, 1993: 229). This last position points to the crucial issue of the relations between habit, deliberation, choice and action. Considering habit as the basic unit when analysing behaviour does not deny the individual capacity to make decisions, including strategic choices. On the contrary, as William James (1892: 139) already points out, habit is a sine qua non of the exercise of intelligence: since "habit diminishes the conscious attention with which our acts are performed", it releases cognitive resources that can be used to cope with original situations and new problems. This argument has been widely acknowledged within Behavioural Economics. 5 Herbert [START_REF] Simon | Administrative Behavior[END_REF] explicitly endorses it, leading him to consider that man"s ability to make decision derives from his propensity to develop and follow habits in similar situations. Friedrich August [START_REF] Hayek | The Sensory Order. An Inquiry into the Foundations of Theoretical Psychology[END_REF] adds to the argument, by identifying neural underpinnings to memory. Hayek argues that "the mind classifies all in-coming data from the external environment into an appropriate category based on the similarity of the incoming data with all previously received data" (Frantz, 2013: 11). These inner processes lead an individual"s past experiences to shape his/her later experiences, perceptions and actions, depending on the activation of different parts of his/her nervous system. Subsequent works in neuroscience and neuroeconomics have corroborated Hayek"s (1952) insights (Festré and Garrouste, 2009: 261-264). In particular it has been shown that "everyday consumer behaviour", such as "routine shopping and consuming", displays features that can be properly observed and analysed at the neural level. For instance, brand loyalty has been interpreted in terms of matching and discounting, two psychological phenomena, which are associated with neurophysiological processes, such as the release of the neurotransmitter dopamine within the reward system, and with the generation of emotional responses (Foxall, 2008: 378). This perspective allows highlighting an important part of habit-based behaviour.

Actually this way of entering the cognitive black box of habits to understand their neural underpinnings echoes William James" (1892) concern who already points out the neurophysiological foundations of habit formation. 6 5 We are grateful to a referee for his/her suggestion to consider the relations between the VEE framework and Behavioural Economics, from Simon and Hayek to Neuroeconomics. 6 James (1892: 134) defines habit in the following terms: "An acquired habit, from the physiological point of view, is nothing but a new pathway of discharge formed in the brain, by which certain incoming currents ever after tend to escape". See [START_REF] Twomey | Reviving Veblenian economic psychology[END_REF] for an extended discussion of how recent results from a number of cognitive sciences are reminiscent of Veblen"s and American pragmatists" ideas.

However, although consistent to some extent, Neuroeconomics and VEE views on habit are rooted in different approaches. In the main, neuroeconomics has tended so far to interpret habit through the lens of decision theory, as a "control system" competing with other individual"s "decision systems" -i.e. "the Pavlovian system" and "the goal-directed system"when facing specific situations of choice, such as "which road to travel during a commute, or addiction" (Fehr and Rangel, 2011: 22). 7 On the contrary, the VEE perspective rests on the principle of continuity, which allows habits to be "fused" with rationality (Kilpinen, 2005: 1). It leads to behaviour being considered as a succession of actions where "motives do not precede action because they enter the scene in the middle of ongoing action processes" (Gronow, 2008: 361). Habitual and intelligent aspects interact during the performance of action [START_REF] Kilpinen | The enormous fly-wheel of society: Pragmatism"s habitual conception of action and social theory[END_REF][START_REF] Kilpinen | Human beings as creatures of habit[END_REF]. Moreover, habit is seen as the very foundation of rationality since "rational choices themselves are always and necessarily reliant on prior habits" (Hodgson, 2004b: 653). The traditional view of rationality is thus reversed: "its role is to hinder rather than further our action process" [START_REF] Kilpinen | A positive correlation between rationality and habituality -What does it mean?[END_REF] 2), and "it is no more action that needs to be explained, it rather is a change in action that demands an explanation" (ibid: 4). Individuals do not necessarily need incentives/motives to start acting; however opposing habitual ways of acting requires incentives/motives. More generally, the VEE framework puts the process of formation, reinforcement and change of habits at the heart of the research agenda, and allows the principle of continuity in human behaviour to be considered from two complementary viewpoints: the diachronic and synchronic dimensions.

From a diachronic viewpoint, one should consider that the characteristics of an individual"s habits, at a given time, are "path-dependent", or dependent on their processes of formation, reinforcement and change. Identifying and analysing these processes is crucial to understand the forming of the individual"s cognitive perceptions, appreciations and normative judgments. Indeed, the process of evolution of habits leads to the development of coherent cognitive frames that shape the way individuals interpret the world and make decisions [START_REF] Dolfsma | The mountain of experience: How people learn in a complex, evolving environment[END_REF]. Hodgson and Knudsen (2004: 36) argue that "a sequence of similar and repeated behaviors creates in each agent a habitual predilection, which can stimulate a "belief" or "conviction" that a particular behavior is appropriate" and, finally, that these "stubborn "beliefs" in the appropriateness of an action … weigh heavily in the decisionmaking process of each agent". There are many types of factors that may combine to shape the habituation processes including factors related to the individual"s personal features, and to his/her natural, material and social environment. A significant change in context may create a disturbance to the individual"s behaviour, which could promote changes to certain habits and alter his/her "beliefs" and "convictions"see the example given by Hodgson (2003: 166) in relation to transportation behaviour. In these circumstances, the individual"s "rationality" is opened to new information and to some reassessment of his/her habits.

Promoting openness and exploiting such "windows of opportunity" is a major challenge for public policy, which aims at increasing people"s receptiveness to economic incentives [START_REF] Maréchal | Not irrational but habitual: the importance of "behavioural lock-in" in energy consumption[END_REF]. Finally, the individual"s social interactions must be considered. 8 This aspect is related to the crucial issue of institutional evolution, or the process of formation, reinforcement and change of institutions. The current paper does not specifically address institutional issues. Our present concern is to outline our VEE framework and to begin to elaborate on its implications for transportation studies. The rest of the paper is devoted to the personal features of an individual"s transportation habits. Exploring these "simple" features is a first (necessary) step to the study of more intricate issues, such as the dynamic interactions between individual habits and institutions. As Hodgson (2010: 14) argues, "once habit is seen as the foundation of preferences or beliefs, we can develop an enriched understanding 8 For a comprehensive account of the interplay between institutions and the individual"s habits and intentions, see [START_REF] Fleetwood | Structure, institution, agency, habit, and reflexive deliberation[END_REF] where he discusses the case of labour market. On the importance of this topic for future research in relation to transportation behaviour, see Dugundji et al."s (2011) of the interaction between individuals and institutions, including of the causal mechanisms involve".

To be more precise, our focus is on the synchronic dimension of personal habits. At a given time, an individual"s ingrained habits are more or less interdependent although they may be related to different fields of activity or thought. Veblen (1990: 39) argues that "[t]he individual subjected to habituation is each a single individual agent, and whatever affects him in any one line of activity, therefore, necessarily affects him in some degree in all his various activities". 9 We consider this point to be crucial for analysing the strength of a habit. We thus test the synchronic hypothesis that the strength of a habit, especially car-use habit, depends on its relations and degree of consistency with other habits.

Material and Methods

Characterizing the nature, manifestation and effects of habits related to commuting behaviour is difficult. [START_REF] Verplanken | Reflections on past behaviour: A self-report index of habit strength[END_REF] use the Self-Report Habit Index (SRHI), to open the "blackbox" of habits and study their automaticity. Following the work of John [START_REF] Bargh | The four horsemen of automaticity[END_REF], automaticity displays four distinct features or the "four horsemen of automaticity": lack of control; lack of awareness; efficiency (i.e. economizing on cognitive resources that can be used for other purposes); and lack of intention. [START_REF] Verplanken | Reflections on past behaviour: A self-report index of habit strength[END_REF] provide evidence of the existenceto an extentof the first three features, which can serve to distinguish the "strength" of different habits. Regarding unintentionality, this requires some qualification: if habits can become "counterintentional" [START_REF] Verplanken | Good intentions, bad habits, and effects of forming implementation intentions on healthy eating[END_REF], the fact that they must be functional (i.e. in line with other on-going actions and beliefs) makes them intentional to some degree [START_REF] Polites | Counterintentional habit as an inhibitor of technology acceptance[END_REF], which means that habits cannot be reduced to pure automatism or reflex-type behaviour.

9 One can also consider the synchronic viewpoint at the institutional level. As Veblen (1915: 374) asserts, "the cultural scheme i.e. the institutional system is, after all, a single one, comprising many interlocking elements, no one of which can be greatly disturbed without disturbing the working of all the rest". While having important implications for transportation issues, these aspects are beyond our current concern.

Although insightful and helpful for empirical investigation of the strength of habits, Verplanken and Orbell"s SRHI is a rather "static" and "discrete" approach that needs to be complemented by some characterization of "dynamic" (namely diachronic) and "systemic" (namely synchronic) features. Also, the degree of automaticity of a habit is not equivalent to its "strength". Starting from the definition of habits in [START_REF] Hodgson | Choice, habit and evolution[END_REF], we assess the strength of habits through the relation between particular classes of situations and the implementation of specific patterns of behaviour. Strength of habits, e.g. the strength of the car-use habit, encompasses both intensity and scope. Intensity refers to the frequency with which a specific pattern of behaviour is activated in a given set of situations. For instance, the frequency of using the individual"s own car to travel between home and work is a measure of the strength (intensity) of his/her car-use habit. Scope refers to the variety of types of situations (e.g. work commuting, grocery shopping, family visits, etc.) when a specific pattern of behaviour (e.g. using the car) is commonly activated.

It is also important to stress that empirical analysis of the notion of habits entails risk of methodological bias, especially because many surveys put the respondent in the position of decision-maker [START_REF] Røe | Qualitative research on intra-urban travel: an alternative approach[END_REF][START_REF] Petit | Cinq logiques de mobilité et leurs conséquences sur la planification des déplacements urbains[END_REF]. Respondents trapped within the researcher"s decision theory framework have to justify their so-called choices and apply transportation mode choices to hypothetical situations. This can make habits appear incidental determinants of daily mobility patterns, or reduce them to the outcome of some choice, which is inconsistent with our view of habits discussed in Section 2.

Accordingly, we developed an original method that combines qualitative and quantitative elements in order to identify some habit-based features of transportation behaviour. In the first step of our empirical study, 15 regular car-users were asked to describe, explain and comment on their daily car journeys. Information was collected at the end of each journey over seven consecutive days, using voice recorders, followed by in-depth interviews with respondents. This first step, which echoes to some extent Middleton"s (2011) and Earl"s (2012) experiential approaches, informed a questionnaire to identify recurring or unexpected elements. 10 In the second stage we conducted a questionnaire survey on a random sample in December 2010 involving 124 respondents who declared using their cars "at least several times a week" and having a close-to-home access to public transportation. The survey was conducted during four periods of two hours in four main squares in Villeurbanne. Residents of Villeurbanne were chosen to be survey respondents. Villeurbanne is the second (in terms of population) city in the metropolitan area of Lyon (142 552 inhabitants in 2008) and forms a continuous urban area with the city of Lyon within a ring road. The average distance from home to the public transportation system for the survey sample was 110m, providing respondents with a convenient alternative to private car use. This allowed examination of the importance of inertia in daily transport behaviours in a urban context. In addition to general information on the respondents (see Table 1), this survey was designed to identify their places of residence and work, their most frequent journeys, their behaviours associated with driving, their frequency of weekly use of other transportation modes than private car, their feelings of constraint or choice associated with the transportation modes used, and to collect their qualitative judgments about alternative solutions. Most frequent journey by car included 31% direct commuting trips, 30% shopping trips, and 31% leisure and free-time activities. 10 Middleton (2011) explores the experiential dimensions of habits of urban pedestrian mobility, on the basis of interview and diary data. Earl"s (2012) approach is quite different, in that it rests on the introspection of the researcher himself, in order to develop a long term diachronic perspective on car-purchasing behaviour.

Variables n Freq

Results

Several quantitative surveys show that modal share and traffic in the metropolitan area of Lyon remained stable between 1995 and 2007 despite major improvements to public transport, especially since 2000 [START_REF] Hubert | Dans les grandes agglomérations, la mobilité quotidienne des habitants diminue, et elle augmente ailleurs[END_REF][START_REF] Urbalyon | Le trafic routier en agglomération lyonnaise: quelles évolutions?[END_REF]. Although the number of car journeys declined between 1995 and 2007, their average length increased. In contrast to the projections of policy makers, car use continues to be significant and has become more complex as car drivers globally tend to integrate more activities per journey [START_REF] Buhler | Should we hang private car drivers? Facing private car driver's resistance to sustainable mobility policies. Lessons from metropolitan Lyon's case and research perspectives[END_REF].

The survey results presented here constitute the core of our research to develop a better understanding of relative behavioural inertia in daily transportation modes, especially caruse, in the metropolitan area of Lyon. We assess the relations between degree of behavioural inertia, strength of the car-use habit, and the characteristics of other habits associated with driving.

We formulated a series of questions to test several hypotheses with respect to potential theoretical explanations for observed inertia in modal behaviours. These explanations mirror two of the three classes of theories referred to in Section 2, that behavioural inertia can be considered the result of rational choice or habits as defined in the VEE perspective. Many analytic and forecasting models used for transportation planning and in academic research, build on cost-benefit analyses to predict behaviours, using the variables time and cost in a generalized cost analysis. This refers to a conception of universal rationality. The first set of survey results assesses the relevance of this cost and time approach to analyse transportation behaviours. Participants were asked to estimate the time and monetary costs involved in their most frequent car journeys and the equivalent costs of alternative public transport. This allows us to compare reported estimations with objectified values and identify the proportion of the population giving an accurate estimation for either alternative. 11

For the variable time, a huge majority (96%) were unable to provide both an accurate (i.e.

within a 20% margin of error) estimation of the time required for their most frequent journeys by car and of the time required for the same journey using public transport, whether they already experienced it or not (see Table 2).

11 Objectified journey times were estimated using information provided on the websites "www.mappy.com" for car journeys and "www.tcl.fr" for public transport journeys, taking into account traffic conditions at the same time on the same day. These results tend to discard the idea that individuals take account of objective time issues and choose on this basis to use their cars for their most frequent trips. Our results are coherent with the findings of other studies which highlight the discrepancies between perceived time and real time, due to a diversity of biases in time perception [START_REF] Kaufmann | Re-Thinking Mobility[END_REF][START_REF] Li | Evaluating the urban commute experience: A time perception approach[END_REF][START_REF] Parthasarathi | Network structure and travel time perception[END_REF]. Since perception of time is highly subjective, it is essential to look at the other component in rational choice, namely the associated financial costs. Respondents were asked to assess the monthly costs of their most frequent car journeys, and then were asked about what these included. A small minority (16%) were unable to answer the question. More than two-thirds of respondents (71%) referred only to fuel costs. 12 In line with the results of [START_REF] Beirão | Understanding attitudes towards public transport and private car: A qualitative study[END_REF], it thus appears that the cost of travel is unlikely to be the main reason for observed inertia in transportation behaviour. Our results support the idea that people do not necessarily resort to a choice scheme based on comparing alternative times and costs, as assumed by decision theory.

To test the role of habits in transportation mode behaviours, we partitioned the survey population into two subsamples, considering the respondents" frequency of use of public transportation services. The first sub-sample, "PT+", includes 38 people who use public transportation services more than once a week, in addition to using their car. Sub-sample "PT-" includes 86 people who rarely use public transport (i.e. once a week maximum). Our segmentation criterion allows us to classify car-drivers according to the strength of their caruse habit, assuming that both samples experience similar classes of situations. 13 This means that the PT+ group habits could be interpreted in two ways. It may be that PT+ drivers use their personal cars and public transport alternately for a particular type of trips (e.g. Correlation tests show no specific link of this habit strength with age, gender, educational level, number of children, occupancy status or employment location (see Table 3). These usual socio-economic variables are weakly correlated with the strength of car-use habit (i.e.

being part of PT-group). What seems to have an influence on the strength of car-use habit, however, is the fact of having a professional activity. This would tend to support the idea developed in [START_REF] Shove | The Design of Everyday Life[END_REF] that time-constrained schedules (e.g. having to conform to widespread institutional arrangements regarding working hours) provide a favourable context for the formation of habits. This line of argument is reinforced by an even more (statistically) significant result from Table 3 which shows that the "feeling of having no choice" for transport mode is strongly correlated with the fact of belonging to the PT-group. Obviously, one must be very cautious in treating such a result linked to a statement that is inherently subjective (i.e. it is but a feeling). Nevertheless, taken together with the influence of having a professional activity, 14 it does suggest that "constrained" car-drivers tend to display stronger car-use habits. This important result is further discussed in Section 5 in connection with other insights from the empirical survey.

As expected, differences in the strength of the car-use habit also correspond to significant differences in the patterns of transportation behaviour. Table 3 shows that, on average, PTdrivers use their personal cars more frequently and for longer journeys than PT+ drivers. This result leads us to examine the links between car-use habit strength and differences in the respective behaviours of the two sub-groups associated directly with driving.

14 Coupling both arguments is compatible with our data since, within the PT-group, 81% of people that declare a "feeling of having no choice for the transport mode" have a professional activity (compared with a proportion of 69.76% in the PT-group as a whole and 50% in the PT+ group). We hypothesized that the development of habits during car journeys might play a role in strengthening the car-use habit. Some of the literature on transportation [START_REF] Mokhtarian | How derived is the demand for travel? Some conceptual and measurement considerations[END_REF] indicates that daily journeys are increasingly used for a series of activities (phoning, texting, chatting, etc.). This trend is based on growing use of communication technology and less clear boundaries between family life and work and leisure activities [START_REF] Urry | Sociology Beyond Societies: Mobilities for the Twenty-First Century[END_REF][START_REF] Laurier | Doing Officework on the Motorway[END_REF]. Investigating these "associated behaviours" provides a better picture of the lived experience of a car journey. The first qualitative step in our protocol (based on in-action voice recordings) identified 13 different recurring associated behaviours mentioned in Table 4. We asked participants to the quantitative survey to estimate the frequency of each according to: "always", "often", "rarely", "never". Respondents had space to indicate additional associated behaviours. Two respondents reported the respective actions of "singing" and "eating while driving". This suggests that, in the main, our list included relevant behaviours.

The first effect identified is the weak influence of gender, social category and age on the nature and frequency of the 13 associated behaviours. In contrast, we found a statistically significant correlation between belonging to one of the two sub-groups (i.e. PT+ versus PT-)

and the nature of these associated behaviours. Although respondents from the PT-group reported on average performing a higher number of associated behaviours (5.70 to 4.92 for the PT+ group) this result remains non-significative. Beyond these quantitative aspects we explore the hypothesis of specific learning effects for the PT-group in their daily driving. To test this hypothesis further, we examine the qualitative features of the associated behaviours.

Table 4 shows that some associated behaviours are more frequent in the PT-group than the PT+ group. Most of the associated behaviours in this category (i.e. "texting", "phoning" and "talking to passengers") require at least one hand to be free, focused cognitive resources, and/or speech. n.s.

Always + Often

Table 4. Practices associated with driving according to the strength of car-use habit

The associated behaviours that are more frequent among the PT+ group or are equivalent for both groupsi.e. listening to music, listening to a particular radio broadcast, thinking about one"s job/daily life, thinking of nothing in particular, looking at the city and the landscapeare passive behaviours, requiring less focused cognitive resources, and show no statistically significant difference.

These results confirm the hypothesis of specific learning effects and possibility of behaviours that require additional cognitive resources for the PT-group. 15 The dexterity (manual dexterity, focused cognitive resources, speech) acquired through these learning processes and frequent journeys enabled this group to integrate a series of associated practices that they correlated with experiencing a "pleasant" journey. As shown by [START_REF] Gardner | What drives car use? A grounded theory analysis of commuters" reasons for driving[END_REF], the performance of related activities is mentioned by cars users in relation with the idea of a "personal space". This private sphere is valued by car users notably as it ensures a "freedom from observation" thereby allowing them to do things such as "singing loudly" (Gardner and Abraham, 2007: 192). The car is also experienced as providing opportunities

for "relaxation and quiet contemplation" which may explain some of the puzzling findings of our study that are explained below. Indeed a logistic regression of the occurrence of the adjective "pleasant" 16 (dependent variable, Table 5) shows that the strength of car-use habit (i.e. PT-or PT+) has a statistically significant impact on perceptions of journey times as "pleasant", even more than real travel time. Mirroring this important result, Table 6 (see appendix 1) displays the results of the logistic regression for the occurrence of the adjective "stressful". Besides real travel time, the only other variable that plays a significant role on perceiving car journeys as being stressful is the fact of belonging to the PT+ group.

15 [START_REF] Middleton | I"m on autopilot, I just follow the route": exploring the habits, routines, and decision-making practices of everyday urban mobilities[END_REF]Middleton ( : 2873) ) points out similar learning effects in case of walking habits, deriving from the everyday practices of urban pedestrians. 16 Respondents were asked to qualify the time spent during each journey with one or several adjective(s): "What do you think about the time spent in car during this particular journey?". Multiple answers were suggested ("long", "short", "useless", "useful", "pleasant", "stressful", "tiring") and the respondents could also suggest another adjective. The impact of transportation mode behaviours on qualitative perceptions of travel time suggests that associated practices may influence drivers" evaluations. The formation and reinforcement of habits through mono-modal experience of car use (PT-) results in a deeper exploitation of travel time and more positive evaluation of car travel experience which contributes to stronger behavioural inertia.

The results of our study confirm the empirical findings in psycho-sociology and behavioural economics and, as the discussion below shows, are particularly consistent with the VEE framework.

Discussion

Habits and perceptions of journeys

Traditional rational choice theory has been challenged by recent empirical findings in behavioural economics (see [START_REF] Gowdy | Behavioral economics and climate change policy[END_REF][START_REF] Dellavigna | Psychology and economics: evidence from the field[END_REF], for a useful survey). In a world of bounded rationality [START_REF] Simon | Administrative Behavior[END_REF], people are unable to absorb all the information available. They must select among it [START_REF] Maréchal | Overcoming inertia: insights from evolutionary economics into improved energy and climate policy[END_REF][START_REF] Arena | Introduction[END_REF] using filters or cognitive frames (for a discussion see [START_REF] Gronow | Not by rules or choices alone: a pragmatist critique of institution theories in economy and sociology[END_REF]. Moreover, individual learning grounded on a personal "mountain of experience" may lead people facing a complex and evolving environment to "cling to beliefs they have, sometimes in the face of evidence to the contrary" (Dolfsma, 2002: 681). How people assess the time and costs of their most frequent journeys (see Table 2) suggests that their transportation behaviour does not derive from a well-informed rational economic trade-off between car use and use of public transport services (see also [START_REF] Kaufmann | Re-Thinking Mobility[END_REF][START_REF] Li | Evaluating the urban commute experience: A time perception approach[END_REF][START_REF] Beirão | Understanding attitudes towards public transport and private car: A qualitative study[END_REF][START_REF] Parthasarathi | Network structure and travel time perception[END_REF]. 17 The information required to assess this trade-off may be available, but it tends to be ignored and people adopt behaviours based on habit.

Various explanations can be proposed to explain this fact. They include the efficiency derived from habit (i.e. developing certain habits allows better management of scarce cognitive resources). Indeed, the possibility that some mental processes become fairly automatic once learned, saves on psychological energy which can be diverted to novel tasks [START_REF] Egidi | Organizational learning, problem solving and the division of labour[END_REF][START_REF] Bargh | The unbearable automaticity of being[END_REF][START_REF] Kahneman | Maps of bounded rationality. Psychology for Behavioral Economics[END_REF]. Furthermore, in line with the suggestion from de [START_REF] Board | The Psychoanalysis of Organizations[END_REF] and [START_REF] Bovey | Resistance to organizational change: the role of cognitive and affective processes[END_REF], it appears that behavioural inertia may act as a defence mechanism to reduce anxiety and reinforce self-control. "Stick-with-what-weknow"-type actions are likely to enhance comfort and security [START_REF] Lindbladh | Habit versus choice: the process of decision-making in health-related behaviour[END_REF], especially in contexts that require individuals to act under time pressures [START_REF] Betsch | Oops, I did it againrelapse errors in routinized decision making[END_REF]. This line of argumentation is corroborated by some of the main insights from our empirical survey as not only do car-drivers with time-constrained schedules display stronger car-use habits (see Table 3) but the fact of having stronger (weaker) car-use habits increases the likelihood of car journeys being perceived as "pleasant" ("stressful") (see Tables 5 and6). Stronger habits thus seem to indeed provide both a way of handling time pressure and a mechanism to reduce the stress linked to driving. This constitutes a reasonable explanation for an otherwise puzzling result of the survey: the same individuals that more often feel to be highly "constrained" in "choosing" their mode of transportation are also those that find car journeys more "pleasant". Building on the recursive perspective on causation between agency and structure which characterises the VEE framework, 18 this may be interpreted as institutionally-constrained individuals exerting a certain degree of agency through taking advantage of a situation they feel they do not have much grip on. This illustrates how habits may combine with individual "reflexivity" [START_REF] Davis | The Theory of the Individual in Economics: Identity and Value[END_REF][START_REF] Archer | Structure, Agency and the Internal Conversation[END_REF][START_REF] Archer | Making our Way Through the World[END_REF]. 19 [START_REF] Middleton | I"m on autopilot, I just follow the route": exploring the habits, routines, and decision-making practices of everyday urban mobilities[END_REF]Middleton ( : 2859) ) provides examples of such an overlapping process, in emphasizing the importance of "specific narratives of everyday urban mobilities [especially pedestrian mobilities]; the significance of how such practices are actually "talked about"; and how these accounts matter in engaging with the experiential dimensions of urban movement". Habituation thus constitutes the mechanism through which "active" agentsi.e.

agents who may "undertake instrumental internal conversations" (Fuller, 2013: 121) adjust 18 This perspective can be subsumed as bearing in mind that "habits are the constitutive material of institutions" while the presence of institutions make that "accordant habits are further developed and reinforced among the population" Hodgson (2007: 107). 19 We thank two anonymous referees for highlighting the benefit of making room for Davis" (2003) and [START_REF] Archer | Structure, Agency and the Internal Conversation[END_REF][START_REF] Archer | Making our Way Through the World[END_REF] approaches to reflexivity in our discussion. Davis (2003: 117-119) makes it clear that the VEE approach to the individual as a socially embedded being is consistent with the acknowledgment of his/her capacity for reflexivity and learning (see also [START_REF] Dolfsma | The mountain of experience: How people learn in a complex, evolving environment[END_REF]. Such a view of the individual is necessary to the development of coherent (non arbitrary) structure-agency models. Finally, [START_REF] Fuller | Reflexivity, relative autonomy and the embedded individual in economics[END_REF] argues convincingly that [START_REF] Archer | Structure, Agency and the Internal Conversation[END_REF][START_REF] Archer | Making our Way Through the World[END_REF] analysis of individual reflexivity may articulate with the VEE approach to habit and [START_REF] Davis | The Theory of the Individual in Economics: Identity and Value[END_REF] insights on the "relative autonomy" of individuals.

their cognitive perceptions, matters of appreciation and normative judgements in coherent structures [START_REF] Lindbladh | Habit versus choice: the process of decision-making in health-related behaviour[END_REF]. As our survey suggests, one way this works is through habits enabling cognitive resources to be devoted to the performance of synchronic habits, rendering journeys more pleasant. This binding nature of those strong habits intimately connected with everyday life may be what causes individuals to feel they have no choice or, more likely, that it would be very effortful for them to do otherwise.

Within the VEE perspective, habits thus are not simply an efficient way to save on cognitive resources expended on searching and processing information. Habits intimately shape the way individuals acquire, filter and manage information in accordance with their stubborn "beliefs" and "convictions" (Hodgson and Knudsen, 2004: 4).

Performance of habits: one habit triggers another

Our results tend to confirm the synchronic hypothesis that the strength of the habit, in this case car-use, depends on its relations with other habits and the degree of mutual consistency among the various habits. For instance, the habit of driving oneself to work may be associated with other habits, such as supermarket shopping after work [START_REF] Shove | The Design of Everyday Life[END_REF]. These habits are mutually strengthening to the extent that the existence of one justifies the existence of the other. This may explain the reluctance to switch to another option such as public transportation even in presence of a convenient service.

Our study focuses on the relations between the habit of car-use and other habits actualized by the driver during a car journey, such as listening to music, phoning or looking at the landscape. These "small habits", developed to an extent by every individual, may appear trivial especially when considered independently. However, we argue that they become significant when considered jointly, and contribute to shaping the travel experience. This travel experience, in turn, plays a major role in the stability of transportation behaviour thereby strengthening the transportation habit.

Our results show that the more the frequency of their actualization increases, the more the car-use habit and its related synchronic habits (reading documents, writing a message, phoning, and so on) tend to be mutually reinforcing in the sense that one habit triggers the others. Table 4 shows that the people in the PT-group are more likely to perform a set of activities that require particular dexterity. In this respect, our empirical findings are in line with results in psychology on the link between behaviour repetition and strengthening of habits (see Danner et al., 2008 among others). Also, the more actualized the car-use habit is, the more smoothly will its related synchronic habits be performed, leaving space for the memorization of new cognitive skills [START_REF] Lazaric | Organizational routines and cognition: An introduction to empirical and analytical contributions[END_REF]. In short, drivers in the PT-group are more prone to being governed by cognitive automatisms enabling combination with additional activities since already established activities (i.e. driving) become more automatic.

Finally, our results shed new light on the relations between habits and the pleasure of commuting [START_REF] Mokhtarian | How derived is the demand for travel? Some conceptual and measurement considerations[END_REF]. They show that an increase in the frequency of actualizations of car-use and associated habits positively affects the pleasure derived from travelling (Table 5). Thus, there seems to be a positive feedback between the development of associated habits, more positive perceptions of the travel experience, and a stronger caruse habit.

These results open new avenues for mobility policy. Transportation habits intermingle with and are performed as part of a large set of habits, and cannot be reduced to discrete phenomena that could be targeted and changed independently by public policy. Promoting change in transportation behaviour thus implies acting on various linked factors that underlie the development and strengthening of bundles of everyday life habits. This not only implies taking account of the main habits associated with the trip-chaining phenomenon, such as grocery shopping [START_REF] Ye | An exploration of the relationship between mode choice and complexity of trip chaining patterns[END_REF], but also "smaller habits", which may play jointly a significant role in shaping people"s convictions and behaviours. 20 This leads us to wonder 20 Admittedly, acknowledging the interrelatedness between habits may significantly complicate the design and assessment of public policies. As pointed out by one reviewer, prohibiting smoking in public transport could well constitute an indirect incentive for smokers to continue driving their cars. Conversely, making it illegal to phone or whether public policies should primarily target car drivers" will to change and their awareness of convenient public transport alternative. According to [START_REF] Bargh | The unbearable automaticity of being[END_REF], disrupting habit-based behaviours requires setting specific goals, and motivation. Similarly, [START_REF] Pelletier | Environmental psychology and sustainability: Comments on topics important for our future[END_REF] argue that people need to be aware of the drawbacks of current transportation schemes in order, in a second stage, to identify solutions. Having chosen an alternative behaviour, its recurrence could result in the development of new habits. The VEE framework is illuminating and almost reverses this argument by considering that awareness of the drawbacks of current transportation behaviour is a secondary factor in the process of change. The primary factor is the appearance of a disturbance to the current sequence of actions that is sufficiently important to trigger a reassessment of current behaviour. One way to motivate change in transportation, to maintain the intensity of this motivation over time and to make it effective, is to disturb the process of positive feedback between actualization of the car-use habit, actualization of associated habits and more positive perceptions of the travel experience.

Conclusion

This paper is intended to show the usefulness of the VEE perspective on habits, and to complement current analyses of transportation behaviour. The results of our empirical study underline the shortcomings of standard economic approaches that consider urban transportation behaviours as the sole result of a decision process guided by a well-informed assessment of the costs and benefits of various alternatives. This confirms the need to investigate the issue of urban transportation through an alternative conceptual lens. In line with the principle of continuity, applying the VEE framework allowed us to explore the importance of the synchronic dimension of habit for strengthening the car-use habit. Our empirical findings show that synchronic habits can significantly affect behavioural inertia in send text messages while driving on the ground of a better safety could lead some car-drivers (i.e. those who are very active on social networks) to use public transport.

this regard and should be taken into account by policy-makers targeting urban transportation.

For instance, this approach to transportation behaviour suggests a new rationale for the importance of "windows of opportunity" or key events when modal changes are more likelye.g. following a house move or the birth of a child [START_REF] Fujii | Development of script-based travel mode choice after forced change[END_REF][START_REF] Stranbridge | Travel behaviour change and residential relocation[END_REF][START_REF] Bamberg | Is a residential relocation a good opportunity to change people"s travel behavior? Results from a theory-driven intervention study[END_REF][START_REF] Bamberg | Using a residential relocation as starting point for breaking car use habits[END_REF][START_REF] Rocci | De l'automobilité à la multimodalité ? Analyse sociologique des freins et leviers au changement de comportements vers une réduction de l'usage de la voiture. Le cas de la région parisienne et perspective internationale[END_REF][START_REF] Schäfer | Breaking habits: linking sustainable consumption campaigns to sensitive events[END_REF][START_REF] Verplanken | Context change and travel mode choice: Combing the habit discontinuity and self-activation hypotheses[END_REF][START_REF] Maréchal | Not irrational but habitual: the importance of "behavioural lock-in" in energy consumption[END_REF][START_REF] Meissonnier | Le déménagement: un déclencheur des changements dans les choix modaux? Le cas de trois agglomérations du nord de la France[END_REF]. Indeed, synchronic habits are found to be partly inactivated during these specific life stages. The underlying thesis is that the most important driver of changes to habits may be disturbances in the current driving experience, including the actualization of habits associated with car-use.

It follows from the conceptual perspective adopted in this paper that there is indeed much more behind a habit than the sole behaviour to which it gives birth. It also comprises the underlying processes which bestow habits a central role in maintaining general coherence of individuals" perceptions, appreciations, and normative judgements, and allow us to fully grasp their intertwining with institutions. In line with the argument put forth in Schwanen et al. (2012: 527), this intertwining makes that displacing carbon-intensive transportation habits "requires changes to "objective conditions", which are notor rather not onlyinfrastructures or pricing structures as most travel behaviour analysts would have it but the customs and institutions that have shaped the habits of body-mind-world assemblages".

Coupling the main results of our survey with the account provided in Pooley et al. (2011: 19), it follows that some of the needed wider societal changes that are relevant for tackling strong car-use habits would be (among others) to increase the flexibility of working hours so that alternative modes "could be more easily fitted into a household routine" and to devise on family welfare policies allowing parents to be "less constrained by time commitments".

Despite some useful findings, the approach adopted in this paper has some limitations which could be addressed in future research. First, our approach to the strength of the car-use habit is quite crude. It would be useful to develop a tool to measure habit strength more precisely, distinguishing between scope and intensity and taking account of the diversity of types of situations individuals face. Our method could be improved by considering some intermediate stages between strong and weak habits, and the possibility of a continuous rather than a discrete measure of habit strength might be considered. Also, the present study considers only personal car and public transportation services as substitutes for urban transport. The survey did include questions about cycling and walking, but the responses were not significant enough to include in our analysis. A measurement tool for research on transportation should include a more comprehensive approach to modal behaviour, including non-motorized modes.

This paper focuses mainly on the synchronic aspects of habits. A barrier to the development of a comprehensive tool to assess transportation habits is the lack of longitudinal approaches to transportation already underlined by [START_REF] Goodwin | The case for heterodoxy in longitudinal analysis[END_REF]. More comprehensive analysis would require a combination of longitudinal and cross-sectional approaches. It should take account of path-dependence effects and habit reinforcement, but avoid confinement in an idiosyncratic conception of the processes at play. Although the specificities of each person"s experience of life are likely to play a major role in developing some specific habits [START_REF] Earl | Experiential analysis of automotive consumption[END_REF](Earl, : 1070)), a typology of generic life paths in relation to transportation could be constructed as a necessary next step in research on habits in transportation behaviour. For instance, one cannot consider in the same way people with a strong car-use habit formed after trying all alternatives, with people who commute exclusively by car and have always done so. 21 The diachronic dimensions of habits should thus be the subject of further work to understand and formulate actions to reduce behavioural inertia in transportation. Such an approach should notably have in view a better understanding of the mechanisms of "reconstitutive downward causation" from institutions to individuals [START_REF] Hodgson | Reclaiming habit for Institutional Economics[END_REF][START_REF] Hodgson | Institutions and individuals: interaction and evolution[END_REF]. Indeed, the impact of low- 21 We are grateful to an anonymous reviewer for his/her suggestion to consider [START_REF] Archer | Structure, Agency and the Internal Conversation[END_REF] typology of modes of "reflexivity" or "internal conversation" (see [START_REF] Fuller | Reflexivity, relative autonomy and the embedded individual in economics[END_REF] as a relevant way of identifying some typical groups of people, the generic life paths of whom are likely to differ from one to the other, with significant implications in terms of mobility habits. Here is a potentially fruitful perspective to explore in further work.

carbon mobility policies also depends on their ability to displace "the cultural meanings and affective atmospheres associated with cars" and to challenge "popular connotations of cars with freedom, power, control" (Schwanen et al., 2012: 528). In this case, as in many other fields, it seems essential to put the processes of formation and evolution of habits and institutions at the heart of the social sciences research agenda, as Thorstein Veblen already urged economists to do one century ago.

  guest editorial in the Special Issue on Transportation and Social Interactions in Transportation Research Part A: Policy and Practice.

  commuting) while PT-drivers (almost) always use their personal cars in the same class of situations and/or that PT+ drivers always use public transport for a certain type of trips which PT-drivers usually accomplish by car. Differences in the strength of the car-use habit between PT+ and PT-drivers may rest on the intensity of the habit (first case) and/or on the scope of the habit (second case). Whatever the case, PT-drivers are considered to have a stronger car-use habit than PT+ drivers.

Table 1 .

 1 Descriptive statistics

Table 2 .

 2 Assessment of travel times for an individual"s most frequent journey by car and for public transport alternative

Table 3 .

 3 Characterisation of two groups of people according to the strength of car-use habit

The reported coefficients are estimated with a multinomial logit with random effects. The significance thresholds are respectively 0.5%(***), 1%(**) and 5%(*)

  Rarely + never Chi-square test

	the PT-group	Reading or re-reading documents Writing a message / SMS	PT+ PT-PT+ PT-	0 [0%] 4 [5%] 1 [3%] 16 [19%]	38 [100%] 82 [95%] 37 [97%] 70 [81%]	p = 0.177 n.s. p = 0.017 *
	in frequent more practices	Phoning Talking to passengers Smoking a cigarette Combing again your hair /	PT+ PT-PT+ PT-PT+ PT-PT+	4 [11%] 24 [28%] 27 [71%] 75 [87%] 7 [18%] 25 [29%] 1 [3%]	34 [89%] 62 [72%] 11 [29%] 11 [13%] 31 [82%] 61 [71%] 37 [97%]	p = 0.033 * p = 0.030 * p = 0.212 n.s. p = 0.140
	Associated	redoing your make-up Looking at passers-by	PT-PT+ PT-	9 [10%] 15 [39%] 45 [52%]	77 [90%] 23 [61%] 41 [48%]	n.s. p = 0.187 n.s.
	in group the PT+	Listening to music Listening to a particular radio broadcast	PT+ PT-PT+ PT-	34 [89%] 72 [84%] 19 [50%] 41 [48%]	4 [11%] 14 [16%] 19 [50%] 45 [52%]	p = 0.402 n.s. p = 0.811 n.s.
	frequent more practices Associated	Thinking about your job / your studies Thinking about your daily-life organization Thinking of nothing special Looking at the city / the landscapes	PT+ PT-PT+ PT-PT+ PT-PT+ PT-	19 [50%] 42 [49%] 26 [68%] 58 [67%] 16 [42%] 24 [28%] 21 [55%] 45 [52%]	19 [50%] 44 [51%] 12 [32%] 28 [33%] 22 [58%] 62 [72%] 17 [45%] 41 [48%]	p = 0.905 n.s. p = 0.914 n.s. p = 0.92 n.s. p = 0.762

Table 5 .

 5 Logistic regression of the occurrence of the adjective 'pleasant' to qualify travel time by car (dependent variable) Perception of quality of time while driving

	Independent variables	Modality	Value p-value significance
		less than 30	0,42	0,32	n.s.
	Age	31 to 40 41 to 60	(ref) 0,31	(ref) 0,48	(ref) n.s.
		more than 61	-1,66	0,08	n.s.
	Car-use habit strength	PT+ PT-	(ref) 1,09	(ref) 0,01	(ref) **
		with family	(ref)	(ref)	(ref)
	Company during the journey	alone roomates colleagues	-0,58 -0,39 -2,29	0,09 0,64 0,14	n.s. n.s. n.s.
		friends	-0,20	0,70	n.s.
	Frequency of the journey	rare daily	-0,07 (ref)	0,83 (ref)	n.s. (ref)
		more than daily	0,55	0,51	n.s.
		T < 10 min.	(ref)	(ref)	(ref)
		11 min. < T < 20 min.	0,72	0,04	*
	Real travel time	21 min. < T < 30 min. 31 min. < T < 40 min.	0,03 1,85	0,96 0,04	n.s. *
		41 min. < T < 60 min.	3,54	0,02	*
		T > 60 min.	0,92	0,15	n.s.

Viewing individuals as mainly passive entities has challenged the ability of "methodological collectivism" to explain the way cultural, social or technological structures are formed, reproduced and changed. In contrast, attempts to include habits in the frame of "decision theory" (see notably[START_REF] Becker | Habits, addictions, and traditions[END_REF] question the very meaning of rational choice, and especially maximizing choice.

A view that also occurs in[START_REF] Becker | Habits, addictions, and traditions[END_REF] approach, as pointed by[START_REF] Hodgson | Reclaiming habit for Institutional Economics[END_REF]Hodgson ( : 653, 2010: 4): 4).

The various approaches in neuroeconomicsin spite of significant discrepanciesare primarily interested in studying choice behaviour[START_REF] Fumagalli | The disunity of neuroeconomics: A methodological appraisal[END_REF][START_REF] Vromen | Where economics and neuroscience might meet[END_REF].

Based on his own experience,[START_REF] Earl | Experiential analysis of automotive consumption[END_REF]Earl ( : 1070Earl ( -1071) ) presents several cases where people are likely to misestimate or disregard the real costs of car ownership and use.

Bearing in mind the broad definition of the main classes of situations considered here (i.e. direct commuting trips, shopping trips, leisure or free-time trips), this is a weak hypothesis.

The results displayed in Table7(see appendix 2) show no statistical difference between the PT+ and PTgroups regarding their accuracy in assessing both the costs and the time related to their most frequent journey by car and to its public transport alternative. We can thus not interpret the transportation behaviour profilei.e. the fact of belonging to one of the two groupsas an indicator of a more or less significant degree of an individual's rationality.


Coura Kane
email: courakane@yahoo.fr

Alioune Kane
email: alioune.kane@ucad.edu.sn

Joël Humbert

Joe ¨l Humbert

Á A Kane

Management of a tropical river: Impacts on the resilience of the Senegal river estuary

Keywords: Watershed management, Resilience, Risk, Estuary, Senegal River

HAL is

Introduction

Due to rapid growth of the population and lack of resources in developing countries, people tend to settle along coastal areas and develop activities in estuaries which receive constant natural and anthropogenic actions. Human beings have not been insensitive to the wide array of opportunities provided by the coasts and have been attracted to them, making the coasts the most favored locations to either live permanently, for leisure, recreational activities or tourism [START_REF] Martinez | The coasts of our world: ecological, economic and social importance[END_REF][START_REF] Culliton | 50 years of population change along the Nation's Coasts, 1960-2010[END_REF][START_REF] Miller | Tourism and coastal development[END_REF]. Thus, Senegal River estuary has undergone significant alterations as a consequence of the hydrodynamic conditions that have prevailed there following human actions established in the area. With the construction of the Diama and Manantali Dams, in addition to the new opening on the mouth of the ''Langue de Barbarie,'' the Senegal River has experienced major changes due to many water planning decisions [START_REF] Kane | Responding to climate variability: the opening of an artificial mouth on the Senegal River[END_REF]). These decisions were made in the context of significant hydro-climatic variability. This resulted in the constant hydrological deficit recorded during the 1970s, but also exploited the higher levels of rainfall, particularly prevalent during those years.

The Senegal River estuary is an extremely fragile environment; it is threatened by natural hazards such as the recurrent floods. This situation stems from the physical environmental conditions. The risk is thus permanent, which can be exacerbated by human presence and its multiple actions on the environment. By interventions was generally failed to seriously predict the effects induced, and they frequently destabilized environments characterized by precarious equilibriums. The new mouth of the Senegal River is causing significant damage to the estuary. This arrangement does not fail to change the hydrological regime of the estuary and especially led to changes in estuarine ecosystems and increased the vulnerability of the estuary. Anthropogenic actions lead, most often, uncertainty of the occurrence of catastrophic events. In this context, the domino effect can be triggered, resulting in, as a result, increasing vulnerability. The impacts associated with climate change will be much more catastrophic in Africa because of the weakness of the economy and the rapid growth of the coastal population [START_REF] Nicholls | Sea-level rise and its impact on coastal zones[END_REF]. Climate change and rising of sea levels will increase the risks to which the estuary of the Senegal River is already exposed, whereas the system has low capacity to absorb disturbance.

Enhancing disaster risk reduction before a disaster occurs, and also during the reconstruction process, requires enhanced knowledge regarding the most vulnerable groups, the areas at risk, and the driving forces that influence and generate vulnerability and risk [START_REF] Birkmann | Risk and vulnerability indicators at different scales: applicability, usefulness and policy implications[END_REF][START_REF] Bogardi | Vulnerability assessement: the first step towards sustainable risk reduction[END_REF]. Better take into account of the physical environment in policy development is essential. Environmental assessment aims to contribute to environmental awareness among proponents and competent authorities and to environmental protection by requiring the ex ante evaluation of the environmental impacts of a wide range of public and private initiatives such as spatial plans [START_REF] Runhaar | Environmental assessment in the Netherlands: effectively governing environmental protection? A discourse analysis[END_REF]. The vulnerability of the Senegal River estuary area was accentuated by the water management of the Senegal River with the opening of the new mouth that was made without prior impact assessment. To a certain extent, the resilience of the system is strongly found to disturb. For this reason, it is essential to carry out detailed studies of the impacts of development on the hydrological regime of the river and the consequences in the upstream and downstream portions. All positive and negative impacts should be considered before making a layout and fairly weigh the consequences of choices.

In this chapter, the consequences of the watershed management of Senegal River in the estuarine area are analyzed. This last appears as a space where a variety of risk factors which may cause significant damage. Its occupation makes these areas vulnerable for local riparian. The latter has a low capacity of resilience from watershed management issues.

Human Pressures and Functioning of the Senegal River Estuary

The presence of a risk linked to existence water resources in an area determines the way the population exploit it; that situation influences strongly the type of structure and management settled consequently [START_REF] Beucher | L'aménagement des territoires face au risque d'inondation : regards croisés sur la Loire moyenne et le Val-de-Marne[END_REF]. The situation of the Senegal River obeys to this concept. Somehow, it could be promoted some existing geo-engineering technologies when used properly have low or no significant negative impacts [START_REF] Olson | Soft geoengineering: a gentler approach to addressing climate change[END_REF].

Climatic variability is determining factors in risk management. In the 1970s, the Sahel region experienced a period of exceptional water deficit in terms of duration and intensity. This period of rainfall decreasing is still on going [START_REF] Dai | The recent Sahel drought is real[END_REF][START_REF] L'hôte | Analysis of a Sahelian annual rainfall index updated from 1896 to 2000; the drought still goes on[END_REF][START_REF] L'hôte | The 1990s rainfall in the Sahel: the third driest decade since the beginning of the century, Reply to discussion[END_REF] although less strong since the mid 1990s than during the 1980s. The average rainfall over the last 37 years has remained below the 1900-1970 average in this region with large-scale consequences [START_REF] Hulme | Global warming and African climate change: a re-assessment[END_REF]. For almost all rivers of West Africa has decreased [START_REF] Mahé | 1896-2006 Sahelian annual rainfall variability and runoff increase of Sahelian Rivers[END_REF]. More than 125,000 people died in the African French-speaking countries during this period, in addition to the successive food shortages of the 1972-1974 period [START_REF] Dauphine | Risques et catastrophes: observer, spatialiser, comprendre, gérer[END_REF]. A loss of 600,000 tonnes of the grain crops was also recorded, equivalent to about a 15 % reduction in average annual income. In livestock farming and during the same period of 1970s, there was an 80 % cattle loss [START_REF] Guillaumie | La sécheresse au Sahel: un exemple de changement climatique[END_REF]. As a consequence, the survival of the socio-ecological system was endangered, in addition to being subject to severe risks that resulted in destructive disasters. Water control appeared to be essential for reducing the system's vulnerability. Thus, the ''taming'' of nature became a significant challenge for riparian states and the construction of the Diama and Manantali dams on the Senegal River (Fig. 1) was the solution adopted to ensure sustainable development and food security.

The Diama Dam resulted in considerable modification of the hydrological regime of the Senegal River downstream of the structure. Freshwaters are stored in the upstream of the dam. They arrived in the estuary during the rainy season (Saint-Louis station), when dam gates are opened and when they are some releases from Diama Dam. In addition, the principle of functioning of the Diama dam is to close it in periods of low water to prevent the intrusion of saltwater and to open in period of floods to allow floodwaters to flow downstream (OMVS 2003). The role of the Diama Dam in controlling the Senegal River is strengthened by the Manantali Dam.

The Sahelian zone has in recent years recorded a return of heavy rainy seasons. In the tropical wet area of Guinea, high basin River, we note abundant precipitation creating an increase in the river water flow. The increase in the river water levels generates overflows which further increase the risks of floods. The town of Saint-Louis stuck between the Senegal River and Atlantic Ocean (Fig. 1) is built on an area dominated by deposits of mud and is particularized by the omnipresence of water [START_REF] Kane | Responding to climate variability: the opening of an artificial mouth on the Senegal River[END_REF]. The digital elevation model (Fig. 2) shows the lower elevations of the city of Saint-Louis and identifies the most sensitive areas to flooding. Low areas represent 53 % of which 17 % have altitudes between -1 and 0 m and 36 % emerging barely above sea level is between 0 and 3 m. They correspond, in fact, the main bed of the river is located in ''Faubourg de Sor.'' The slopes are also very low; they are between 0 and 9 %. In ''Faubourg de Sor,'' the slopes are practically nil and they vary between 0 and 2 %. Also, at this point, changes in the water level will be low in case of flooding and lead to greater variation in the extent of water. Many cities are located in flood areas as they are likely to be places favorable for urban development [START_REF] Tingsanchali | Urban flood disaster management[END_REF]. The town of Saint-Louis is in a similar situation to many others throughout the world, for example, in Ethiopia where floods occur regularly in all parts of the country at times of rising water levels downstream of large basins and in low ground [START_REF] Ayalew | Analyzing the effects of historical and recent floods on channel pattern and the environment in the Lower Omo basin of Ethiopia using satellite images and GIS[END_REF]. In Saint-Louis, following many years of drought, the local population occupied much of the river bed normally under water; consequently, many districts may be flooded in the event of a major rise in water level. This raises the important issue of population relocation during rainy seasons, as it is also a limitation to the smooth flow of people and goods. This weakening of the socio-economic system in the face of the threats of flooding prompted the Senegalese authorities to open a new mouth on the ''Langue de Barbarie'' in 2003 to enable a rapid escape of excess water toward the sea.

Thus, since July 1986, the Diama Dam has controlled most of the water influx into the estuary. An average annual of 255 m 3 /s is released into the river below the dam during this period. Water flows through the dam since 1986 (Fig. 3) enable determination of the estuary's monthly and annual flows, showing two periods:

• a 4-month period (August to November) during which the estuary is invaded by flooding water and • a 8-month period during which the estuary is subject to marine influence. The dam's highest water flow is in October. The highest flows are in August, September, October, and November, and the lowest usually in December, January, February, March, May, April, June, and July.

In order to ensure that instructions on the dam's proper management are followed, occasional releases can be carried out by managers to guarantee its safety. The openings of floodgates are related to the support of raising water levels at the Manantali dam, which at certain times requires substantial water releases to be taken from reserves intended for other needs, while higher than turbine output [START_REF] Bader | Le soutien de crue mobile dans le Fleuve Sénégal, à partir du barrage de Manantali[END_REF]. Moreover, with the operation of the hydroelectric station at the Manantali Dam since 2002, the floodgates of the Diama Dam are regularly opened to avoid overflow of the river banks. Although insignificant, the river's water level seems to have experienced a slight increase in recent years, with an average of 129 cm in the 1992-1993 period. As a whole, there is a gradual fall from 1964 to 1971 and since about 1997.

The monthly water levels at the Saint-Louis station vary according to the totals discharged by the dam. They reach their maximum in October with, respectively, 1.32 m NGI1 ; 1.19 and 1.13 m NGI. There is nearly steady flow throughout the entire flooding period, becoming irregular after November. The minimum heights are recorded in May

Difficulties in the Management of the Diama Dam Following the Opening of the New Mouth

The opening of the new mouth has lowered water level at the Saint-Louis station. The opening of the new mouth has changed the flow conditions downstream of the dam to a share by lowering the average level downstream side making it more sensitive to the effect level of the tide (OMVS, Coyne and Bellier 2005). Thus, at the Diama Dam, the tidal range has significant changes. Before the opening of the new mouth estuarine, tides were semidiurnal with heights rarely exceeding the meter during whitewater and 0.5 m during neap tides. Currently, the maximum daily tidal range recorded downstream of Diama Dam has tripled from 0.30 to 0.93 m near is noticeable throughout the year, including during the rainy season, which makes it difficult handling the valve system. This had an impact on the management of the Diama Dam. Since tides are the determining factors that influence the final stretch of the river, their variation determines water levels. Water levels downstream of the dam were substantial due to flood waves, whereas the upstream part experienced a sharp decrease in water level. This difference in height may cause the dam's failure. It has also become difficult to maintain water levels upstream for irrigation needs while ensuring the dam's safety. Following [START_REF] Omvs Coyne | Etude d'évaluation des impacts de l'ouverture d'une embouchure artificielle à l'aval de Saint-Louis sur le barrage de Diama[END_REF], management rules prior to the opening of the new mouth are shown in Table 1.

The new context has required various studies (IRD 2 -OMVS 2004; [START_REF] Omvs Coyne | Etude d'évaluation des impacts de l'ouverture d'une embouchure artificielle à l'aval de Saint-Louis sur le barrage de Diama[END_REF] financed by the OMVS in order to revise the rules of management at the Diama Dam. These studies have enabled a more flexible management of the dam to enable levels of 1.50 m to be reached upstream.

These installations on the Senegal River estuary have strongly disturbed natural hydrological function and strikingly pose the issue concerning the risk of serious flooding in the case of a rise in seawater level, hypersalinization of water in the lower estuary, the development of halophytic plants, the degradation of ecosystems, etc. In conclusion, these installations, which aim at alleviating extreme events of climatic origin, actually pose significant risks.

Consequences of Water Management for the Senegal River Estuary

The changes introduced by the new mouth have clearly resulted in environmental impacts through disturbances in river water circulation. The new river mouth has resulted in a shorter residence time of freshwater since the part located downstream of the new mouth has evolved into a lagoon. The quality of water in the lower estuary remains close to that of the sea. According to [START_REF] Troussellier | Water quality and health status of the Senegal River estuary[END_REF], salinity reached 35 % more than 12 km south of the town of Saint-Louis. Since this section of the river has developed into a lagoon, exchange between marine and river water remains very limited. In fact, perturbations in the exchange between marine and river water and releases of salt from old deposits will certainly increase this salinization. The estuary is thus controlled by the Diama Dam and the marine dynamics by the opening of the new mouth. From the 2003 annual evaporation average (1,207 mm), we have calculated the volume of water evaporated from the lagoon, which corresponds to more than 13 million m 3 . This strong evaporation explains the excessive salinization of the lagoon, which will certainly impact the groundwater recharge of the lower estuary. The average salinity level measured in 2007 in the lower estuary was 47 g/l, which is higher the concentration than in seawater. In addition, groundwater changes related to the developments of rivers are seen as a major issue in many parts the world. In Morocco, for example, following the construction of the Mohamed V Dam, the coastal wetlands of Moulouya are no longer subject to flooding from rivers, which is responsible for the recharge of groundwater used for irrigation. Both water and soil have become increasingly saline as arable lands are abandoned; this has caused farm workers to emigrate [START_REF] Arthurton | AfriCat: LOICZglobal change assessment and synthesis of River catchment-Coastal Sea interactions and human dimensions in Africa[END_REF]. Groundwater contaminated by saltwater intrusion makes inappropriate irrigated crops. The population around the lower estuary is highly dependent on groundwater which constitutes their irrigation system for market gardening. The problem remains severe since the opening of the new mouth; the salinization of groundwater has reached critical levels, causing an extension of the salt marshes and a progressive loss of farmland.

Important changes were noted in the coastal hydrodynamics such as the tidal patterns. During high tides, there is a large increase in marine water with an important evacuation of the river flows during low tides. The acceleration of coastal erosion cause real threats to the area. The mangrove swamp has shown a progressive degradation following the opening of the new mouth. The area of the mangrove swamp has fallen by 636 ha (or 59 %), from 1086 ha in 1979 to 450 ha in 2003 [START_REF] Kane | Vulnérabilité du système socio-environnemental en domaine sahélien: l'exemple de l'estuaire du fleuve Sénégal. De la perception à la gestion des risques naturels[END_REF]. The area of the island of Doun Baba Dièye decreased by 8 % between 2003 and 2007, from 257 to 236 ha and now has completely damaged since October 2012.

Many villages such as Dieule Mbame, Tassinere and Keur Bernard (Fig. 2) are under threat. Resettlement of populations constitutes indeed a real problem as rehousing in the lower estuary is a big issue relatively to the availability of land.

Box 1: The opening of two new mouths Recently, in October 2012, it has been reported that the two natural openings which are distant of 150 m have accelerated the process of dismantling of the sandy spit of ''Langue de Barbarie,'' and the disappearance of the coastal habitats. After having amalgamated with the breach opened in 2003, they reinforced the problem of the salinization of the grounds in the lower estuary.

Risks Linked to Water Management in the Senegal River Estuary

The major risk in this area is linked to population settlements and the impacts of the various changes to the ecosystem and the resources. The Senegal River estuary appears to be a risk-prone area, where the conjunction of several factors can produce incommensurable effects. Physical conditions (geomorphological, lower altitude) determine an area where risk is permanent and expose populations to hazard. Moreover, communities may aggravate the intensity or frequency of certain phenomena and the reach of their effects.

To prevent the occurrence of flooding requires the means to contain floods, such as the construction of dams or river dikes and the establishment of non-structural measures such as alarm systems for flood prevention, public participation, and institutional commitment [START_REF] Tingsanchali | Urban flood disaster management[END_REF]. However, the new opening of the mouth, which aims at reducing population vulnerability within the estuary, has been favored over precautionary principles. Also, questions concerning the human impact on risk aggravation are inevitably directed toward inquiring about negligence during establishment of these new developments, weaknesses in protection systems including alert and prevention, safety devices, and toward any negligence which may obstruct proper control and handling of these phenomena as well as reduction in the consequences.

Considering the impact of an extremely vulnerable area at the opening of ''Langue de Barbarie,'' which has had the consequences described above, the occurrence of a natural disaster will only increase the fragility of the system. It is probable that rising of sea levels will intensify environmental pressures and undermine sustainable development. This will result in higher risks of flooding and greater ingress of seawater while the lower estuary is already experiencing groundwater salinization due to exchanges between river and marine water following the opening of the new mouth. Populations located in low and coastal areas as well as in deltas and which are estimated to make up approximately 10 % of the world's population will be particularly vulnerable to climate change [START_REF] Nicholls | Ranking port cities with high exposure and vulnerability to climate change extremes exposure estimates[END_REF][START_REF] Mcgranahan | The rising tide: assessing the risks of climate change and human settlements in low elevation coastal zones[END_REF]). Simulations conducted by [START_REF] Durand | L'impact de l'ouverture de la brèche dans la langue de Barbarie à Saint-Louis du Sénégal en 2003 : un changement de nature de l'aléa inondation ? Cybergeo[END_REF], based on the potential height of water at the end of the rainy season and carried out for several scenarios for the rise in sea level, show that a 0.5-m sealevel rise would be enough to flood part of the town of Saint-Louis at times of annual high waters and that it would be almost completely submerged in the event of a 1m rise.

Figure 5 represents the probable evolution of submergence of St-Louis based on increase in sea-level areas. It should be noted that the island and the sandy spit of ''Langue de Barbarie'' will be less affected than the ''Faubourg de Sor.'' The surface losses are 3 and 126 ha or 4 % of the territory up to 1 m. However, as we mentioned earlier, the ''Langue de Barbarie'' currently experienced recurring phenomena of coastal erosion. This situation can be explained by the importance of altitudes of more than 6 m. It is mostly in the ''Faubourg de Sor'' that flood risk will be greater due to lower altitudes. The occurrence of extreme events is increasingly determined by the coincidence of hydro-climatic events affecting a changed environment by human activities [START_REF] Bogardi | Hazards, risks and vulnerabilities in a changing environment: the unexpected onslaught on human security?[END_REF]). This area is experiencing unplanned occupation which only exacerbates the risk of flooding in the case of sea-level rise. This shows that the ignorance of risk is a source of vulnerability. At a height of 1 m of submersion, 71 % of the city will be under water. The entire area east of the city will be particularly affected.

The realization of large-scale new developments in sensitive areas may show a lack of perception by communities of the risks involved. [START_REF] Lefèvre | Les risques naturels majeurs[END_REF] stress that extreme difficulty in risk recognition and management is due to their limited perception by human communities. The exposed population's lack of perception of their own vulnerability to natural disasters is itself a factor of vulnerability which will constrain the establishment of preventive measures [START_REF] Leone | La vulnérabilité, un concept fondamental au coeur des méthodes d'évaluation des risques[END_REF]. Thus, the estuary of the Senegal River appears vulnerable and subject to the shock of unforeseeable and uncontrollable devastating events.

Conclusion

Management of tropical rivers can increase risks and vulnerability. In the Senegal River, it has resulted in consequences that impacted seriously the functioning of the estuary. The sea-level rise and the rapid erosion of beaches are not always mastered. In spite of the estuary's particular nature, it has been subject to developments carried out without preliminary studies in regard to their impact. The resulting changes may be determining factors in the evolution of the local environment. In the Sahel, despite the large climatic variability which causes events of extreme nature, risk-related consequences have not been sufficiently considered. Preventive measures seem not to be related to knowledge of risk, but to knowledge of this perception.

The new mouth has created a dysfunctioning which, in the long term, will have much more significant repercussions. The consequences of these modifications generally erode resilience of the system, which has difficulty resisting, to adapt to a major change. The rise of sea level in the lower estuary would flood most low ground, causing damage to the socio-ecological system. Such a rise would constitute a serious threat to the socio-ecological system in addition to having serious consequences geomorphological and socioeconomic. Nevertheless, knowledge and perception of risk play a fundamental role in the policies of prevention concerning risks. Even if the latter appear to have been controlled, there has not been sufficient consideration of the consequences associated with the prevalence of risk, which can generally lead to irreversible actions. Considerations of risk in the various management policies are of significant interest in a context characterized by an increase in disasters.

Fig. 1

 1 Fig. 1 Location of the Senegal River basin and estuary

Figure 4

 4 Figure4shows changes in water level at the Saint-Louis station from May 1964 to April 2012, showing a succession of peaks which reflects the strong inter-annual variability of the flow. Water flows have reduced since the 1970s with a clear decline in water level, from a peak of 281 cm in 1965-1966 to an average of 54 cm in 1972-1973 reflecting changes in the climatic conditions in the Senegal River basin. Although insignificant, the river's water level seems to have experienced a slight increase in recent years, with an average of 129 cm in the 1992-1993 period. As a whole, there is a gradual fall from 1964 to 1971 and since about 1997.The monthly water levels at the Saint-Louis station vary according to the totals discharged by the dam. They reach their maximum in October with, respectively, 1.32 m NGI 1 ; 1.19 and 1.13 m NGI. There is nearly steady flow throughout the entire flooding period, becoming irregular after November. The minimum heights are recorded in May

Fig. 3

 3 Fig. 3 Senegal River flows at Diama dam station (1986-2010) (OMVS Data) Organisation pour la Mise en Valeur du Fleuve Sénégal: Organization for the Development of the Senegal River

Fig. 5

 5 Fig. 5 Flooded areas in the town of Saint-Louis in case of sealevel rise

Table 1

 1 Rules of management of the Diama Dam before the opening of the new mouth

	Period	Objective	Level of reserve
	Low water level	Increased level in the reservoir	+2.10 to +2.20 m NGI (since the low water level of
			2001-2002)
	Beginning of flood waves	Maintenance of an increased level for as long	+2.10 to +2.20 m NGI then lowering to +1.50 m NGI
		as possible, then lowering preventive when
		flood waves arrives reach Bakel
	Period of flood waves	Level lowered in reservoir	+1.50 m NGI as long as flows are such that it is necessary
			to entirely open close the floodgates
	End of flood waves	Raising the level of the reservoir as soon as	+1.50 m NGI then rising to +2.10 to +2.20 m NGI
		possible

2 Institut de Recherche pour le Développement: Research Institute for Development.

NGI measurements constitute the National Geographical Institute measurements of the various water levels.