Xiao Bai

Marin Bertier

Rachid Guerraoui

Anne-Marie Kermarrec

Toward Personalized Peer-to-Peer Top-K Processing

We present the first personalized peer-to-peer top-k search protocol for a collaborative tagging system. Each peer maintains relevant personalized information about its tagging behavior as well as that of its social neighbors, and uses those to locally process its queries. Extensive experiments based on a real-world dataset crawled from del.icio.us shows that very little storage at each peer suffices to get almost the same results as a hypothetical centralized solution with infinite storage.

INTRODUCTION

A central task in information retrieval consists in processing queries in order to obtain the top-k items with the highest scores under a monotonic function. This is particularly appealing in collaborative tagging systems, also called folksonomies, such as Flickr, del.icio.us and CiteULike, which have become highly popular for publishing and searching contents. Yet this can turn into a nightmare because of the unstructured nature of tagging: there is usually no fixed ontology and users typically choose their tags in a free and possibly ambiguous manner.

One way to introduce some structure in such a scheme is to capture affinities between users with common tagging behaviors and then leverage these in the top-k processing. More relevant results for a given user could be achieved should the search be directed in a restricted network of users exhibiting similar tagging bahaviors. For example, a computer scientist and a Keanu Reeves fan might probably not be interested in the same results when Googling 'matrix'. User affinities can disambiguate these situations and alleviate the need for reformulating the same query through several steps with more refined keywords.

Very elegant centralized approaches have recently been proposed to capture the personalization through social scoring models that compute the user-centric correlation among different tags in order to improve information retrieval quality [START_REF] Schenkel | Efficient top-k querying over social-tagging networks[END_REF][START_REF] Xu | Exploring folksonomy for personalized search[END_REF]. In Amer-Yahia et al. [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF], the score of an item only depends on how users sharing similar preferences have tagged it. The reference retrieval solution consists in maintaining one inverted list per (tag, user), but this is extremely space consuming. Alternative solutions to save storage space do exist but their processing time is increased.

We argue that scalability calls for fully decentralized solutions to process top-k queries. In fact, decentralized solutions have indeed been proposed. In Michel et al. [START_REF] Michel | Klee: a framework for distributed top-k query algorithms[END_REF], precomputed inverted lists are distributed across peers and partial information is transmitted in the network progressively to approximate the top-k results. In Cuenca-acuna et al. [START_REF] Cuenca-Acuna | Planetp: Using gossiping to build content addressable peer-to-peer information sharing communities[END_REF], a gossip scheme is used to implement distributed content search and ranking. None of these decentralized solutions is however 'personalized'.

This paper presents, to our knowledge, the first decentralized and personalized top-k processing scheme. In our scheme, each user maintains its own inverted list by periodically discovering its network. 1 Our approach alleviates the storage space problem while enabling highly efficient local query processing. We use a gossip-based network management protocol to identify users' personal networks in a peerto-peer way; once these personal networks are established, users locally process their queries using a classical top-k algorithm, namely NRA (No Random Access). Interestingly, only a subset of (well selected) users suffices to provide the most relevant items while preserving their relative order for a given query. We explore various strategies to prevent overloading individual users by limiting the size of their personal networks. This results in little degradation in the quality of the top-k results.

We have implemented our decentralized and personalized top-k processing scheme in PeerSim [START_REF] Jelasity | The Peersim simulator[END_REF] with a dataset crawled from del.icio.us. Interestingly, if all qualified users are kept as neighbors, after 50 cycles of gossips, we retrieve at least 8 relevant items out of 10 with a negligible storage overhead with respect to the idealized centralized solution of [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF] (called Exact). Thanks to our optimization strategy which simply keeps the closest neighbors, more than 35% storage space is further economized for each user.

The rest of this paper is organized as follows: Section 2 establishes the social model of our system and provides a brief review of relevant top-k processing and network-aware search techniques. Section 3 describes our peer-to-peer per-sonalized top-k processing scheme and some optimization techniques. Section 4 presents our experimental setup and results. Section 5 concludes the paper.

SOCIAL NETWORK MODEL AND BACK-GROUND

Social Network Model.

In a social network model, collaborative tagging sites are typically presented as information space U × I × T , where U denotes the set of users. Each user has a profile that expresses his endorsement of visited items by tagging them. The profile is described in the form of i{t 1 , ..., t n }, meaning that an item i is tagged by tags t 1 , ..., t n . I is the set of items that appear in the system and T is the set of all related tags. T agged(u, i, t) captures the fact that user u tags the item i with the tag t.

We model the social network as a directed graph where a node corresponds to a user and an edge presents the relationship between two users. We use Link(u, v) to indicate the existence of a directed edge from user u to user v. For a user u ∈ U , N etwork(u) is the set of u's neighbors, i.e., N etwork(u) = {v|Link(u, v)}.

There are many possibilities for establishing Link(u, v) according to user preferences. We do not assume any particular semantics; as such we follow the criterion in [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF] that Link(u, v) exists if and only if v tags a sufficient number of items with the same tag as u, i.e., |{i|∃t, T agged(u, i, t) ∧ T agged(v, i, t)}| > threshold, where threshold is a predefined number. The number of commonly tagged items is also used to measure the strength of Link(u, v), denoted by Strength Link(u,v) .

Query And Scoring Model.

We consider a query Q = {u, t 1 , ..., t n }, issued by a user u with a set of tags t 1 , ..., t n . Returned items for a query should be ranked according to its overall score. Our score is user-specific and network-aware. The score of an item i for user u and tag t j is defined as the number of users in u's network who tags i with t j , i.e., Score tj (u, i) = | N etwork(u) ∩ {v | ∃t j , T agged(v, i, t j )} |. The overall score of an item i for user u is the sum of all the scores related to the query, i.e., Score(u, i) = tj ∈Q Score tj (u, i). We use the same scoring functions as in [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF] for ease of comparison. (Note that alternative functions can be used in our social network model.)

Top-k Processing.

Given a query, a top-k processing algorithm (e.g., [START_REF] Fagin | Combining fuzzy information: an overview[END_REF]) aims at retrieving the k most relevant items. The goal is typically to minimize the time it takes to come up with these items as well as the amount of storage needed to perform the actual computation.

Information is organized in per tag inverted lists. Each entry of the inverted list contains the identifier of an item and its score for that tag. Inverted lists are sorted in a descending order of scores. Below we review the well-known NRA algorithm, as this is considered particularly effective: it underlies our work as well as that of [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF]. For a query of n tags, NRA scans the n inverted lists in parallel. In order to do so, NRA maintains a heap of candidate items. Each candidate has a score lower-bound and a score upper-bound, which are the overall scores it can attain based on the information available at the moment. The score lower-bound takes the most pessimistic assumption that if an item has not been seen in some lists, then it does not exist in them. Alternatively the score upper-bound takes the most optimistic assumption that its scores in those lists equal to the scores of last seen items in them. Once an item is seen, it is either added to or updated in the heap. The score upperbounds of items already in the heap are also updated. Candidate items are sorted based on their score lower-bounds. For those with equal lower-bounds, the one with a larger upperbound is ranked ahead. The processing stops when none of the items out of the top-k items has an upper-bound larger than the lower-bound of the kth item.

Network-Aware Search.

Amer-Yahia et al. [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF] proposed several strategies to achieve efficient network-aware top-k processing. Again, for a network-aware search, the score of an item only depends on the querier's network. The most straightforward strategy, called Exact, is to build one inverted list for each (tag, user) pair. Query Q generated by a user u is processed on the (t j , u) lists, where t j ∈ Q, using a traditional top-k processing algorithm such as NRA. However, storing these lists is prohibitive space-wise for a single server. Therefore, they explore another strategy, Global Upper-Bound, that maintains user-independent inverted lists whose entries only contain the max scores over all users. The exact score of an item is computed at query time. With Global Upper-Bound, considerable storage space is saved but much more time is required for query processing. To strike a balance between the two extremes, users are then clustered and only score upperbounds over all clusters are maintained in per (tag, cluster) pair inverted list.

PEER-TO-PEER SOLUTION

In our decentralized scheme, each user maintains a set of neighbors that form its personal network. A query is processed with the information available in the querier's network to get personalized top-k results. If its network fails to provide any satisfactory results, a default search mechanism will be activated. Here we concentrate on the personalized processing. In our setting, a key problem is how to build a personal network for each user in a peer-to-peer way as quickly as possible so as to guarantee the quality of top-k results.

Personal Network Construction

In our scheme, the personal network is discovered and maintained through a two-layer gossip protocol. The bottomlayer gossip protocol, typically known as a random peer sampling protocol (RPS) [START_REF] Jelasity | Gossip-based peer sampling[END_REF], is in charge of keeping the overlay connected. Basically it provides each peer with c random peers. On top of the peer sampling protocol, a top-layer protocol is in charge of tracking the similarity between users' profiles and discovering new neighbors. Once the top protocol has stabilized, it is still possible to discover new related peers through the peer sampling protocol.

We assume that each peer executes the same protocol in the same manner every T time units, referred to as a cycle. A gossip protocol, bottom and top, consists of two threads: an active thread initiating communication with other peers, and a passive thread waiting for incoming messages. A gossip protocol is fully characterized by three functions: (i) the peer selection (choice of gossip target); (ii) the data exchange (which data are exchanged in a gossip interaction) and, (iii) the data processing (which data are kept after the interaction). In this paper, the data exchanged are peers (IP addresses and profiles of peers).

At the beginning of each cycle, the peer selection selects a neighbor with the oldest TimeStamp as a gossip destination. Once a peer is picked, its TimeStamp is set to zero and other neighbors' TimeStamps increase by one. The variable TimeStamp ensures that all the neighbors have a comparable chance of participating in gossiping. The data exchange function selects gossip-size neighbors from the views of both layers and sends the profiles of selected peers to the gossip destinaton. When the gossip destination receives this information, its passive thread is activated. It also selects gossip-size neighbors and sends their profiles to the peer from who the message came. The data processing function selects the network-size closest peers according to some predefined metric. Two users are added to each other's view if they tag a minimum number of items in common with at least one same tag. Note that the top layer provides each peer with its personal network.

Inverted Lists and Query Processing

In the process of gossiping, the profiles of a user's neighbors are stored by the user. To enable efficient top-k processing, inverted lists for each (tag, user) pair are also computed and stored by the user itself. When a user generates a query, it first checks whether the inverted lists for the tags in the query already exist in its cache and whether they should be updated to reflect new neighbors. Once all the related lists are up-to-date, the user processes its query locally with NRA to get the top-k results.

The inverted lists are constructed lazily so that an inverted list is computed only when it is necessary for the query processing. There is no need to pre-compute the inverted lists for the tags that may never be queried in the Exact centralized case. Note that the computation of inverted lists are not more expensive than Exact when changes occur in the network. In contrast, gossip-based protocols enable to capture such dynamics.

Personal Network Optimization

We adapt the criteria in [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF] for choosing neighbors in our setting. (Note that any metric that captures the affinity of users will work as well with our gossip-based personal network construction protocol). Since the common-item-based neighbor choosing criteria may cause a scalability problem when the network grows and users add more contents in their profiles, we propose several optimization strategies that reduce the size of user's network without degrading the top-k quality.

Instead of maintaining all the users that meet the criteria to be a neighbor, only n of them are kept in a user's personal network. The effect of n will be evaluated in the next section. These strategies can also be used independently of the criteria in [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF] to form personal networks.

Random n random neighbors are chosen from the candidate users. The intuition is that a random sample is usually representative of the population from which it is drawn.

Biased Random Like Random, only n users are randomly selected as neighbors. However the probability that a user is kept as neighbor is proportional to the strength of the link between the two users. So users with stronger links have better chances of being chosen as neighbor.

Nearest As users are more likely to enjoy what is preferred by the users having similar preferences, and the similarity of user preferences is measured by the strength of the link between them, this strategy has confidence in users possessing strongest links with the gossip initiator and chooses them as neighbors.

Nearest With Enhanced Link Strength (Nearest-ELS) Similar to Nearest, users having similar preferences are also chosen here. In collaborative tagging sites, similarity of user preferences depends on the tagging behaviors exhibited in their profiles. Normally, the overlap of used tags in users' profiles implies their common interests on topics, while an overlap of tagged items reveals specific objects they prefer. As a tag can be used for several items and a same item can receive different tags from different users, it is more accurate to compare tagging behaviors by the number of (item, tag) pairs in common rather than the number of items tagged by both users with same tags. The intuition is that the more common tags are used for an item, the more similar the way on which the users understand and judge the world is. For this reason, we re-define the strength of Link(u, v) as | {(i, t j ) | T agged (u, i, t j ) ∧ T agged (v, i, t j )} |. For each candidate neighbor v of user u, we re-compute Strength Link(u,v) and keep the n users having strongest links with u as neighbors.

EVALUATION

Experimental Setup

Data Set and Query Generation.

We have implemented our decentralized and personalized top-k processing scheme in PeerSim, an open source simulator for peer-to-peer protocols. All experiments presented here are executed on a cluster of servers including 10 Dell PowerEdge 1855 machines equipped with Bi-pro Intel Xeon processors with 3.40GHz CPU and 4GB memory.

The dataset used in our evaluation was crawled from del.icio.us. It contains 13,521 distinct users who participate in 31,833,700 tagging actions in the form of T agged(u, i, t). 4,741,631 distinct items and 620,340 distinct tags are concerned by these actions. We randomly picked 10,000 users from the dataset and built their profiles with the items and tags used by at least 10 distinct users. Note that this does not affect the top-k results and processing time because only the items ranked at the tail of the inverted lists are dropped. After removing uncommon tags, the interference of the noisy and often meaningless tags is also eliminated. The remaining dataset contained a total of 101,144 items, 31,899 tags and 9,536,635 tagging actions.

In our experiments, each user processed exactly one query. We randomly picked an item from a user's profile and composed the query with the tags used by the user to annotate this item. This is motivated by the reasonable observation that the tags in the query reflect the user's understandings of this item and their combination within the same query is meaningful.

Evaluation Metrics.

We were mainly interested in the implicit semantic relations exhibited by users' tagging behaviors. As in [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF], there is a link between two users if they tag two items in common with at least one same tag. The personal relationships are formed as the network converges. If all users have their complete networks, the same top-k results would be obtained for a given query. We assume that there is neither arrival nor departure of user for ease of comparison with Exact and Global Upper-Bound in [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF], which are considered ideal in terms of processing time and storage space respectively. Important questions related to our decentralized setting are then how fast the personal network of each user can be established and what is the influence on the top-k results.

A user begins building its personal network by first discovering the IP address of any user currently in the system with some bootstrap mechanisms. At any following time, each user has a number of neighbors thanks to gossiping. The ratio of this number to the user's network-size in a centralized setting implies how close its current personal network is to the target. We measure the speed of convergence with the average ratio of all users, i.e., Speed attains 1 when all users find their personal networks in the centralized implementation.

Speed = 1 |U | u∈U |Current N etwork(u)| |N etwork(u) in centralized setting| .
Our goal is to show that an efficient network-aware topk processing can be achieved in a peer-to-peer way, so we take the top-k results in [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF] as a reference. Note that all algorithms proposed in [START_REF] Amer-Yahia | Efficient network aware search in collaborative tagging sites[END_REF] provide the same top-k items and only differs from each other in processing time and storage space. We then try to obtain as similar top-k items as possible for the same query with less time and space consumption in our setting.

We use the accepted metric, recall, in information retrieval to evaluate the quality of top-k results. The recall R k is the proportion of the total number of relevant items that are retrieved in the top k: R k = N umber of Retrieved Relevant Items T otal N umber of Relevant Items

Recall quantifies the coverage of the result set and varies between 0 and 1.

The space overhead for each user is estimated by the number of entries in the inverted lists and the number of entries in the profiles of its neighbors. The query execution time for a query is quantified by the number of sequential accesses of related inverted lists. Both metrics are highly dependent on the query and the user, so we are more interested in the relative improvement compared to the centralized implementation of Exact and Global Upper-Bound for a given user and query.

Convergence of Personal Networks

We start with some observations on how the network converges and how fast our algorithm enables users to find their own networks, which are considered as the basis of our personalized top-k processing. The gossip-size is set to 20 and 50 respectively for both layers in two different experimental settings. We can see from Figure 1 that exchanging more information provides higher convergence speed. After a small number of cycles, users can get almost its whole personal network in a centralized setting.

We run top-10 processing in a centralized implementation of Exact and take the 10 returned items for each query as relevant items and compare our top-10 results with them. The following results are from the setting of gossip-size set to 50. Figure 2 plots the evolution of R 10 in the process of network converging. At cycle 50, more than 77% of queries get exactly the same results as in the centralized implementation and the rest of the queries retrieve at least 8 relevant items. R 10 continues improving as time passes and about 98.5% of queries obtain all their relevant items at cycle 250.

Comparison of Optimization Strategies

Constructing personal networks in a peer-to-peer way can provide almost the same personalized top-k results as the centralized implementation can do. However, when the number of users and the items tagged by each user increase, it is possible that users' local disks become fully occupied, because too many profiles of neighbors and inverted lists need to be stored. With our own dataset, average network-size and maximum network-size over all users are listed in Table 1. On average, each user should maintain about 18% of other users as neighbors to construct inverted lists for top-k processing, which is infeasible in large-scale networks. It is therefore important to choose neighbors in a more intelligent way.

Figure 3 compares the performance of the four strategies we proposed in Section 3.3 The horizontal axis corresponds to the maximum number of neighbors a user can have. We consider Recall with no less than 0.7 as satisfactory top-10 results. The vertical axis shows how many queries receive such results. In this figure, only the queries sent by users having corresponding number of neighbors are taken into account, because others will get the same results as in the centralized implementation using the users' whole personal networks. It is seen that the strategy Nearest With Enhanced Link Strength outperforms the others in terms of top-10 quality. When the network-size is relatively small, the difference among these strategies is more pronounced, while the difference decreases when fewer limits are set on network-size. This difference also conveys the fact that users with similar tagging behaviors are more representative and contribute more to the final top-k results.

Space Overhead And Response Time

The maximum space overhead for each user is attained when its complete personal network is established. Figure 4 compares individual user's space overhead with that of Exact and Global Upper-Bounds. As mentioned earlier, the individual user's space overhead depends on the total length of its inverted lists Figure 4(a) and entries in all its neighbors' profiles Figure 4(b). Users are ranked in ascending order of their space overhead. As expected, no user needs to store as much information as a centralized database even for Global Upper-Bounds. Space is no longer a severe problem with our decentralized storage.

Figure 4(c) illustrates the response time at cycle 50. Shorter inverted lists do not necessarily mean less execution time because the lack of information may decrease the scores of certain items and as a result, more entries in the lists may need to be checked to get the final top-10. However, the penalty in time consumption is not too expensive. On average, only 3% more time is required to process these queries.

With the optimization strategies to limit network-size, fewer profiles and shorter inverted lists are stored by each user. If we use the Nearest-ELS strategy to choose neighbors and fix the network-size to 500, there is no difference for about 28% of the users whose network-size is 500 or smaller. For the others, on average storage space for 34.5% of the entries in inverted lists and 54.2% of the entries in neighbors' profiles are saved. As the inverted lists are changed, the number of sequential accesses to get the top-10 items is no longer the same as before. About 28% of queries need the same time to retrieve the results while 35.9% consume less time and 36.1% require more time. On average, there is no great difference as before in terms of processing time.

Scalability

As shown in Table 1, the personal network-size grows linearly as the number of users increases using the criteria in the centralized implementation to choose neighbors. With our optimization, we find that the necessary number of neighbors to obtain top-10 results of same quality remains stable even when the number of users continues increasing (see Figure 5). Clearly, the larger the network, the smaller the ratio of necessary network-size over number of users. Our optimization scales well, and similar phenomena can also be found in other optimization strategies. This suggests that there is no need to maintain a large network to obtain good personalized top-k results. Well selected neighbors conserve the relative order of relevant items to a query even though their overall scores decrease.

CONCLUSION

We presented in this paper the first peer-to-peer approach to personalized top-k processing. We described the design and implementation of our approach and investigated its performance. The experimental results are encouraging and we believe that decentralization is the right way to provide personalized top-k processing in a scalable manner.

We are exploring several complementary research directions, including system dynamics, in terms of churn as well as frequent changes in tagging behaviors. We are also considering various techniques to improve privacy as well as reducing redundant information within a cluster.

Figure 1 :

 1 Figure 1: Convergence speed

Figure 2 :

 2 Figure 2: Recall evolution (gossip-size=50)

Figure 3 :

 3 Figure 3: R 10 for the four strategies with varying network-sizes

  Number of sequential accesses per query

Figure 4 :

 4 Figure 4: Space overhead and processing time at cycle 50 (log scale)

Figure 5 :

 5 Figure 5: Recall of Nearest-ELS in systems of different size

Table 1 : Network-size in different systems

 1

	Number of users Average network-size Max network-size
	1000	180	750
	5000	897	4165
	10000	1801	8350

Note that this is different from a social network in the traditional sense, as neighbors in our network are those with similar tagging behavior and might be and remain unknown.


Patricia Lejoux
email: patricia.lejoux@entpe.fr

Aurore Flipo
email: aurore.flipo@entpe.fr

Nathalie Ortar
email: nathalie.ortar@entpe.fr

Nicolas Ovtracht
email: nicolas.ovtracht@laet.ish-lyon.cnrs.fr

Stéphanie Souche-Lecorvec
email: stephanie.souche@univ-lyon2.fr

Razvan Stanica
email: razvan.stanica@insa-lyon.fr

Coworking, a Way to Achieve Sustainable Mobility? Designing an Interdisciplinary Research Project

Keywords: mobilities, mobility studies, transport, transportation studies, interdisciplinary, coworking

Sustainable mobility has been one of the central paradigms of research in the field of transport and mobility for several decades. However, the implications of adopting the concept of "sustainable mobility" for the conduct of interdisciplinary research has been little discussed within the relevant research community. Research in the field of transport and mobility has nevertheless been the setting for major debates in recent years on the question of interdisciplinarity, or even transdisciplinarity, with the emergence of mobility studies as opposed to transportation studies. The objective of this paper is to show, empirically, how researchers who are specialised in mobility and transport issues, but who belong to different disciplines (anthropology, computer science, economics, geomatics, sociology and urban planning) have sought to build an interdisciplinary research project-which is currently ongoing-around the links between the development of coworking, which is a new way of organising work, mobility and sustainability. This paper sets out to highlight cross-fertilisation between disciplines, the issues raised, and the difficulties encountered. As such, it provides an account that is as faithful as possible to our experience of conducting interdisciplinary research in the area of sustainable mobility.

Introduction

For several decades, research on transport and mobility has addressed the economic, environmental and social issues associated with mobility, and it has made the concept of "sustainable mobility" one of its central paradigms [START_REF] Banister | The sustainable mobility paradigm[END_REF][START_REF] Hickman | Planning more for sustainable mobility[END_REF][START_REF] Low | Making Urban Transport Sustainable[END_REF][START_REF] May | Urban transport and sustainability: The key challenges[END_REF][START_REF] Bulckaen | Sustainability versus stakeholders preferences: Searching for synergies in urban and regional measures[END_REF]. For Morency [START_REF]Mobile Lives Forum[END_REF], a sustainable transport system is: "[ . . . ] [A] system that will come with reasonable costs, that will operate efficiently, and that will offer all populations a choice between different transport alternatives. So, one of the characteristics this system needs to have is the ability to limit polluting emissions, waste, etc., so as to try and ensure that the planet's ability to absorb all of this is not exceeded." The adoption of this concept has helped revitalise theoretical frameworks by departing from the traditional approaches developed in the contexts of transport engineering and transport economics. By looking not only at the contribution of mobility to economic growth, but also its impact on the environment and social inequalities, approaches based on the concept of "sustainable mobility" have helped to undermine the view that individuals are purely rational and that their behaviour can be explained solely by their desire to maximise their utility. They have allowed researchers to distance themselves from a definition of mobility as a simple demand derived from activities and from an explanation of mobility behaviour that is based solely on trade-offs between financial and time costs [START_REF] Banister | The sustainable mobility paradigm[END_REF]. However, the implications of adopting the concept of "sustainable mobility" for the conduct of interdisciplinary research has been little discussed within the transport and mobility research community. Under the pressure of strong social demand, scientific debates have focused more on ways of applying this concept than on studying it. In the field of transportation studies, the work has consisted of examining the advantages and limitations of the different ways of implementing sustainable mobility. In some cases, reviving a technocentric vision of mobility [START_REF] Kębłowski | All transport problems are essentially mathematical: The uneven resonance of academic transport and mobility knowledge in Brussels[END_REF][START_REF] Gössling | Why sustainable transport policies will fail: EU climate policy in the light of transport taboos[END_REF], they have focused on transport policies that promote a modal shift towards public transport and active modes (reallocating space to public transport, parking controls, road pricing, speed reduction, etc.), urban planning policies that result in a reduction in the distances travelled (densification, mixed-use development, Transit Oriented Development, etc.), technological innovations (improvement of the performance of internal combustion engine vehicles, use of electric, autonomous or shared vehicles), the development of digital technology to reduce travel needs (teleworking, e-commerce) or optimise travel (smart mobility). In the field of mobility studies, research has highlighted the need to take into account the complexity of social behaviours in order to achieve a transition to more sustainable mobility practices: "It can get us beyond the idea that transport "consumers" need to stop driving cars and start taking the bus or riding bikes. There is a recognition that the underlying social organisation of mobility needs to be changed in other ways" [START_REF] Sheller | Mobilizing the new mobilities paradigm[END_REF] (p. 18). Only a few researchers have recently undertaken a reflexive examination of the implications of this concept for interdisciplinarity by drawing attention to the low level of consideration given to socio-political issues (class, gender, ethnicity, etc.) and its very normative nature [START_REF] Green | The travelling citizen: Emergent discourses of moral mobility in a study of cycling in London[END_REF]. They highlight the lack of a critical perspective in research on sustainable mobility and the need to (re-)embed mobility issues within political economy [START_REF] Kębłowski | All transport problems are essentially mathematical: The uneven resonance of academic transport and mobility knowledge in Brussels[END_REF][START_REF] Reigner | Nouvelles Idéologies Urbaines[END_REF].

While interdisciplinarity appears to be an inherent feature of this field, which is built around a single research topic, the nature of disciplinary cross-fertilisation (between the engineering sciences, the human and social sciences and the digital sciences, etc.) and the intensity of this cross-fertilisation (multidisciplinary, interdisciplinary, transdisciplinary) [START_REF] Stock | Defining Terms for Integrated (Multi-Inter-Trans-Disciplinary) Sustainability Research[END_REF] have evolved over time. This issue of the nature of interdisciplinarity in the field of transport and mobility research has assumed great importance in recent years as a result of the emergence of the field of mobility studies and the repositioning it has imposed on transport studies. By encouraging all human and social sciences to take up the "new mobilities paradigm" [START_REF] Sheller | The new mobilities paradigm[END_REF], the ambition of mobility studies is to create a "postdisciplinary" [START_REF] Sheller | The new mobilities paradigm[END_REF] (p. 214) or "transdisciplinary" [START_REF] Sheller | The new mobilities paradigm for a live sociology[END_REF] (p. 791) field. Employing innovative concepts and methods, it attempts to bring together disciplines as diverse as anthropology, cultural studies, geography, migration studies, science and technology studies, sociology of tourism, and transport studies: "[ . . . ] [W]hat mobilities research does is to break down disciplinary silos and thereby enable us to begin to see the connections across topics and scales [ . . . ]" [START_REF] Sheller | The new mobilities paradigm for a live sociology[END_REF] (p. 803). While not all the human and social sciences seem willing to see themselves as part of this paradigm [START_REF] Ortar | Circulations, Mobilités. Nouveaux Enjeux Épistémologiques et Conceptuels à L'épreuve Du terrain[END_REF], transportation studies have been greatly influenced by the emergence of mobility studies, due to the similarity of the research issues and the way the concept of mobility gained ascendancy over that of transport during the 1990s and 2000s. For example, this has led transport geography to engage in a reflexive examination not only of its place within the humanities and social sciences, but also as a sub-discipline within geography [START_REF] Shaw | Making links: On (re)engaging with transport and transport geography[END_REF][START_REF] Schwannen | Geographies of transport 1: Reinventing a field?[END_REF]. Traditionally closer to transport engineering and transport economics, with which it shares positivist assumptions and methods of data collection and modelling, transport geography has entered into a dialogue with mobility studies. This has resulted in an acknowledgement of their mutual contributions [START_REF] Grieco | Mobilities: New Perspectives on Transport and Society[END_REF][START_REF] Prytherch | Introduction: Transportation, Mobilities, and Rethinking Urban Geographies of Flow[END_REF][START_REF] Cresswell | Mobilities I: Catching up[END_REF][START_REF] Knowles | Transport Geographies: Mobilities, Flows, and Spaces[END_REF][START_REF] Shaw | Transport, geography and the 'new' mobilities[END_REF][START_REF] Hall | Transport geography and new European realities: A critique[END_REF], and the identification of topics for reflection that could be conducive to interdisciplinarity: The different ways of understanding travel, a better understanding of the methodologies used and the contribution of this body of research to public action [START_REF] Shaw | Transport, geography and the 'new' mobilities[END_REF].

This brief overview of research developments in the field of transport and mobility, due to the adoption of the "sustainable mobility" paradigm and interdisciplinarity, raises questions about the possibilities of developing an interdisciplinary approach to sustainable mobility. How does the adoption of the concept of sustainable mobility affect the nature of interdisciplinarity in the field of transport and mobility research? Which disciplines and methods should be used to understand it? What cross-fertilisation should be set up? The objective of this paper is to contribute to this thought process. It shows, empirically, how researchers who are specialised in mobility and transport issues, but who belong to different disciplines (anthropology, computer science, economics, geomatics, sociology and urban planning) have sought to build an interdisciplinary research project around the issue of sustainable mobility. This project aims to explore the links between the development of a new way of organising work -coworking -, mobility and sustainability. Coworking spaces, which first appeared in San Francisco in 2005, have spread rapidly throughout the world and represent a new way of organising work based on the sharing of a workspace and a network of workers that encourages collaboration [START_REF] Gandini | The rise of coworking spaces: A literature review[END_REF]. Within the category of "third places" [START_REF] Oldenburg | The Great Good Place: Cafes, Coffee Shops, Bookstores, Bars, Hair Salons, and Other Hangouts at the Heart of a Community[END_REF] coworking spaces differentiate themselves through their collaborative dimension. They advocate for inventing new methods of working, which are often less formal, based on exchange, collaboration and co-creativity. In France, the creation of coworking spaces goes hand in hand with a very proactive discourse on the part of the public authorities, who encourage, and even financially support, their development. Their number has increased significantly in recent years: There are currently 1800 coworking spaces in France [START_REF] Levy-Wailtz | Faire Ensemble Pour Mieux Vivre Ensemble. Rapport de la Mission Coworking: Territoires, Travail et Numérique[END_REF]. The main justification for this support is presumed (but not proven) beneficial effects of coworking in terms of sustainable mobility. By offering workers the opportunity to work close to home, coworking spaces are expected to reduce the negative impacts of commuting (congestion, pollution, etc.) and promote a better work-life balance. According to the French Ministry of Territorial Cohesion, coworking would cut down travel time by one hour and ten minutes per teleworking day; which would represent 5 billion m 3 of CO 2 annually [START_REF] Levy-Wailtz | Faire Ensemble Pour Mieux Vivre Ensemble. Rapport de la Mission Coworking: Territoires, Travail et Numérique[END_REF]. ADEME, the French Environment and Energy Management Agency, for example, has identified coworking spaces as "immobility solutions" that have the ability to encourage mobility with a "reduced environmental footprint". Indeed, both mobility plans and coworking are seen as a way of resolving home-to-work mobility problems [START_REF] Esztergár-Kiss | Stakeholder Engagement in Mobility Planning[END_REF]. The objective of the COWORKWORLDS project is to examine the evidence for the sustainability of coworking in terms of mobility in different geographical contexts. Through the study of this interdisciplinary project, the paper helps to draw attention to cross-fertilisation between disciplines, the issues raised, and the difficulties encountered. As such, it provides an original record that is as faithful as possible to our experience of the practice of interdisciplinary research on sustainable mobility, with its strengths and limitations.

The Construction of an Interdisciplinary Research Project on Sustainable Mobility

The findings presented in this paper are based on work undertaken as part of an ongoing research project which is being funded by the French National Research Agency for a 36-month period. Known as COWORKWORLDS, it sets out to study the sustainability and spatiality of coworkers' mobility practices in different geographical contexts (metropolitan areas, medium-sized cities, peri-urban and rural areas), based on an interdisciplinary approach. The project's key hypothesis is that the sustainability of coworkers' mobility practices depends on their economic situation (job type, income level, career, etc.), their social position (social category, household type, etc.) and the spatial context (location, residential trajectory). These practices, thus, depend on the job and residential markets, working practices, transportation infrastructures, and social aspirations. The aim of the project is to identify the diversity of these factors and their impact on sustainable mobility based on knowledge of coworkers' mobility practices, an understanding of the choices that lead to their adoption and a consideration of the spatialities they create. In order to study the sustainability and spatiality of coworkers' mobility practices, the project developed a methodology that combines qualitative and quantitative methods:

•

A database of coworking spaces in the Auvergne-Rhône-Alpes region of France was created, and mapping was performed.

•

A qualitative survey was conducted among the creators of coworking spaces. The aim was to better understand their locational strategy and how they take sustainable mobility issues into account. It was also a way to collect first-hand information about coworkers' profiles and their mobility practices.

•

A questionnaire survey about coworkers' working and mobility practices was carried out. The purpose was to collect information about the subjects' socio-economic characteristics (age, gender, place of residence, level of income, etc.) and their working practices (one or several workplaces, coworking space attendance, occupation, etc.) and travelling (daily mobility, business trips, etc.).

•

The subjects were observed in two coworking spaces, one in the centre of a metropolis and the other in a small town. The objective was to better understand what coworkers do in coworking spaces in order to improve our understanding of what coworking involves.

•

A qualitative survey of coworkers is being conducted. The aim is to place the choice of coworking in a wider context by exploring the workers' professional, family and housing pathways and their attitudes towards sustainable mobility.

•

The final step will be devoted to collecting GPS mobility traces for a week by installing an application on coworkers' mobile phones. The aim of this is to consider the spatialities that shape the coworkers' mobility practices with regard to urban sustainability While, as we have seen, the debates on interdisciplinarity in the field of transport and mobility research have been the subject of numerous publications, no work has been carried out that explains how to implement in practice such interdisciplinarity in the field of sustainable mobility. Based on the limited work that addresses this issue in other research fields [START_REF] Stock | Defining Terms for Integrated (Multi-Inter-Trans-Disciplinary) Sustainability Research[END_REF][START_REF] Bark | Evaluating an interdisciplinary research project: Lessons learned for organisations, researchers and funders[END_REF][START_REF] Sumpf | ENERGY Reflexive Review of Interdisciplinary Working[END_REF], in the COWORKWORLDS project, we have identified three components that are conducive to interdisciplinarity:

•

Defining a common research goal which required cross-fertilisation between disciplines and methods from both transportation studies and mobility studies; • Inclusion within the organisation of the project of a specific task focused on achieving interdisciplinarity; and

•
The existence of relationships of trust between researchers from different disciplines.

The project is marked first and foremost by a common focus on research, namely the sustainability of coworkers' mobility practices. Indeed, it was deemed essential to set up a team that could develop a variety of approaches (spatial, social and economic) and methods (qualitative and quantitative). The aim was not only to collect information about coworkers' travel behaviour, but also to take into account attitudes and norms, both of which are important [START_REF] Von Behren | An Integrated Survey Approach for Clustering Urban Mobility Types[END_REF]. The COWORKWORLDS project, thus, harnesses the expertise of researchers in the disciplines of anthropology, computer science, economics, geomatics, sociology and urban planning. It provides an opportunity for these researchers to bring their diversity of approaches into play: Planners and geomaticians have developed a spatial approach, the first using qualitative methods and the second, quantitative ones; sociologists have drawn the team members' attention to the contribution of urban sociology and used their methodological skills to obtain real-life accounts; economists have provided their expertise in statistical analysis and knowledge of transport policy. The researchers have applied a variety of methodologies: Descriptive statistics and econometric processing, spatial analysis, digital trace collection and analysis, semi-structured interviews and participant observation. To encourage cross-fertilisation between disciplines and methods, each task in the project has involved researchers from different disciplines and used a variety of methods.

When the project was organised, a specific task was created whose purpose was to develop an interdisciplinary approach. This consisted, for example, of identifying the current state of knowledge at the beginning of the project by asking each researcher to present the contributions of their discipline and to explain them. It also included the organisation of regular meetings to which all the researchers were invited, irrespective of how closely the agenda touched on their work.

Finally, relationships of trust gradually developed between researchers from different disciplinary horizons. These were facilitated by the fact that they share a common focus on research-both it's mobility and transport. In addition, many of the researchers belonged to the same laboratory. Although they had never all worked together, a certain amount of collaboration had already taken place they were all involved in the same research seminar, which was interdisciplinary by its very nature. The COWORKWORLDS project represented an opportunity to implement this cross-fertilisation between different disciplinary approaches.

While the characteristics of this project appear to be favourable to the practice of interdisciplinarity, this is never a given [START_REF] Stock | Defining Terms for Integrated (Multi-Inter-Trans-Disciplinary) Sustainability Research[END_REF]. It is on the basis of the questions raised by contact between the disciplines and methods that its extent can truly be assessed. These questions are presented in the following section, based on the notes taken during the meetings which prepared the response to the call for projects, then during its implementation, as well as in the context of the research seminars.

Results

How Can We Define Mobility?

One of the first questions we were faced with when we began this interdisciplinary project was what definition of mobility to use. Researchers in economics, geomatics and computer science, whose approaches are more rooted in transport studies, see mobility as moving from point A to point B [START_REF] Cresswell | Mobilities I: Catching up[END_REF]. Their aim is to quantify trips, to characterise them using a number of indicators (purpose, distance, duration, frequency, transport mode, financial cost, etc.) and to identify the individual determinants of this demand. Naturally, the modalities used to quantify and characterise these trips vary from one discipline to another, particularly between the economists and the computer scientists. In the context of the project, the researchers in these fields consider the mobility of coworkers on the basis of their daily trips, whether for work or other purposes. Researchers who are specialised in spatial and urban planning see mobility first and foremost as spatial. It is a means of understanding the interactions between locations in geographical space, across different spatial and temporal scales (daily mobility, long journeys, residential mobility and migration) [START_REF] Gallez | Aux racines de la mobilité en sciences sociales[END_REF]. They are, therefore, interested not only in the daily movements of coworkers, but also in their business trips or their choices in terms of residential mobility, or even migration. Moreover, the interactions in question can be physical, through real movements, or virtual, through the practice of teleworking, of which coworking is one form. Finally, researchers in sociology and anthropology, who are closer to the field of mobility studies, examine mobility through its social and spatial dimensions. Their objective is to understand mobility practices, how they are built and interrelate with residential and professional mobility in order to understand how the patterns and practices of daily life are interrelated and erode and reinforce each other [START_REF] Shove | Governing transitions in the sustainability of everyday life[END_REF]. Within the framework of the project, this essentially involves using the life history method to follow the residential, professional and family trajectories of coworkers in order to better understand how they came to adopt coworking, and to highlight the norms and values behind those choices, as well as the constraints that inform them. This method, usually used by sociologists and ethnologists, aims to identify the changes in people's mobility practices that take place throughout their lives, as well as the factors that affect them by describing their occupational, family and housing histories. Analysing this biographical material leads to a better understanding of the changes they have undergone and the factors that are significant in their life story.

In view of the diversity of these approaches, how should mobility be defined in the context of our project? Since the aim is to increase interdisciplinarity, it was decided to adopt a broad definition of mobility, where the different disciplinary approaches are considered in a continuum [START_REF] Shaw | Transport, geography and the 'new' mobilities[END_REF][START_REF] Cresswell | Towards a politics of mobility[END_REF]. The mobility practices of the studied coworkers include their daily work-related and non-work-related travel, business trips, residential mobility and life-cycle migration, as well as virtual mobility, i.e., activities carried out at a distance during coworking. This broad definition shows that the links between coworking and mobility can have different meanings. For example, our first results highlight the fact that in metropolitan areas, coworking is often associated with daily mobility issues (choice of transportation mode, changes in travel behaviour, etc.), whereas in rural areas it is more related to life-cycle migration factors (a way of bringing about a certain residential outcome, a lifestyle choice, etc.). These elements have implications both for research and public action. On the one hand, they are an example of cross-fertilisation between transportation and mobility studies by helping to highlight the different ways of understanding movement [START_REF] Shaw | Transport, geography and the 'new' mobilities[END_REF]. On the other hand, they show that local development policies, as well as transport policies, affect coworking, which can be considered as a new way to attract inhabitants and economic activities to rural areas.

How Can We Define Sustainability?

The second question that was raised when the different disciplinary approaches came into contact was how to define sustainability. One of the first areas of discussion involved the environmental, economic and social dimensions of sustainability. Regardless of their discipline, all the researchers involved in the project thought that coworkers' mobility practices have an environmental impact. One of the challenges for the project is to examine the evidence for the environmental sustainability of coworking in different geographical contexts, as coworking could reduce the environmental impact of work-related travel by reducing the distances travelled or by increasing the use of less polluting modes of transportation. In this regard, our preliminary findings point up that rural areas -usually associated with car dependency -can also implement environmentally friendly mobility solutions. For example, some coworking spaces located in these areas offer shared mobility solutions (electric bicycles or cars, cargo bikes, etc.). However, in some cases, the results also highlight the disparity between pro-environmental attitudes and car-based travel behaviour [START_REF] Von Behren | An Integrated Survey Approach for Clustering Urban Mobility Types[END_REF].

For the planners, anthropologists and sociologists, an important challenge for the project is also to evaluate the social and economic sustainability of coworkers' mobility practices. Many studies in the social sciences show that the adoption of environmentally sustainable mobility practices is facilitated by the objective advantages enjoyed by the most affluent segments of the population, examples being the possibility of living in the city centre, flexible working hours and sizeable financial resources [START_REF] Kębłowski | All transport problems are essentially mathematical: The uneven resonance of academic transport and mobility knowledge in Brussels[END_REF][START_REF] Reigner | Nouvelles Idéologies Urbaines[END_REF][START_REF] Coulangeon | A Case Study of the Carbon Footprint of Individual Transportation Practices in France between 1981 and 2008[END_REF]]. If, for example, coworkers' mobility practices prove to be more environmentally sustainable, is this not ultimately because they belong to this creative [START_REF] Florida | The Rise of the Creative Class: And How It's Transforming Work, Leisure, Community and Everyday Life[END_REF], urban, mobile, skilled and connected class? In this case, does encouraging coworking because of its virtuous effects on the environment not amount to promoting a mobility solution that can only benefit the most affluent categories of the population and heighten social inequalities? Various methods were applied to identify the socio-economic profile of coworkers. These included those used by the planners, anthropologists and sociologists (semi-structured interviews, life histories), and those used by the economists, who included questions on the socio-economic characteristics of coworkers in their questionnaire surveys. The first results of the survey highlight the diversity of coworkers' socio-economic profiles: While they share some characteristics (a high level of education, being self-employed), their income levels are varied. At the same time, some of the low-income workers we interviewed were thinking about stopping coworking on the grounds of its cost.

The second area of discussion related to the meaning of sustainability. Should the sustainability of coworkers' mobility practices be assessed in terms of individuals or in terms of areas? This debate contrasts the approach of anthropologists and sociologists on the one hand, and that of urban planners on the other. With regard to the environmental dimension of sustainability, anthropologists and sociologists focus, for example, on the mobility practices of coworkers (modes of transport used, etc.) and the extent to which these practices are compatible or incompatible with their environmental awareness. Planners, on the other hand, assess whether these mobility practices are in keeping with a degree of spatial proximity that has less impact on the environment. With respect to the economic and social dimensions, anthropologists and sociologists will, for example, study the residential and professional mobility associated with the adoption of coworking to see whether they belong to upward paths, and whether they are endured or chosen and by whom. Planners, for their part, will analyse the contribution of such residential and professional mobility to the economic and demographic vitality of the areas in question, as a result of the creation of economic activities and the influx of new inhabitants. These different approaches raise the question of the purpose of sustainable mobility, in view of the fact that the spatial and individual outcomes of sustainable mobility often seem contradictory. In some cases, the mobility associated with coworking may have positive effects on an area (sustainable urban or rural revitalisation), but negative effects on individuals (precarious employment, greater family constraints, car dependency, etc.). The value of the project is to create a dialogue between these approaches, because it is the conjunction of these perspectives that should provide a clearer picture of the complexity of the concept of sustainable mobility. This ties in with the definition of sustainability given by Shove and Walker which includes: "[ . . . ] [T]he importance of attending to all requisite elements of practice, to forms of practical know-how, bodily activities, meanings, ideas and understandings, as well as to materials, infrastructures and sociotechnical configurations" [START_REF] Shove | Governing transitions in the sustainability of everyday life[END_REF] (p. 476).

How Can We Define Coworking?

The third question raised by the development of an interdisciplinary approach concerns the definition of coworkers, and more broadly of coworking. Should we focus on coworking spaces or coworking practices? Planners, economists, and geomaticians tend to favour a spatial approach, centred on the coworking spaces. This ties in directly with mobility concerns. The aim is to see whether or not working in another workplace, located near their home, can help coworkers reorganise their mobility, and what effects coworking can have on sustainability. This approach is in line with research in the field of transport and mobility on the links between ICTs and mobility (homeworking and telecentres, etc.). The social approach, which examines coworking practices, appeals more to anthropologists and sociologists, and even computer scientists who are interested in coworkers' practices with regard to digital technology. The aim is to understand who these coworkers are, why they have adopted coworking, what this new way of organising work consists of and to place their mobility practices and sustainability within this context. Faced with the diversity of definitions found in the literature and the vagueness of the concept of coworking in practice (possibility of coworking in associations, the subsidiaries of large real estate groups, cafés, hotels, business centres, etc.), we decided to first identify coworking spaces, then to focus on the coworkers working in them. This led us to favour the spatial approach, and to develop a definition of coworking spaces. In the context of the project, these are defined as third places [START_REF] Oldenburg | The Great Good Place: Cafes, Coffee Shops, Bookstores, Bars, Hair Salons, and Other Hangouts at the Heart of a Community[END_REF] that are in an intermediary position between the home and the traditional workplace, whose main role is to provide users with a workspace that includes shared services, such as meeting rooms and printers, and to encourage interaction and collaboration. This definition has allowed us to identify the coworking spaces in the studied area (Auvergne-Rhône-Alpes): Of the 164 locations claiming to be coworking spaces (cafés, hotels, etc.), 122 were selected and then mapped in order to analyse their geographical contexts. However, the anthropologists and sociologists subsequently suggested that it would be worthwhile to refine this definition, by looking more closely at what coworking encompasses in terms of exchange and collaboration. It was, therefore, decided to add two participant observations, which were not initially a planned part of the project, carried out over a period of six months in two coworking spaces, one located in the centre of a metropolis and the other in a small town.

How Can We Combine the Different Methods?

The researchers also discussed combining the methods used by different disciplines. The first issue concerned the conduct of the questionnaire survey, which aims to collect information on coworkers' socio-economic characteristics and work and travel habits. One of the major challenges in obtaining a sufficient number of questionnaires for the survey to be usable. As it was clear from the literature that online co-worker surveys had low response rates, the economists initially opted for a telephone survey, the idea is to outsource the conduct of the survey to a private firm. This firm was tasked with contacting coworking space managers to obtain coworkers' telephone numbers in order to interview them for the survey. Such methods are quite commonly employed by transport economists who often use service providers to obtain sufficiently large samples. However, on the basis of the first qualitative surveys conducted among coworking space managers, the planners and sociologists expressed reservations about the possibilities of conducting these telephone surveys. As coworking spaces receive a great many calls, from students and journalists, for example, their managers are strongly committed to protecting their coworkers from disturbance. This made them reluctant to give their clients' telephone numbers to a survey company. After discussions between the economists, the planners and the sociologists, it was finally agreed to use more traditional methods. The researchers contacted the coworking spaces directly and gave them the opportunity to participate in a questionnaire survey. Once their agreement was obtained, they personally administered the questionnaire to the coworkers by visiting the coworking spaces. This method made it possible to attain the required number of completed questionnaires-360 were obtained between November 2018 and March 2019 at coworking spaces registered in our database. However, the method proved costly and time-consuming, due to the need to recruit an additional person to administer the survey.

The second issue related to combining the qualitative surveys of coworkers conducted by the anthropologists and sociologists and the collection of digital traces by the computer scientists and geomaticians. The aim of the qualitative surveys was to reconstruct the residential, professional and family paths of coworkers using the life history method and to elucidate the values and representations that underlie their mobility practices. The collection of digital traces aimed to collect the GPS trace, and therefore, the movements, of coworkers over a week by means of an application installed on their smartphone. This method is increasingly used in the field of transport and mobility studies. However, while it collects all movements, it provides little information on their characteristics (purpose, etc.), which is needed in order to interpret the traces, or their time variability. This explains the value of combining this method with semi-structured interviews. Two concerns were raised when the methods were considered side-by-side. The first was identifying the information to be obtained during the interviews in order to interpret the traces. Many discussions were necessary between the anthropologists, sociologists, planners, computer scientists, and geomaticians in order for them to gain an understanding of each other's methods. These discussions reached the conclusion that a series of tests are necessary to identify the information that is required, and these tests are currently in progress. The second concern was the order in which the surveys should be conducted. Should the process begin with the semi-structured interviews or collection of the digital traces? The first solution that was considered was to start by collecting the digital traces, so that at the end of the interview, they could be presented to the respondents in order to elicit their reactions. However, after several attempts at drafting the interview guidelines, it proved difficult to reconcile the objectives of the two methods. Furthermore, contacting coworkers with a proposal to participate in a survey protocol that was deemed to be intrusive could result in a large number of refusals. It was, therefore, decided to do this in two phases: First, conducting the life story interview with the coworkers, and asking them at the end if they would agree to be part of a digital trace collection operation, then conducting the digital trace collection and recontacting the respondents by telephone to elicit the missing information needed to interpret the traces.

Discussion

Our presentation of the questions raised by the implementation of this research project has suggested three avenues for reflection when attempting to develop interdisciplinary approaches to sustainability in the field of transport and mobility.

The first relates to the importance of defining the terms used by researchers from different disciplines. The key role of language in the practice of interdisciplinary research has already been underscored, particularly in the context of dialogue between the natural sciences and the human and social sciences [START_REF] Bracken | What do you mean?' The importance of language in developing interdisciplinary research[END_REF]. However, initially one might think that this recommendation does not apply to a narrower field of research, such as transport and mobility. In this field, those involved in research deal with (almost) the same topics, and adopt an approach that claims to be interdisciplinary. However, the questions raised in the COWORKWORLDS project show that, depending on the discipline, the concepts commonly used in this field, such as mobility or sustainability, do not have the same meaning and need to be explained in order to avoid ambiguities or misunderstandings. This stems from the fact that there is little agreement on the exact meaning of sustainable mobility in the context of research on transport and mobility [START_REF] Bulckaen | Sustainability versus stakeholders preferences: Searching for synergies in urban and regional measures[END_REF]. It highlights the need not only to define the concept of sustainable mobility as such, but also how to conduct research into it. This need to define terms seems particularly important when research in transport and mobility is concerned with the practical application of interdisciplinarity and is open to other disciplinary fields, such as the digital sciences. Furthermore, working on a common definition of sustainable mobility can help to further the dialogue that has been initiated between transport and mobility studies [START_REF] Grieco | Mobilities: New Perspectives on Transport and Society[END_REF][START_REF] Prytherch | Introduction: Transportation, Mobilities, and Rethinking Urban Geographies of Flow[END_REF][START_REF] Cresswell | Mobilities I: Catching up[END_REF][START_REF] Knowles | Transport Geographies: Mobilities, Flows, and Spaces[END_REF][START_REF] Shaw | Transport, geography and the 'new' mobilities[END_REF][START_REF] Hall | Transport geography and new European realities: A critique[END_REF].

The second avenue for reflection concerns time management when developing interdisciplinary approaches. At the collective level, implementing these approaches requires more time than traditional research: Time to explain the concepts and methods used by each discipline, time for negotiation, time for coordination, time to discuss the results and the strategy for exploiting them, etc. However, the current funding mechanisms for research projects may not yet make sufficient allowance for this additional time, despite strong calls for the pursuit of interdisciplinary research. In the field of research on transport and mobility, projects are generally funded for a period of 12, 24 or 36 months. These periods are quite short in view of this need for additional time for interdisciplinary work, especially when projects involve time-consuming qualitative methods. How can this reflexive attitude be maintained throughout the project when it is necessary, at the same time, to implement the different survey phases, engage in communication activities in connection with the project, write reports and disseminate the results within the academic sphere and among the social and economic community? At the individual level, the implementation of these interdisciplinary approaches also requires researchers to make an additional time investment. Interdisciplinarity is about being able to understand each other and needs clear objectives on all sides [START_REF] Sumpf | ENERGY Reflexive Review of Interdisciplinary Working[END_REF]. It requires a reflexive examination of one's own disciplinary practices, an effort to familiarise oneself with the concepts and methods of other disciplines, availability to participate in projects that are not necessarily central to one's own research. However, in a system where researchers' publications and career evaluations are still discipline-based, the question of return on investment may arise. The problem of the recognition of interdisciplinary publications in some disciplines is a case in point: While in some disciplines these publications will be counted, in others, where the journals are strongly focused and ranked, they will not. Under these circumstances, why should a researcher spend time on such publications?

The third avenue for reflection relates to the need to carry out further work on the implementation of interdisciplinarity in the field of research on transport and mobility. As we have shown, an increasing number of publications deal with the issue of interdisciplinarity and the need to develop a deeper dialogue between transportation studies and mobility studies [START_REF] Shaw | Transport, geography and the 'new' mobilities[END_REF]. However, so far, little work has been done on how to implement this interdisciplinarity in the field of transport and mobility research. This is the area in which this paper has sought to make a contribution, by describing the experience of the COWORKWORLDS project. It has brought together, with a common research focus, a team of researchers from different disciplines who continue to engage in dialogue within the framework of the project, to combine quantitative and qualitative methods and to provide a multidimensional perspective on mobility and sustainability. However, this first phase of the project was also marked by the need to rapidly implement the various field surveys in order to meet the research deadlines, which left little room for reflexivity. It is undoubtedly in the next phase, when the results will be pooled, that the project will become genuinely interdisciplinary in nature.

Sustainability 2019, 11, 7161; doi:10.3390/su11247161 www.mdpi.com/journal/sustainability

Funding: This research was funded by the French National Research Agency (ANR), grant number ANR-17-CE22-0004. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the drafting of the manuscript, nor in the decision to publish the results.

Conflicts of Interest:

The authors declare no conflict of interest.


In this article, Julien Bétaille presents his views on the recognition of the rights of nature and its implications for environmental protection.

Improving the legal protection of the environment in general is not simply a technical problem but raises political issues that deserve to be discussed democratically, not just by legal experts. For some years now, following the recognition of nature as a legal person in Ecuador's 2008 Constitution, a number of Western lawyers and scholars have been using their legal expertise to promote the "rights of nature" or the recognition of nature as a legal person. They present this type of reform as a simple legal technique, whose effects would nonetheless be spectacular… It is true that this was originally a relevant idea. In the early 1970s, Christopher D. Stone proposed considering nature as a legal person. This would enable it to defend its procedural and substantive rights in court, through human "guardians" appointed to represent it. While this iconoclastic idea may have come as a surprise, no one today disputes its technical feasibility. It is legally possible to recognize the legal personality of nature. From then on, this issue was no longer a matter for debate between legal experts only. It entered the political arena.

Rather than knowing whether such recognition is possible, the question is to determine its possible usefulness to better protect the environment. It is on this point that the proposal may seem anachronistic in the European or North American context. Since the mid-1970s, other legal avenues have been used to fulfill the same functions. Access to justice has been widely opened up. Every day, nature is defended before the courts, notably by environmental NGOs, with no need to personify nature. The advocates of this personification seem to ignore it, and in so doing they are pushing an open door and, in a way, deceiving the general public. This is a recurring theme. This is how they present the fact that Ecuadorian judges banned mining in a national park in the name of the rights of nature as a great legal advance. But let's be serious. This is certainly progress in Ecuador and it is of course important to welcome it, but using this argument to promote the personification of nature in Europe is simply inadequate. Traditional environmental law has long used this type of prohibition in national parks. It is of course possible to prohibit economic activities in the name of environmental protection. There is no need to resort to the rights of nature. We should certainly create more national parks and protected areas, but this is not a question of legal technique.

The debate is also distorted when it comes to the supposedly superior performance of "nature's rights".

There are few articles on this topic that do not begin by explaining that environmental law is incapable of properly protecting nature. However, where it does exist, and when it is properly implemented, it produces undeniable results. Just think of the progress that has been made over the last fifty years or so (reduction in pollution, return of large predators, surface of protected areas), even if this remains largely insufficient given the scale of environmental problems. The problem, then, is not to replace the techniques of traditional environmental law with those of the "rights of nature", as their supporters suggest. The real issue is the possible extension and radicalization of the rules that protect nature. There is no point in hiding this political issue behind a legal cloak. It is a question that should not be monopolized by legal experts, but should be debated in the political arena.

Indeed, the promotion of the "rights of nature" tends to push political issues into the background.

Radicalizing the rules protecting the environment necessarily has consequences for individual and collective freedoms. It is therefore better to have an open democratic debate on this subject than to focus on a simple idea, too simple. Supporters of the "rights of nature" tend to deny it when they take part in public debate, but it is obvious that as soon as rights are recognized to nature, the question of how they compare with those of humans is raised. On the other hand, the preachers of this theory are more assertive about their project, following the example of the Earth Jurisprudence and Wild Law movements which intend, without any real nuance, to "redefine" human rights by subjecting them to a pseudo-natural order, nothing less… What is most disturbing when reading these arguments is the common emphasis on the idea of "biological" laws or laws "based on the functioning of living organisms". So, there's no room for political choice, it is just a matter of following -without discussion -an injunction from nature. In other words, this is a return to the naturalistic fallacy, a deliberate confusion between the laws "of nature" and legal laws. But nature, as such, does not set any standards. Of course, humans observe nature and science describes regularities in it, deriving "scientific laws" from it. But there is nothing normative about these "laws". For example, science describes the decline in biodiversity and offers an explanation, but it does not prescribe the solutions to be adopted, at most it suggests them. By contrast, based on these scientific results, humans may (or may not) decide to impose legal standards on themselves, to the benefit of nature. It is then a self-limitation of humans in favor of nature, not the consequence of a "natural order". This is precisely what classic environmental law has been doing for a long time, on the basis of scientific findings but also as a result of democratic deliberation. This is the real "paradigm shift", which already took place with the affirmation of environmental law in the 1970s. Humans have collectively decided, albeit imperfectly and tragically inadequately, to limit themselves. But with or without "rights of nature", nature as such decides nothing. This clarification is important because there is a great risk that this idea of "laws of nature" will be manipulated, especially in a context where we have to make radical decisions in favor of the environment. Indeed, history has taught us that we must be wary of arguments based on the "laws of nature", even when they are presented as liberal and well-intentioned. In his day, Aristotle justified slavery in this way. Today, some people oppose abortion or same-sex marriage in the name of these same "laws of nature". At the end of the day, these laws are an argument of authority used by a few to impose a prescription on the majority, hiding behind a pseudo normative "natural order". In the case of the "rights of nature", the risk is that a minority of nature's "guardians" will decide "in the name of nature" against most of the people. We would then be moving from the sovereignty of the people to a form of tyranny.

  NATURE AGAINST PEOPLE'S SOVEREIGNTY?