Philippe Gagnepain

Marc Ivaldi

Catherine Vibes

The industrial organization of competition in local bus services

Introduction

In most countries, local transport services by bus, which are a major component of regional passenger transportation systems, are subject to the scrutiny of policy makers for at least two contextual reasons. First, while the passenger transport services have always been highly regulated, the public transportation policy is now experiencing deregulation and/or privatization in an industry where urban transport companies are heterogeneous in their ownership status, which can be public or private, as well as in the diversity of transport modes they offer (bus, train, underground, and tramway). Second, while the modal share of bus transport services has been declining for several decades in most developed economies, the growing environmental concern raises the calls for promoting urban mass transit (as opposed to private car). This chapter is aimed at deepening our understanding of the functioning of competition in the local bus transportation industry and to evaluate its effectiveness. It provides an overview of the competitive constraints that are at work in the industry as discussed in the economic literature, and sketches empirical tests to check whether the intuitions provided by the economists are in line with the reality of the industry. To address these various issues, the first three sections of this text survey the economic literature on bus competition, emphasizing the case of UK which is used as a benchmark. We suggest that earlier contributions, proposed in the late eighties, (i.e., just after the deregulation of the industry) are very often based on unrealistic assumptions, mainly chosen because the authors lack of a sufficient perspective on the effects of deregulation. Hence, we focus on the most recent literature, which we attempt to survey as completely as possible. The objective is to draw the main conclusions or results which are shared by the analysts or researchers on how this economic activity functions. In the last section, we propose some methods to empirically test these main predictions of the economic literature.

The analysis is summarized as follows. Section 1 reviews what it is empirically known about the technological features and the economic performance of bus operators. i Section 2 focuses on the analysis of demand for urban transport services. Section 3 analyzes the competition in local bus service industry, leading to the conclusion that operators do not compete in prices but mainly in frequency.The last section concludes.

Cost and production analysis

The cost and production analysis addresses three main issues. First it is aimed at measuring economies of scale, economies of density and economies of scope which are key structural elements to describe in economic terms the technology behind an industry.

Second, it should provide a measure of the level of technical efficiency to evaluate the performance of firms. Finally, it discusses the definitions for the output variables. In the literature, either supply indicators (e.g., vehicle-kilometres or seat-kilometres), demandrelated output measures (e.g., passenger-kilometres or the number of passengers) or multidimensional output definitions are used. (See De Borger and Kerstens (2006) for a discussion on the choice of output measures.)

Technological characteristics

Let us first review some characteristics of the technology for bus transport services, such as returns to scale, economies of density and economies of scope. It is a common result that bus companies experience increasing returns to scale. (See [START_REF] Filippini | The Influence of Ownership on the Cost of Bus Service Provision in Switzerland -An Empirical Illustration[END_REF][START_REF] Farsi | Cost efficiency in regional bus companies: An application of new stochastic frontier models[END_REF][START_REF] Farsi | Economies of Scale and Scope in Local Public Transportation[END_REF]) More specifically, it seems that smaller firms benefit from increasing returns to scale, as opposed to larger firms which exhibit constant or even decreasing returns. (See [START_REF] Viton | The main determinants of the demand for public transport: a comparative analysis of England and France using shrinkage estimators[END_REF][START_REF] Matas | Technical Characteristics and Efficiency of Urban Bus Companies[END_REF][START_REF] Kerstens | Decomposing technical efficiency and effectiveness of French urban transport[END_REF] For the British bus industry, [START_REF] Cowie | Organisation Form, Scale Effects and Efficiency in the British Bus Industry[END_REF] estimate that small companies of fewer than 200 buses experience some economies of scale. They also find that the size of such returns varies with the company type whether it is public limited, private limited, or municipal. [START_REF] Sakano | Re-examination of inefficiencies in urban transit systems: A stochastic frontier approach[END_REF] find increasing returns to scale for the U.S. urban transit industry. Overall a significant number of empirical studies are in line with a U-shaped average cost function exhibiting increasing returns to scale for the smaller operators, which become constant and finally decreasing as companies' size increases.

In most empirical studies, economies of density are frequently found regarding the bus companies' technology. As already pointed out, the distinction between economies of density and economies of scale is very important in industries that provide their services over a network. In these cases, the firm size is more closely related to the size of the network than to the output provided over that network. For this reason it is important to distinguish cost changes that occur because of output changes only and cost changes that occur because of a proportional network and output change. Among studies which estimate that bus companies fail to operate at an efficient density are [START_REF] Farsi | Cost efficiency in regional bus companies: An application of new stochastic frontier models[END_REF] and [START_REF] Filippini | The Influence of Ownership on the Cost of Bus Service Provision in Switzerland -An Empirical Illustration[END_REF] on the Swiss market, [START_REF] Matas | Technical Characteristics and Efficiency of Urban Bus Companies[END_REF] for Spain and Shaw et al. (2005) for Taiwan. It appears that bus operators could obtain cost-saving benefits by extending their output scale. Some articles have focused on the multi-modal side of the industry and have asked whether a bundling of operations from different urban transport modes (bus, train, metro for instance) is preferable to a separated configuration. They converge to the conclusion that economies of scope are significant in the industry, and that their results are in favour of integrated multi-modal operators. [START_REF] Farsi | Economies of Scale and Scope in Local Public Transportation[END_REF] conducted a study in Switzerland and found increasing returns to scale in almost all outputs. They consider that these returns, combined with cost complementarities, can be considered as a suggestive evidence for natural monopoly. [START_REF] Viton | How big should transit be? Evidence on the benefits of reorganization from the San Francisco Area[END_REF] also finds positive economies of scope and concludes that together with the nature of economies of scale, they support the formation of larger multimodal systems in the San Francisco Bay Area.

Efficiency

The recent literature on performance of operators of local bus services shows that there still exists a substantial level of inefficiency in this industry. However, huge differences exist over time and across countries. [START_REF] Cowie | Organisation Form, Scale Effects and Efficiency in the British Bus Industry[END_REF] find a high degree of inefficiency in the British bus industry which they interpret as an indicator of wasteful competition. However, [START_REF] Cowie | Acquisition, efficiency and scale economies, an analysis of the British bus industry[END_REF] estimates that the average efficiency has improved in the U.K., suggesting that mergers may have allowed existing group companies to operate closer to the optimal level of output. [START_REF] Heseltine | The Effects of Bus Deregulation on Costs[END_REF] for the British operators find that the main total unit cost reduction was achieved by productivity improvements. Working on a sample of Spanish cities, [START_REF] Garcia-Sanchez | Technical and Scale Efficiency in Spanish Urban Transport: Estimating with Data Envelopment analysis[END_REF] finds that a majority of municipalities are technically inefficient, mainly due to scale inefficiency. This is a similar result to [START_REF] Kerstens | Decomposing technical efficiency and effectiveness of French urban transport[END_REF] who indicates that inadequacies in scale are the major source of poor performance in her sample of French urban transport service operators. Some studies though are more optimistic in their measurements of efficiency, in particular in the UK. [START_REF] Viton | The main determinants of the demand for public transport: a comparative analysis of England and France using shrinkage estimators[END_REF] finds that 80% of bus systems are efficient in the U.S. Wunsch (1996) who compares 178 European urban transport companies claims that two British firms, in the cities of Manchester and Sheffield, are among the first on his list in terms of technical performance. However, he takes into account only dominant bus companies and he admits that his result depends crucially on data quality. Most studies underline the dispersion in the efficiency measures they obtain within the same country or area. (See Kerstens 1999, De Borger andKerstens, 2006, with the exception of Salas, 1998, who finds that, in Sweden, the levels of efficiency are very similar among companies.)

Private / Public ownership

Contrary to a common argument, there is substantial evidence in the literature that private bus companies do not operate more efficiently than public companies. Ownership type does not seem to be a crucial determinant in the firms' performance, as shown in [START_REF] Odeck | The Relative Efficiency of Public and Private Bus Companies?[END_REF] and [START_REF] Garcia-Sanchez | Technical and Scale Efficiency in Spanish Urban Transport: Estimating with Data Envelopment analysis[END_REF] on the Norwegian market and Viton (1997) who shows that U.S. public and private systems share the same distribution of technical efficiency. [START_REF] Fazioli | Cost-Structure and Efficiency of Local Public Transport: The Case of Emilia Romagna Bus Companies[END_REF] found no relation between technical efficiency and ownership among a sample of Italian urban transit firms precisely because of the absence of effective competition for both public and private operators and strong regulation. [START_REF] Filippini | The Influence of Ownership on the Cost of Bus Service Provision in Switzerland -An Empirical Illustration[END_REF] underline that the results in their study on a Swiss sample depend on the specification of output and network variables. However, if we can assert there is no strong evidence of a higher efficiency for private firms, some studies do estimate they perform better. [START_REF] Cowie | Organisation Form, Scale Effects and Efficiency in the British Bus Industry[END_REF] find privately owned firms are not more technically efficient, although they exhibit a considerable level of managerial efficiency. They find that values of increasing returns to scale for small companies not only vary with the ownership type (public/private) but also with the actual form of private ownership. Relevance of ownership as a determinant for performance is also found in [START_REF] Kerstens | Technical efficiency measurement and explanation of French urban transit companies[END_REF] and De Rus and Nombela (1997) on the French and Spanish market respectively. At this point, the literature is considered inconclusive regarding the impact of ownership type on efficiency.

Subsidies

There is some evidence that subsidies are associated with an increase of operating costs. In particular, [START_REF] Kerstens | Technical efficiency measurement and explanation of French urban transit companies[END_REF] corroborates this assertion when analysing a sample of French urban transit companies. [START_REF] Sakano | Re-examination of inefficiencies in urban transit systems: A stochastic frontier approach[END_REF] on U.S. transit systems report that subsidies lead to excess use of labor relative to capital and excess use of fuel relative to capital and labor.

Incentive contracts

Several recent studies have revealed the positive effects of incentive contracts on technical efficiency. In [START_REF] Kerstens | Technical efficiency measurement and explanation of French urban transit companies[END_REF], empirical findings confirm the importance of appropriate incentives in contracting for monopoly. Risk-sharing agreements seem to stimulate the performance of organizations. These results for French operators are confirmed by Gagnepain and Ivaldi (2002a) who develop a method which should help to clarify the choice of regulation in the urban transport industry. They conclude that cost-plus contracts are dominated by any type of second-best contract. These results are in line with those of [START_REF] Roy | Contractual practices and technical efficiency: The case of urban public transport in France[END_REF] who find that operators under cost-plus contracts exhibit a higher level of technical inefficiency than operators under fixed-price agreements.

De Borger and Kerstens (2006) survey other European studies which exhibit that highpowered incentive contracts improve efficiency.

Competitive tendering

The available evidence suggests that competitive tendering may improve performance. These results are exhibited by [START_REF] Hensher | Competitive tendering as a contracting mechanism for subsidising transport -The bus experience[END_REF] who review the international successes and failures of competitive tendering from ten developed countries.

De Borger and Kerstens (2006) in their survey give a more detailed description of the effects of competitive tendering.

Methodologies / Discrepancies

It is important to bear in mind that all these performance analyses differ in several aspects. First, there exist several approaches to estimate efficiency on the basis of observed data. Efficiency, as measured by a deviation from the unobserved cost or production frontier, can be estimated by means of parametric and non-parametric methods aimed at determining the production or cost frontiers. On the one hand, parametric methods require the specification of a functional form for the frontier, a popular one being the flexible translog cost function. On the other hand, non-parametric approaches do not need to specify a functional form; they construct the frontier by enveloping the data on inputs and outputs by piecewise linear hyperplanes, as proposed by the extensively used data envelopment analysis (DEA) method. Both methodological strands have advantages and weaknesses, related to the presence (or not) of measurement errors or the requirement to specify functional forms. A detailed description and discussion of these frontier methodologies are presented in [START_REF] Lovell | Production frontiers and productive efficiency[END_REF] and [START_REF] Brons | Efficiency of urban public transit: A meta analysis[END_REF] respectively.

A second source of differences in the measurement of efficiency comes from the definition of the output variable. A significant number of studies conclude that operators' performances differ substantially depending on the output specification considered. Supply indicators (e.g., vehicle-kilometres or seat-kilometres) or demand-related output measures (e.g., passenger-kilometres or the number of passengers) have been used.

A third crucial aspect in the model specification for measuring efficiency is that models should account for relevant measures of service and network characteristics. Bustransit services have been recognized as very heterogeneous across countries and even cities. This is confirmed by [START_REF] Brons | Efficiency of urban public transit: A meta analysis[END_REF] and De Borger and Kerstens (2006) who find significant and consistent effects of the type of database, region and output measurement method. Fourth, some authors underline the need to decompose the measures of efficiency into two components (allocative and technical). For example, [START_REF] Viton | The main determinants of the demand for public transport: a comparative analysis of England and France using shrinkage estimators[END_REF] suggests that the result of similar efficiency distributions between private and public firms might hide the fact that private systems would be more allocatively efficient. Also, according to him, the distinction between managerial and organisational efficiency seems relevant in this industry, particularly in measuring the impact of ownership type on efficiency. This conclusion is confirmed by [START_REF] Cowie | Organisation Form, Scale Effects and Efficiency in the British Bus Industry[END_REF]. (See also Gagnepain and Ivaldi, 2002b.)

Further research

Although the literature on measuring efficiency in the urban transport industry is extensive, some aspects still have to be investigated more thoroughly. An international comparison on the effects of deregulation and competition on efficiency would be of high interest. Also, only a few studies take into account the presence of other transport modes on the market. Indeed, the presence of economies of scope and the call for limiting private car traffic to the benefit of urban modes because of environmental policies make this multioutput aspect of the industry particularly relevant. Further analysis of the decomposition of efficiency into its several components to better understand the effects of ownership and deregulation on efficiency seems to be a next step in the research agenda.

Demand analysis

In this section, we review the values of bus demand elasticities found in the literature. We discuss different types of elasticities. First, we look at the own price elasticities. Note that the own price elasticity of the demand that a firm faces is always more elastic than the aggregate elasticity of market demand. This is because there are fewer substitutes for a product at the market level than at the firm level. An example would be the substitution between competing bus services on a market as opposed to substitution between different transport modes on this market. Second, we present the measures of cross-price elasticities of demand which evaluate the substitution patterns between competitors (transport modes or services). For example, the cross-price elasticity from bus to car tells us the percentage increase in car demand following a one percent increase is bus fare. In the bus-transit industry competition can come from other bus operators, as well as other transport modes (train, metro, car...). Finally, we report the values of income and service quality elasticities found in the literature.

Own price elasticities

It is a common result in the literature that the demand for bus-transit is not elastic in the short run. Most studies on bus-transit own-price elasticities agree on a value of -0.4 and this result is summarized in four surveys on urban demand by [START_REF] Goodwin | A review of new demand elasticities with special reference to short and long run effects of price changes[END_REF], [START_REF] Oum | Concepts of price elasticities of transport demand and recent empirical estimates[END_REF], [START_REF] Dargay | Bus Fare Elasticities[END_REF] and Balcombe et al. (2004).

These studies emphasize that authors now agree on the necessity to consider dynamic changes in these own-price elasticities. All studies allowing elasticities to vary over time, that is to say, allowing demand to adjust to changes in price in the long-run, have agreed that demand in the long run is more elastic than demand in the short run. The role of dynamics in urban transport demand is the objective of the survey conducted by [START_REF] Goodwin | A review of new demand elasticities with special reference to short and long run effects of price changes[END_REF] who estimates that long-run elasticities range between 1.5 to 3 times higher than short-run elasticities. He concludes that a time-dependent specification for the demand is preferred. In the literature [START_REF] Goodwin | A review of new demand elasticities with special reference to short and long run effects of price changes[END_REF], Balcombe et al. 2004) the long-term response should be expected in a period of 5 to 20 years according to the authors. Only Matas (2004) on the Spanish market finds that 95 percent of the effects are realized within 3 years. As shown in Table 2.1, the values for long-term own-price demand elasticities vary from -0.4 to -1.3. However, values significantly greater than 1 are rare in the literature. Among the articles displaying the highest values are [START_REF] Romilly | Subsidy and Local Bus Service Deregulation in Britain: A Reevaluation[END_REF], [START_REF] Dargay | Bus Fare Elasticities[END_REF] and [START_REF] Gilbert | The demand for travelcards on London regional transport[END_REF] on the British market. On the other hand, a study conducted by [START_REF] Deb | Public bus transport demand elasticities in India[END_REF] on the Indian market leads to relatively small values of long-run elasticities which the authors interpret as the effect of the low level of development in India and the fact that public transport is still a necessity there. That long-term are higher than short-term elasticities has the following implications.

First, the full behavioural response to fare changes cannot be properly identified by means of unlagged time-series models. Now demand models estimated on cross-section data can only reveal long-run price elasticities. Second, in this industry, the range of responses open to people is larger in the long run. Car ownership decisions require time to be implemented.

It is well known that this dynamic aspect of demand is an important consideration in implementing policy strategies.

Another important finding of the literature on own-price elasticities for bus-service demand is that the estimated measures vary with the type of ticket purchased by customers.

The common result is that demand for a single ticket is more elastic than demand for a travel pass. Instead of building a price index to analyse the impact of a change in this price on demand, some authors have disaggregated these effects with respect to the different categories of tickets available to the customers. De Rus (1990) estimates fare disaggregated elasticities for bus-transit in Spanish cities and finds that data disaggregated by ticket fare provides a deeper understanding of demand responses. As he finds that price sensitivity decreases as we move from single tickets to the travel pass, he concludes that an aggregate approach fails to allow explicitly for shifts in demand between ticket types and that the role of cross-effects between ticket-types is key for the pricing policy. These results are in line with other studies on aggregate data, like Tegner and Holmberg (1998) on the Swedish market, and on micro data as in [START_REF] Hensher | Establishing a fare elasticity regime for urban passenger transport[END_REF] and [START_REF] Taplin | Preserving the symmetry of estimated commuter travel elasticities[END_REF]. However, these last two analyses report smaller values for the elasticities. Matas (2004) in a more recent article with aggregate data on the Spanish market confirms these previous results and concludes that there is scope for a more efficient non-uniform pricing policy with positive effects on demand while minimising the negative effects on revenue.

Cross-price elasticities / Substitution effects

A change in fare for a transport mode can lead a customer to switch to another competitor, within the range of all available urban transport modes available (private car, train, bus, metro, or others). These substitution effects between travel modes are important when analysing competition and we present here the main literature findings on these measures.

The common result in the literature is that these substitution effects between modes are of a small magnitude in the short run. However, some authors consider that these findings, combined with higher long-run own-price elasticity for car and bus use, make modal shifts more feasible than often assumed [START_REF] Goodwin | A review of new demand elasticities with special reference to short and long run effects of price changes[END_REF]. [START_REF] Hensher | Establishing a fare elasticity regime for urban passenger transport[END_REF] who distinguishes between fare classes finds that, in the Sydney metropolitan area, the largest cross-elasticity between private car and train travel pass is 0.335 in the event of an increase in the price of car utilization. He also finds that there are more changes between modes for a given fare class than between fare classes within modes. The strongest cross-mode substitution for a given fare class (excluding car) occurs between train and bus single tickets with cross-elasticities of 0.067 and 0.057 for train-to-bus and bus-to-train respectively. [START_REF] Taplin | Preserving the symmetry of estimated commuter travel elasticities[END_REF] who aim at improving the methodology presented in [START_REF] Hensher | Establishing a fare elasticity regime for urban passenger transport[END_REF] estimate that the most significant differences observed between the two approaches are a large decrease in the elasticity of demand for car with respect to the price of a ticket for a single trip called Bus Single (from 0.066 to 0.018), and a large increase in Bus Single with respect to car cost (from 0.116 to 0.212). Matas (2004) looks at cross-price elasticities between ticket types and between transport modes. According to his results, bus users are sensitive to both bus and underground prices and quality, whereas underground users are only sensitive to underground characteristics. However, he also concludes that there is not enough information to understand the impact on modal shifts from car to public transport. [START_REF] Dargay | Bus Fare Elasticities[END_REF] observe that the cross-elasticity between bus patronage and motoring costs appears to be negligible in the short run and about 0.3 to 0.4 in the long run. According to them there is some price substitution between bus and car use, although comparatively small. Balcombe et al. (2004) find that, in urban areas outside London, public transport use is sensitive to car costs but car use is much less dependent on public transport costs. [START_REF] Oum | The structure of travel demands in the Netherlands: an application to predict modal shares under the sustainable development goals[END_REF] in a study of the Dutch urban market estimate that the relative price of private car must rise significantly to induce a significant number of car drivers to switch to public transport modes.

Trip purpose / Peak and off-peak demand

Fare elasticity is different for different journey purposes. Trips made to go to work or to school are considered as peak demand, whereas trips for leisure or shopping are much more flexible in the time of the day and correspond to an off-peak demand. One would expect fare elasticity to be higher for off-peak demand than for peak demand where customers do not have much choice but to travel. In their review of the literature, Balcombe et al. (2004) observe that the mean off-peak elasticity for buses (precisely, -0.5) is at least twice the peak elasticity (i.e., -0.2). This is in line with the World Bank report by [START_REF] Oum | A Survey of Recent Estimates of Price Elasticities of Demand for Transport[END_REF].and a literature review by Fowkes et al. (1993). [START_REF] Ivaldi | A Parsimonious Approach to Multidimensional Choice Models of Urban Transport[END_REF] also find significant differences in urban trip purposes. [START_REF] Dargay | Bus Fare Elasticities[END_REF] observe that, in the UK, the income elasticity for bus services, which includes car ownership effects, is negative in the long run. This is in line with the literature and suggests that bus transport is an inferior good. (See Bresson et al., 2003, Balcombe et al. 2004.) The negative long-run elasticity reflects the effect of income through its positive effect on car ownership and use, and the negative effect of the latter on bus patronage. They estimate that income elasticity ranges between -0.5 to -1 in the long run. However, as car ownership approaches saturation the income elasticity can be expected to become less negative. [START_REF] Romilly | Subsidy and Local Bus Service Deregulation in Britain: A Reevaluation[END_REF] finds a positive value of 0.61 for his long-term income elasticity, suggesting that the economic growth has outweighed the inferior good aspect of the service. Matas (2004) also finds a positive value for the income elasticity (precisely, 0.15) in Spanish cities. He explains the difference with [START_REF] Dargay | Bus Fare Elasticities[END_REF] by the higher population density of Spanish cities which makes them better suited to public transport use than to car use.

Income elasticities and car ownership effect

Service Elasticities

Regarding service elasticities, Matas (2004) estimates a service elasticity of 0.24, although he explains that, in aggregate studies, a very crude proxy for the quality of service is used, and it is difficult to give an adequate interpretation of the estimated elasticities.

Quality is defined in different ways in different studies making uneasy the comparison of their values which are ranging between 0 and 1. De Rus (1990) finds a high coefficient of variation between the different cities. According to [START_REF] Deb | Public bus transport demand elasticities in India[END_REF] and as expected from the literature, service quality is the most significant policy variable as it has the largest impact on travel demand. Bresson et al. (2003) show that, in France and in the U.K., fare and service elasticities are of a similar magnitude (although opposite in sign), so that an increase in fares combined with an equivalent increase in service (vehicle kilometres) would have only marginal effects on patronage.

Methodologies / Discrepancies

Several approaches are used in the literature to compute reliable measures for urban transport demand elasticities. There is common agreement that variances in values for the different elasticities are influenced by several factors, both related to methodological aspects and to features of the industry. In particular [START_REF] Nijkamp | Meta-analysis for explaining the variance in public transport demand elasticities in Europe[END_REF] have carried out a comparative analysis of elasticity values of transport demand resulting from twelve studies in various countries. Their analysis indicates that the difference between aggregate (macro-) and disaggregate (micro-) models, as well as with other assumptions, explain the variance in the values of elasticities across studies. They also find that the country involved, the number of competitive modes, and the type of data collected are important factors in accounting for the level of elasticities. These conclusions confirm the findings of [START_REF] Oum | Concepts of price elasticities of transport demand and recent empirical estimates[END_REF] who survey the elements that impact the estimation of demand elasticities in different studies. Oum et al. emphasize the need to take into account intermodal competition because, otherwise, own-price and cross-price elasticities are biased given that they ignore some of the competing services. They also underline that different functional forms can result in widely different elasticity estimates, even with the same set of data. Note that models also differ with the choice of the definition of the dependent variable (whether one considers journeys or passengers-kilometres) and the way fares are aggregated into a price index. They observe that results differ according to the area or country under analysis, which have their own features (in particular for their urban-transit services). This is why they highlight the fact that disaggregated data would lead to a wide range of elasticities as they would reflect unique market conditions. [START_REF] Dargay | The Demand for Local Bus Services in England[END_REF] find a considerable variation in the fare elasticity across counties, ranging from 0 to -3 in the long run. Bresson et al. (2003) in their comparative study between France and the U.K. confirm the relevance of taking into account countries' heterogeneity. The study by [START_REF] Dargay | Bus Fare Elasticities[END_REF] corroborates the findings of [START_REF] Oum | Concepts of price elasticities of transport demand and recent empirical estimates[END_REF] and [START_REF] Nijkamp | Meta-analysis for explaining the variance in public transport demand elasticities in Europe[END_REF]. First they find a large variance of elasticities across counties in the U.K.; second, they conclude that estimated elasticities from different studies are not directly comparable. More precisely, they assess it is inappropriate to apply the value of an estimated elasticity for different circumstances or to average the values of elasticities from different studies.

Further research

The preceding review of the literature on urban transport demand highlights some areas for improvement in the methodologies adopted so far. First, models for disaggregated data have rarely been estimated and they would constitute a considerable enhancement in urban demand studies. They would allow us to capture the specific effects of the markets under scrutiny, such as different ticket fares, trip purposes, and customer categories. An aggregate elasticity hides these specific effects. Second, more structure could be applied to the models and the interaction between supply and demand could be taken into consideration. Third, the literature suggests that a comprehensive representation of the market is important as we observe significant differences in characteristics across cities.

Competition from other modes should be taken into account to avoid bias in the measures of elasticities. Fourth, functional forms have to be chosen carefully as they can lead to very different results, even applied to a same dataset. Econometric testing of different model alternatives would be a useful part of the research agenda.

Competition analysis

Entry

As a general rule, a firm enters the market only if it can earn positive profits. When entry happens on a significant scale, it is expected that the incumbent reacts. In the early deregulation period, the literature focusing on bus competition suggested that entry may be a relevant issue and has shed light on several cases of entry in local markets. Entry usually occurs on the periphery of the incumbent's main market area, particularly if the incumbent has a local reputation. ii Some smaller operators have attempted to enter on a small scale hoping not to invoke a response from the incumbent firm. The literature suggests however that entry strategies have been unsuccessful in most cases. (See Preston, 1988, for an early analysis. Note that the literature does not provide any further evidence of successful entry in the 90s or the 00s.)

To explain why entry was unsuccessful in the early deregulation period and why it was scarce in the years following deregulation, an important argument is that the industry is not perfectly contestable. As an indication that the industry is not contestable, we list as a first step the usual conditions which guarantee that a market is perfectly contestable. As a second step, we discuss why these conditions seem not to be met in the bus transportation industry.

According to [START_REF] Shepherd | Contestability versus Competition[END_REF], [START_REF] Baumol | Natural monopoly and contestable market analysis[END_REF][START_REF] Banister | Bus deregulation in the UK[END_REF], a perfectly contestable market requires the following conditions:  Entry is free;  Entry is perfectly reversible, i.e., sunk costs are zero;

 The incumbent and the entrant have access to the same technology;

 The incumbent and the entrant have equal access to all customers in the market, i.e., consumers are not loyal to the incumbent's products; the services of the incumbent and the entrant can be easily accessible (for instance bus terminals can be used by all operators);  There is an active second hand market for capital assets (e.g., the entrant has access to "cheap" buses for its rolling stock);  The regulator imposes time lags to prevent sudden changes in prices or withdrawal of services by the incumbent firm, e.g., "hit and run" strategies, where the entrant enters the market over a short period and enjoys high prices, can be implemented.

In a contestable market, any attempt by incumbent firms to earn excessive profits would be unsuccessful. Furthermore, even if there is just one firm offering the service, this firm would be engaged in average cost pricing and have zero profits. If positive profits were obtained, competitors would enter the market and undercut the incumbent's prices and profits. Hence, the important idea is that the mere threat of entry forces the incumbent not to behave as a monopoly despite the intrinsic properties of the market which enable it to do so.

The economic literature is unanimous in stating that the local bus transportation industry is not perfectly contestable. In the very first years of deregulation, [START_REF] Preston | Regulation, competition and market structure: A literature review of the stage bus industry[END_REF], [START_REF] Button | Contestability in the UK bus industry, experience goods and economies of experience[END_REF], [START_REF] Beesley | Collusion, predation and merger in the UK bus industry[END_REF][START_REF] Evans | Are urban bus services natural monopolies?[END_REF] suggest that many factors prevent the markets from being contestable:  Existence of barriers to entry: The access to bus stations and the use of travel cards have acted as barriers to entry; the incumbent may have more convenient terminal positions; entrants may not obtain access to bus stations; information points may be manned solely by the incumbent firm's staff and entrant firms may be located at the least attractive stands in the bus station; other practices include the blocking of a rival's bus, occupying a stand or using couriers to persuade customers to use one company's buses in preference to another. Barriers to entry may have been underestimated at the moment of deregulating the market.

 Existence of sunk costs: Trained staff (managerial, administrative and platform) is costly. An entrant finds it difficult to hit and run if its employees are not highly qualified. iii  Practices which raise rivals' costs: Operators may withhold surplus buses from the second hand market, hoping that the price of old buses would increase as their availability decreased, hence making entry to the local market more difficult.

 Economies of experience, economies of scale, economies of density, and economies of scope: The incumbent may have a larger network than the entrant and may therefore be able to offer more attractive area-wide tickets than the entrants; the incumbent may be better known.

 Incumbents can reduce prices very quickly (usually within 24 hours).

During the 90s, these initial intuitions are confirmed. [START_REF] Evans | Competition and the Structure of Local Bus Markets[END_REF][START_REF] Evans | Are urban bus services natural monopolies?[END_REF] insists on the fact that the incumbents can change their prices immediately in response to entry since operators are allowed to change fares without notice. As a result, incumbents can enjoy super-normal profits on high density routes. Moreover, the "experience" input is essential to explain the tactical advantage of the incumbent firm, given that it is usually better informed about different aspects of providing the service. [START_REF] Beesley | Collusion, predation and merger in the UK bus industry[END_REF] claims that barriers to entry are numerous: For instance, garage locations and other property rights play a key role since they directly affect the likelihood that local markets can be opened to competition. The law may itself impede entry: For instance, entrants are required to remain at least 6 weeks in the market; sub-contracting to drivers is restricted. [START_REF] Banister | Bus deregulation in the UK[END_REF] contributes to this view of the industry when he states that the characteristics of the industry and the strategic actions of the incumbent both impede the local transport market from being contestable. In addition to the previous factors, Banister proposes the following characteristics:

 The need to replace the ageing bus fleet, which requires greater capital investment than the smaller companies are able to obtain or willing to risk;  The fear of competitive disadvantage of the smaller operators against the larger operators is significant in the bus industry;

 Large and small companies do not have access to finance on equal terms.

Incumbents tend to have weaker risks of bankruptcy than entrants because they have a larger size and have a bigger purse. The incumbent may own routes elsewhere which earn high profits which can be used to cross subsidize more less competitive routes;

At the same time, Banister sheds light on the possible actions to be taken by the incumbent to reduce the arrival of entrants; these actions are:  Build up consumer loyalty;  Establishing a reputation for toughness by maintaining a presence in the market;  Reorganize the network so that economies of scale and density can be obtained;  Maintain ownership of fixed assets such as terminal, booking and maintenance facilities.

Banister thus concludes unambiguously that "the theory of contestable markets does not apply to the bus industry. In 1985 it may have been attractive to accept the contestability arguments, but this does not seem to be true anymore ten years after, since the size of operations seems important. The role of the small operators is reduced to competing through the tendering process for the socially necessary services."

After 2000, the initial propositions listed above, on why the industry is not contestable, are corroborated and new claims are made on this issue. First, De Borger and Kerstens (2006) suggest that the rolling stock capital of entering firms has the characteristics of a sunk cost. More importantly, the incumbent's strategic actions impede entry:

 Incumbents can easily cut prices and adjust schedules;  The incumbent operates the fixed facilities (a central bus station for instance) available that are crucial to exploit network economies (interconnections between different lines or sets of lines), given that the demand structure is characterized by complementarities between lines.

Second, [START_REF] Langridge | Contestability in the UK bus industry? The National Bus Company, and the Tilling Mark II effect[END_REF] emphasize the idea of the economies of experience enjoyed by the incumbent. They note for instance that the confederation of Passenger Transport in the UK (the major lobbyist for bus operators, see http://www.cptuk.org/) believes that the incumbent operator always has an advantage over the entrant though knowledge and experience, resources (staff), infrastructure, and reputation.

New strategic behaviours are emphasized as well. Some of them are related to the idea of combining competitive services and subsidized concessions allocated to operators though competitive tendering. In particular, [START_REF] Langridge | Contestability in the UK bus industry? The National Bus Company, and the Tilling Mark II effect[END_REF] note that entrants could minimize barriers related to lower knowledge and experience by entering from a contiguous market in which they had already gained some knowledge and experience and/or entering a local bus market on a small scale, which could be achieved by obtaining contracts with the local authority. iv At the same time, many incumbents are eager to enter into the new quality partnerships with local authorities, even if this entails supporting high costs of investment in new vehicles and related infrastructure. This suggests that they are looking for long term partnerships through the creation of local monopolies.

Finally, as suggested by [START_REF] Van Der Veer | Entry deterrence and quality provision in the local bus market[END_REF], under entry threats, the incumbent may run more buses and increase the frequency of the service (compared to a situation where it is protected from entry) to avoid leaving profitable gaps. [START_REF] Wang | A game theoretic analysis of competition in a deregulated bus market[END_REF] corroborate these findings; they suggest that deterrence through an increase of the service level is a dominant strategy for an incumbent under various market conditions, which in turn explains the high levels of service in many industries. Accommodation occurs mostly on routes where demand is high. Blockaded entry occurs on routes where demand is low.

Competition in price or frequency

Early theoretical models on bus competition have usually been based on strong assumptions which were in most cases unrealistic: (See [START_REF] Evans | A Theoretical Comparison of Competition with other Economic Regimes for Bus Services[END_REF][START_REF] Preston | Regulation, competition and market structure: A literature review of the stage bus industry[END_REF], for a survey.) Moreover, it has been suggested that service quality matters and is therefore a key factor in bus competition. (See Dodgson et al. 1992 and 1993, Dodgson and Katsoulacos 1988, Bly and Oldfield 1986, and Glaister 1985 and 1986. v ) In particular, minibuses have been considered as relevant actors in theoretical frameworks with quality differentiation, where competition can be implemented on a horizontal perspective where firms compete in fixed time schedules and prices. Regular buses were thought as cheap and slow services, while minibuses were associated with lower travel time and higher prices. These different assumptions have been, to various degrees, criticized later on. The most important criticisms have been related to the assumptions of quality differences and price competition. [START_REF] Preston | Regulation, competition and market structure: A literature review of the stage bus industry[END_REF] suggests that consumers have difficulties in perceiving quality differences. Moreover consumers' loyalty to a particular firm seems to be unrealistic: Users usually board the first bus that arrives. A model's outcome of two firms offering distinct qualities of service and charging different fares has not been as common as might be expected. vi Such a model of competition would probably be more relevant in explaining inter-modal competition. Thus, it seems to have been accepted that competition has tended to take the form of service wars with fares matching. Passengers board the first bus that arrives, hence making frequency the key factor for competition. Competition in fares has been mainly restricted to branded ticketing such as system passes, return ticketing, multi-rider tickets or discount vouchers; branded ticketing is thus seen as a tool for operators to increase the consumer's incentives to be loyal to one specific company.-it is an attempt by operators to develop strategic barriers to entry. (See Fernández and Muñoz, 2007.) Later on, many authors, such as [START_REF] Van Reeven | Stable service patterns in scheduled transport competition[END_REF] and [START_REF] Wang | A game theoretic analysis of competition in a deregulated bus market[END_REF] have confirmed these early intuitions. Price competition (and therefore price reduction) is not particularly prevalent in the bus industry. vii Operators have limited scope for meaningful product differentiation that could make consumers loyal. However, on long distance services such as intercity bus services, consumers' loyalty and price competition (through higher services quality) are more relevant. In this case, quality matters, and ticket prices constitute an important fraction of the generalized price paid by consumers. Hence, product differentiation on long-distance routes makes entrants resistant to pricing and scheduling responses of incumbent operators. Scheduling competition is more stable in this case.

Random schedules

The previous section suggests that competition mostly takes the form of frequency wars. Analysts then go a step further when they explain that the arrival time of a bus at a stop is random. [START_REF] Oldale | Local bus deregulation and timetable inability[END_REF], [START_REF] Ellis | British Bus Deregulation: Competition and Demand Coordination[END_REF], [START_REF] Van Reeven | Stable service patterns in scheduled transport competition[END_REF] and [START_REF] Gomez-Lobo | Why Competition does not Work in Urban Bus Markets: Some New Wheels for Some Old Ideas[END_REF] all agree on the fact that the incentives for price competition are smaller than what was expected, even if more than one operator is present on a local transport market.

Two main reasons explain this result: First, users do not particularly care for quality difference, and second, they incur a cost if they want to shop around for the lowest priced bus. Contrary to [START_REF] Evans | A Theoretical Comparison of Competition with other Economic Regimes for Bus Services[END_REF] which assumes that operators' services are scheduled, these authors consider some degree of uncertainty surrounding arrival times at bus stops. In their model, users arrive at a stop and will wait for the arrival of the next bus; an important assumption is that the distribution of passengers across time is uniform, i.e., there are no masses of passengers clustered around departure points. The optimal reaction of the bus operators consists then of randomizing arrival schedules at the bus stop, and setting the highest possible prices.

Given that consumers do not differentiate one bus company from another, random frequencies have to be expected for the following reasons: Some buses may bunch together or some may be alone at a given position in time and space. In the first case, each operator has an incentive to drive just in front of the others. Thus, bunching cannot be an equilibrium. A profile where each bus is alone in a position cannot be an equilibrium either, since buses have an incentive to fall back and drive just in front of the next bus that is following behind. These techniques are known as head running and leapfrogging. viii Hence, randomizing the arrival at a bus stop is the best strategy for each operator competing on the same route, and this forces the rivals to guess the arrival time of their competitors. A striking example is the case of Manchester in the UK, where the first two years of deregulation were characterized by services changing between 1500 to 2000 times annually. Bus companies cannot credibly provide timetable information. In these conditions, competition does not guarantee low prices.

A research agenda

The economic literature proposes a number of arguments as to why bus competition might be limited in liberalized industries. Several reasons could explain such a situation: First, the technology used in the industry favours large and experienced operators and therefore impedes the entry of new competitors on an equivalent scale. Second, the fact that the transportation service occurs on short distances restricts the incentives of the consumers to look for the cheapest operator and/or the company offering the highest quality standards.

Price competition is therefore likely to be very limited, even on routes where more than one operator is present. Note that, where there is no regulation providing incentives to bus companies to comply with the time schedules, there is no guarantee of a proper coordination of consumers at bus stops, which again limits the scope for competition.

As pointed out above, most of the contributions drawn from the economic literature are theoretically derived, although based on experts' knowledge and experience. Although these theoretical arguments are intuitive and convincing, they often wait to be empirically validated. There are potential avenues of investigation.

First, one should test the contestability of the market. It is well known that, if a technology exhibits increasing returns to scale, then the associated industry is highly concentrated or is operated by a single firm. In this case, the presence of too many production units prevents the efficient size of the industry being reached, which could be socially costly. Without entering into the details of this theory, this result invites us to evaluate the level of economies of scale and scope, which is usually performed by means of the estimation of cost functions. To do so one can use data at the depot or firm level. Now, the question of the adequate level of disaggregation to measure output in a network industry is still open and subject to research. However, if estimating firms' cost functions is crucial to characterize the economic fundamentals of the bus industry, recall that such an exercise is not immune from other constraints that the firms are facing, like the regulatory conditions. For instance, the local regulators may impose different quality targets, or the driving conditions may vary from one urban network to another, and this may explain cost differences across local areas.

Second, another set of evidence supporting the non constestability of the bus industry can be achieved though the detection of predatory pricing: Following [START_REF] Motta | Competition Policy, Theory and Practice[END_REF], predatory pricing implies that the incumbent sets low prices for a period and sacrifices short-run profits, so that the entrant believes that positive profits cannot be obtained. When the entrant leaves the market, the incumbent then increases prices and reaches high profits again, which in the long run outweigh possible losses incurred by foreclosing entry. Note that observing that entry occurs is not enough to conclude that the market is competitive or that there are no predatory practices. To properly detect predation, prices should be compared to marginal and average costs. Following Motta, a test of predation could be implemented as follows. First, from the estimated cost function, we can evaluate total and marginal costs. Second, actual prices must be compare to these estimated costs: i) If the price is above total average costs, then the presumption is that the firms are not taking predatory actions; ii) if the price is below total average costs but above marginal costs, then predation should not be presumed, but the burden of proof is on the side of the competition authority; iii) If the price is below marginal costs, then there is a case for predation. Again note however that these tests should not be applied without taking into account the regulatory and competition constraints. Indeed regulation of prices and services or competition from other transport modes could clearly affect the pricing strategies of bus companies.

Third, one should test the theoretical prediction that, companies do not compete on prices at the route level in the short run. There are at least two ways to test this assertion.

There is a direct approach that consists of estimating a structural model of the industry that comprises a demand function and a pricing equation. This approach could be implemented using models specified along the line of the econometrics of differentiated products markets. (See [START_REF] Davis | Quantitative Techniques for Competition and Antitrust Analysis[END_REF], for a presentation of these models.) There is also an indirect approach that it is easier to carry out. It is indirect in the sense that it tests a necessary condition not a sufficient condition, namely that the number of firms on the market has no effect on the price level. This approach relies on the structure-conductperformance paradigm which states that the structure of a market determines the operators' pricing conduct and therefore their profitability. The ability to obtain significant profits is inversely related to the number of firms and/or their market share, and thus is positively correlated with concentration. If operators do not compete on price, as suggested by the economic literature, a non-significant long-lasting relationship between the price and the variable measuring the degree of presence of firms on the market should be obtained. Note that one should similarly test for the impact of the market structure on frequencies.

The research agenda is thus particularly rich. It is also urgent as the effectiveness of competition in the bus industry becomes a crucial issue in many countries over the world. Wunsch, P., 1996, "Cost and Productivity of Major Urban Transit Systems in Europe: An exploratory Analysis", Journal of transport Economics and Policy,30,[171][172][173][174][175][176][177][178][179][180][181][182][183][184][185][186]



  All operators face the same costs;  All operators and passengers have complete information about services and fares;  Operators have information about demand;  Each passenger has a preferred departure time and is indifferent between immediate backwards and forward rescheduling;  Traffic conditions are such that journey times are the same throughout the day;  Departure times and fares of other operators are fixed.


D H Whalen
email: whalen@haskins.yale.edu

Sara Giulivi

Hosung Nam

Andrea G Levitt

Pierre Hallé

Louis M Goldstein

Biomechanically Preferred Consonant-Vowel Combinations Fail to Appear in Adult Lexicons and Spoken Corpora

Keywords: phonotactics, word frequency, type/token, Articulatory Phonology, Frame

.

Plausible biomechanical explanations have been proposed, but it is also possible that infants are mirroring the frequency of the CVs that they hear. As noted, previous assessments of adult language were based on dictionaries; these "type" counts are incommensurate with the babbling measures, which are necessarily "token" counts.

We analyzed the tokens in two spoken corpora for English, two for French and one for Mandarin. We found that the adult spoken CV preferences correlated with the type counts for Mandarin and French, not for English. Correlations between the adult spoken corpora and the babbling results had all three possible outcomes: significantly positive (French), uncorrelated (Mandarin), and significantly negative (English).

There were no correlations of the dictionary data with the babbling results when we consider all nine combinations of consonants and vowels. The results indicate that spoken frequencies of CV combinations can differ from dictionary (type) counts and that the CV preferences apparent in babbling are biomechanically driven and can ignore the frequencies of CVs in the ambient spoken language.

INTRODUCTION

Although the sound systems of the world's languages show a great deal of diversity, they also exhibit some underlying commonalities. Comparisons of phonological inventories have shown that consonants at three places of articulation (labial, coronal and velar) are nearly universal, as are vowels at three degrees of fronting (front, central, back) [START_REF] Jakobson | Child language, aphasia and phonological universals[END_REF][START_REF] Lindblom | Phonetic universals in vowel systems[END_REF][START_REF] Maddieson | Patterns of sounds[END_REF][START_REF] Maddieson | Phonetic universals[END_REF]. Such universal tendencies have been attributed variously to linguistic features [START_REF] Jakobson | Child language, aphasia and phonological universals[END_REF], listener constraints [START_REF] Lindblom | Explaining phonetic variation: A sketch of the H&H theory[END_REF], or coupled articulatory and auditory factors [START_REF] Honda | Organization of tongue articulation for vowels[END_REF], but determining the source of these universal tendencies remains an active area of investigation.

Phonotactic combinations of consonants and vowels have also been found to show some universal tendencies. One surprising pattern was first observed by MacNeilage and Davis [START_REF] Davis | The articulatory basis of babbling[END_REF][START_REF] Macneilage | The frame/content theory of evolution of speech production[END_REF][START_REF] Macneilage | Acquisition of speech production: Frames, then content[END_REF]) in infant babbling. When they plotted the incidence of each of the three consonant types against the three vowel types, they found three preferred patterns of associations between adjacent consonants and vowels, as measured by ratios over 1.0 for the observed versus expected combinations. Labial consonants cooccurred with central vowels more often than would be expected by chance, as did coronal consonants with front vowels and dorsal consonants with back vowels.

Infants explore a broad range of phonetic possibilities during babbling, so finding such regularities is unexpected, even though babblers do exhibit some universal tendencies as well, such as the use of point vowels ([i a u]) and pulmonic egressive stops (e.g., [START_REF] Oller | The emergence of the speech capacity[END_REF][START_REF] Vihman | From babbling to speech: A re-assessment of the continuity issue[END_REF].

Because the CV combination patterns are observed before anything resembling true language is in evidence, biomechanical explanations seem plausible. MacNeilage and Davis proposed the 'Frame then Content' (F/C) account in which jaw movement plays the primary role, (see also MacNeilage & Davis, in press), while an account within Articulatory Phonology (Giulivi, Whalen, Goldstein, Nam, & Levitt, in press; Whalen, Giulivi, Goldstein, Nam, & Levitt, in press) proposes that synergy among articulators is the foundation.

Although a biomechanical explanation is possible for these CV preferences, there is a simpler explanation of their occurrence that has not yet been ruled out:

Infants could be sensitive to the presence of such patterns in adult speech and therefore incorporate them into their babbling. The ambient language is known to influence some aspects of babbling even very early stages [START_REF] Boysson-Bardies | A crosslinguistic investigation of vowel formants in babbling[END_REF][START_REF] Whalen | Intonational differences between the reduplicative babbling of French-and English-learning infants[END_REF], so perhaps this pattern reflects only such an influence. Indeed, two studies have shown that the three preferred co-occurrence patterns are also somewhat preferred in adult languages. [START_REF] Maddieson | Syllable structure and phonetic models[END_REF] studied type frequencies (dictionary counts) in five languages with small phoneme inventories (Hawaiian, Rotokas, Pirahã, Eastern Kadazan and Shipibo). They did not find any direct evidence of preference for articulatory "convenience" (p. 55), i.e., synergy; however, their raw numbers ("vowel deviation scores") are consistent with the ratios found in babbling. [START_REF] Macneilage | The motor core of speech: A comparison of serial organization patterns in infants and languages[END_REF] found that the CV syllables preferred in babbling were also preferred in dictionary counts of 10 languages, namely English, Estonian, French, German, Hebrew, Japanese, New Zealand Maori, Quichua, Spanish and Swahili. They found that the ratios of observed to expected frequencies are generally greater than 1.0 for the CV types that are favored in babbling and early speech: labial consonant-central vowel (in 7 languages out of 10), coronal consonant-front vowel (in 7 languages out of 10) and dorsal consonant-back vowel (in 8 languages out of 10). [START_REF] Rousset | Structures syllabiques et lexicales des langues du monde: Données, typologies, tendances universelles et contraintes substantielles[END_REF] found similar results for 15 languages: 11, 10 and 11 out of 15 for labial, coronal and velar CVs respectively (p. 194). Thus there is some reason to think that the adult language might be providing a template for the CV preference pattern in babbling.

The frequency with which an infant will hear certain syllables, however, is based on the frequency of words in the language (a "token" count) rather than on the total number of syllables within the words of a language (a "type" count). Some frequent words in English, for example, have preferred combinations ("but", "said", "go"), but many do not ("be", "do", "get"). Thus the frequency of the CVs that the infant hears could differ between dictionaries and spoken corpora.

Infants pay special attention to speech that is directed at them with modifications from typical adult-directed language (e.g., [START_REF] Fernald | Four-month-old infants prefer to listen to motherese[END_REF][START_REF] Werker | A cross-language investigation of infant preference for infant-directed communication[END_REF], but they are exposed to more adult-directed speech than they are to infant-directed speech (ref). There are, to our knowledge, no large corpora of infant-directed talk. Therefore, the best estimate we could obtain of the CV ratios in the infant's language environment was to be found in the analysis of adult spoken language corpora.

The one previous study of language corpora that we have found was less supportive of the role of preferred combinations in adult speech. Written data from five languages (Finish, Turkish, Latin, Latvian and Setswana) were studied by [START_REF] Janson | Cross-Linguistic Trends in the Frequency of CV Sequences[END_REF]. He found support in each language for a dental/alveolar place for the C with front vowels and velars with back vowels, both in accord with the babbling results. Labials, however, also preferentially patterned with back vowels. Most of the corpora were relatively small (Latin: 11,803 CVs,Turkish: 33,135 CVs;Latvian: 16,665;Setswana: 20,325), while one was substantial (Finnish: 125,126 CVs). All sources were written texts as well, which could introduce differences from the spoken language. Janson's results must therefore be taken as an important first step but insufficient to provide a clear answer to the question of how CVs pattern in adult spoken languages.

Although the F/C account is meant only to describe the pattern of the preferred combinations (those on the diagonal in a 3x3 table of place of articulation for the consonant and the vowel), one further aspect of CV combinations is that there is systematicity in the less preferred (off diagonal) combinations as well. Modeling results show that this pattern in the off-diagonals is well-predicted by AP, which can calculate the synergy between any two phones, but not F/C (Nam, Giulivi, Goldstein, Levitt, & Whalen, submitted). The dictionary counts have, to date, examined only the diagonal, so we will expand the investigation to the off-diagonals here.

In order to test whether spoken adult language would provide a model for the CV preferences within babbling, we examined CV combinations in spoken corpora for the three languages used in our own study of babbling: English, French and Mandarin [START_REF] Giulivi | Vowels and consonants favored co-occurrences in language development[END_REF]Giulivi, et al., in press). The adult spoken corpora we examined are larger than those used in Janson's study, which should provide a more stable indication of the existence of CV combination preferences in token counts of adult language. In addition to our spoken corpus count, we also compare dictionary counts for English and Mandarin, to allow us to study the patterning of all nine CV combinations rather than just the three preferred patterns.

TESTING FAVORED CV CO-OCCURRENCES IN TWO SPOKEN ENGLISH CORPORA

The primary measure that has been used to demonstrate the CV combination preferences is a ratio of the observed CV combinations to the expected number based on the overall occurrence of C and V independently (e.g., [START_REF] Davis | The articulatory basis of babbling[END_REF]. In previous work, all vowels were classified as Front, Central or Back, and stop and nasal consonants were classified as Labial, Coronal or Velar. Fricatives were excluded from the babbling studies due to their low rate of occurrence, as were the liquids /r, l/. The glides /w, j/ were included in their counts, with /w/ counting as a labial. To maintain comparability, the same criteria were used with the adult corpora.

Method

The consonants that were included were the stops, nasals and glides (i.e. excluding fricatives and liquids) and were grouped, according to place of articulation, into labials (p, b, m, w), coronals (t, d, n, j) and velars (k, g, ŋ). Vowels were grouped with reference to the front-back dimension of the vocal tract, into central (ø, ´, a), front (i, ɪ, e, ɛ, ae) and back (u, ʊ, o, ɔ).

The first English corpus was the Switchboard database of English spoken by 542 different talkers [START_REF] Godfrey | Switchboard-1 Release 2. from Linguistic Data Consortium[END_REF]. Each speaker produced short monologues in response to one of 70 prompts on various topics (e.g., taxes, pets, movies). This resulted in 3,068,137 words available in the transcriptions, with 1,474,728 CV syllables that met the criteria for inclusion here.

The second English corpus was the Buckeye database [START_REF] Pitt | Buckeye corpus of conversational speech (2nd release)[END_REF], containing recordings and transcriptions of interviews with 40 American English speakers of different ages from a single dialect area (Central Ohio). A total of about 300,000 words were transcribed, resulting in 143,756 CV syllables that met our criteria.

Results

Table 1 presents the ratios for the Switchboard corpus, and Table 2, for the Buckeye corpus. In both cases, only one of the three diagonals is greater than 1. This finding is quite different from the previously reported results for both babbling and English type (dictionary) counts.

The diagonals have been shown to represent only a portion of the systematicity present in the data (Nam, et al., submitted). A fuller account can be achieved with a Pearson correlation between all nine cells of one measure with another. With such an analysis, the results from the two spoken English corpora are quite similar; a correlation of the ratios gives an r of .96 (p < .001). The correlations with the original ratios from the babbling data [START_REF] Davis | The articulatory basis of babbling[END_REF], on the other hand, are not only small in magnitude but negative as well (r = -0.23 for the Switchboard, -.11 for the Buckeye, neither significant). The correlations with the English babblers of our own study (Giulivi, et al., in press) are also negative and large enough to be significant (r = -0.77 for the Switchboard, -0.70 for the Buckeye, p < .05 for both). Because the previously reported dictionary counts included only the diagonal values, not the off-diagonals, we recalculated dictionary counts from two sources so that we could explore all nine cells of the CV combinations with those of the dictionary data. The first was the Shorter Oxford English Dictionary (SOED) as made available in the MRC Psycholinguistics Database [START_REF] Coltheart | The MRC psycholinguistic database[END_REF], the same one used by [START_REF] Macneilage | The motor core of speech: A comparison of serial organization patterns in infants and languages[END_REF]. It contains phonetic transcriptions for 38,420 words (58,862 CV syllables used for our analysis). The second dictionary was the Carnegie Mellon University (CMU) Pronouncing Dictionary (CMU_Speech_Group, 2007), which contains over 112,131 words for our search, yielding 133,797 CV syllables for analysis. Tables 3 and4 present the ratios of observed to expected occurrences, presented as before. of English (CMU database).

English--Switchboard

English--SOED

The ratios are similar to those reported by MacNeilage et al. (2000). There are discrepancies in the absolute magnitudes for the SOED, even though that was their source as well as ours. We do not have an explanation for this difference. The correlation of the nine ratios from the two dictionaries was good (r = 0.89, p < .01),

showing that the difference in size and dialect had little effect on the ratios.

Although the diagonals show general agreement between the babbling (token)

and dictionary (type) counts, the correlation of all nine cells does not. Neither dictionary was significantly correlated with either the MacNeilage and Davis data (r = 0.02 (SOED) and 0.24 (CMU)) or with our English results (r = -0.09 (SOED) and 0.05 (CMU)).

The correlations between the nine ratios from the spoken corpora to the corresponding dictionary-based ones were not significant. Neither spoken corpus was significantly correlated with the SOED (r = 0.40 for both), with similar results for the CMU (r = 0.37 for the Switchboard corpus, r = 0.31 for the Buckeye, n.s.). These low correlations indicate that the frequencies of certain words affect the rate at which the combinations preferred in babbling appear in adult speech. Notably, such words as "because" and "got" (velar-central), "you" and "know" (coronal-back), and "we" and "well" (labial-front) are quite common and occur in non-preferred cells.

Discussion

The pattern of preferred CV combinations in English babbling is significantly different from the CV combination preferences in the ambient spoken language. This finding suggests that universal tendencies have a stronger influence on babbling than does the language environment.

In addition, the ratios obtained from spoken English (token counts) and those obtained from dictionaries (type counts) were not significantly correlated, and it is only the dictionary counts that provide substantial evidence in support of the preferred combinations in babbling. Thus, even for the relatively weak comparison of the diagonal ratios, only one of the three CV combinations preferred in babbling was preferred in the adult token counts (the labial-central combination). This result stands in contrast to the diagonals of the type counts, which were generally supportive of the babbling pattern, as previously reported (2 of 3 diagonals for the SOED, all three for the CMU dictionary).

On our more stringent test of comparing all nine cells of the spoken corpora with those of the dictionary and babbling results (thus including secondary regularities), the correlations were either nonsignificant or, in two cases, negatively correlated. Thus it appears that the frequency of usage in spoken English fails to reflect the slight tendency for word types to exhibit the preferred combinations.

CV FAVORED CO-OCCURRENCES IN TWO FRENCH CORPORA

Another of the languages in our babbling study was French (Giulivi, et al., in press), so a further comparison was made with two corpora of spoken French.

Method

The first corpus was that of [START_REF] Tubach | Un corpus de transcription phonetique (300.000 phones[END_REF]. They aggregated three corpora: one of presentations and discussions at a conference, one of conversations between educated adults, and one of sixteen brief conversations among adults, which yielded 3,265 CVs for analysis. The second corpus was the Lexique 3 database, version 3.2 (c.f. New, [START_REF] Nam | Computational simulation of CV combination preferences in babbling[END_REF]. Although a large part of this dataset is based on written texts, we analyzed the segments that were taken from subtitles for films. These were thus scripted dialogs, but were, in general, attempts at replicating spoken language. That this is a reasonable approximation to spoken language is supported by a recent study for Dutch, in which it was found that frequencies based on subtitles were, in fact, capable of explaining more variance of lexical decision reaction times than counts based on published texts [START_REF] Keuleers | SUBTLEX-NL: A new measure for Dutch word frequency based on film subtitles[END_REF]. Here, the same criteria for classifying the syllables were used as before, and ratios were calculated in the same way. 184,012 syllables were available for analysis.

Results

Table 5 shows the results for the Tubach and Boë data. Here, the diagonals are in good agreement with the preferences in babbling; indeed, they are noticeably larger than those found in babbling. The correlations for the full set of nine cells were calculated for the two French corpora and the babbling data. The corpora correlated significantly with each other (r = 0.83, p < .01), as did each with the MacNeilage and Davis babbling data (r = 0.72 and 0.76, p < .05, for the Tubach/Boë and Lexique, respectively). The Tubach/Boë also correlated with our babblers in the French environment (r = 0.78, p < .05), but the Lexique did not (r = 0.58, n.s.). Neither corpus correlated significantly with our babbling data from the other two environments (English and Mandarin).

French-Spoken, Token counts

In order to have a full set of ratios for a type count i , we analyzed the French subtitles by type rather than token (Table 7). This is similar to a dictionary count, but applies to the same material as that of the token count. Again, the values on the diagonals are greater than 1.0, demonstrating the same pattern of preferences observed in the babbling data, but of a greater numerical magnitude. The Tubach/Boë ratios were marginally correlated with this type count (r = 0.66, p = 0.055), and the Lexique token ratios were significantly correlated with the type counts from that same data (r = 0.87, p < .01). This last correlation is no doubt higher than would be expected to obtain with a type count based on an unrelated set of texts.

French-Subtitles

Discussion

The ratios for the French film subtitles are somewhat higher along the diagonal than those in the babbling data (Giulivi, et al., in press) and in the French dictionary data (1.2, 1.6 and 1.6 for the diagonals; [START_REF] Macneilage | The motor core of speech: A comparison of serial organization patterns in infants and languages[END_REF]. They are thus in general accord with the previously reported results. The correlations with the babbling results (MacNeilage and Davis's and our French-learning infants) are positive. In isolation, we would have taken this finding as evidence of early attunement to the language environment in babbling, especially because the correlations between the spoken corpora and our English-learning infants were not significant. Set against the English results, however, it appears that this correlation is due to a greater agreement between spoken French and the universal pattern than any direct attunement. Adding a third case, here, Mandarin, will help elucidate this issue.

CV FAVORED CO-OCCURRENCES IN A MANDARIN CORPUS

The final language in our babbling study was Mandarin (Giulivi, et al., in press), so a comparison was made with a corpus of spoken Mandarin. Because previous dictionary counts did not include Mandarin, we provide a count based on a searchable internet dictionary of Mandarin (CC-CEDICT, 1997).

Method

The Chinese Annotated Spontaneous Speech (CASS) version 3.2 [START_REF] Li | CASS: A phonetically transcribed corpus of Mandarin spontaneous speech[END_REF] database was used here. It was designed to explore the phonetic realization of Mandarin words in spontaneous speech and contains narrow transcriptions of 3 hours of speech from 7 talkers and includes 50,782 syllables. As before, those syllables with fricatives, affricates and liquids for onsets were excluded, leaving 24,766 syllables.

The consonants were classified as labial (b, p, m, w), coronal (t d n y) or velar (k g).

The vowels were classified as front (i, ei, ü, ia, ie, iao, iu), central (a, ao, e) or back (o, ou, u, ua, uo, uai, ui). Note that the /i/ or /u/ onglide in some of the vowel combinations is treated as a vowel by the phonotactics of Mandarin, not as a consonant; we thus classified those as vowels.

Our comparison for the dictionary count was taken from the on-line Chinese-English Dictionary -(CEDICT). For this, all 84,303 syllables with pinyin transcriptions were examined. Those syllables with fricatives, affricates and liquids for onsets were excluded, leaving 32,715 syllables. The same criteria as CASS corpus were used for CV classification.

Results

Table 8 shows the results for the spoken corpus of Mandarin. The diagonals resulted in two that were greater than 1 (the coronal-front and velar-back combinations). The labial-central ratio was well below 1. Thus there is limited evidence of the preferred syllables in spoken Mandarin. The correlations between the Mandarin token ratios and the babbling ratios were not significant (r = 0.04 for the MacNeilage/Davis data, -0.08 for our Mandarinlearning infants). (Neither of the correlations with our other two language groups was significant.)

Mandarin--Spoken

Table 9 shows the results for the dictionary of Mandarin. Here again, two of the three values on the diagonals were greater than 1, indicating some evidence for the preferred combinations in lexical counts. In this case, the labial-central value was relatively close to 1. The pattern of ratios for the spoken (token) corpus correlated with the dictionary (type) ratios (r = 0.73, p < .05). As could be expected, then, the dictionary ratios did not correlate with the babbling results (r = 0.03, n.s., for MacNeilage/Davis, -0.36, n.s., for our Mandarin-learning infants).

Mandarin--Dictionary

Discussion

Two of three diagonal ratios were greater than 1 in both Mandarin spoken corpora, whereas the other was less than 1. Thus there is limited support for the persistence of CV preferences into adult spoken language for Mandarin. However, the ratios for Mandarin were much more similar between the dictionary and spoken corpora than were English and French. This pattern stands in contrast to the marginal results for French and the lack of a correlation for English.

Neither the type nor token ratios from adult Mandarin correlated with the babbling results.

GENERAL DISCUSSION

Previous work had found that the CV combinations that are preferred in babbling are similar to those combinations as they occur in type (dictionary) counts for a variety of languages. In the present study, we instead focused on spoken corpora as being more representative of the language that infants would hear. The patterns of preferred CV combinations were similar for babbling and spoken corpora only for French. For Mandarin, there was no correlation, and there was a negative correlation for English. This indicates that the CV preferences are likely to have a biomechanical source since they appear in all these language, that is, regardless of the frequency of the CVs in the input to the infant. In addition, we found that the previously reported presence of similar preferences for these three combinations in dictionary (type) counts for various languages [START_REF] Macneilage | The motor core of speech: A comparison of serial organization patterns in infants and languages[END_REF] was supported to the same extent (10 out of 12 comparisons) in the type counts examined. However, the connection to the babbling data was less consistent. The correlation between all nine CV combinations of the babbling (token) and dictionary (type) counts was not significant for any of the three languages (English, French and Mandarin), even though simulations of the Articulatory Phonology (AP) biomechanical account had been found to correlate with all nine cells (Nam, et al., submitted).

The likelihood that there is a biomechanical source for the CV preferences in babbling leads to the expectation that the pattern will be universal. Although the CV preferences have been found in most of the languages that have been investigated for this pattern (English, French, Mandarin (though cf. [START_REF] Chen | Consonant-vowel co-occurrence patterns in Mandarin-learning infants[END_REF], Italian, [START_REF] Zmarich | The frequency of consonants and vowels and their co-occurrences in the babbling and early speech Italian children Proceedings of the 15th International Congress of Phonetic Sciences[END_REF], Korean [START_REF] Lee | Frame Dominance" and the serial organization of babbling, and first words in Korean-learning infants[END_REF]), there have been reports that Dutch babbling does not follow the pattern [START_REF] Schauwers | The characteristics of prelexical babbling after cochlear implantation between 5 and 20 months of age[END_REF][START_REF] Zink | De verwerving van de klankproductie tijdens de brabbelperiode bij vier Vlaamse kinderen[END_REF]. These studies found that children in Dutch-speaking environments had the expected preference for coronal/front combinations, but that labials occurred more often with back vowels. Velar consonants did not have cooccur with any vowel class at greater than chance levels. Part of the discrepancy is that the vowel in utterances such as "papa" was counted as back [START_REF] Zink | De verwerving van de klankproductie tijdens de brabbelperiode bij vier Vlaamse kinderen[END_REF], even though particular instances might have been central. Another part of the discrepancy is that individual ratios were tested for significance. The labial/central combination for the normal hearing group of [START_REF] Schauwers | The characteristics of prelexical babbling after cochlear implantation between 5 and 20 months of age[END_REF], for example, gives a ratio of 1.05; this is above 1, as expected, but it is not significantly so by their measure. The velar/back combination, on the other hand, has a (nonsignificant) ratio of 0.8, making Dutch at best one of the languages that has two out of three preferred combinations. In short, there is too little data to be confident that the pattern is universal, but the plausibility of the AP biomechanical account and the appearance of the pattern despite contrary frequencies in the ambient language input (English and Korean) leads to the expectation that most languages will conform to the pattern.

The appearance of the preferred CVs in the adult dictionaries remains unexplained. The F/C account does not predict the adult pattern, and it indeed posits a second mechanism. [START_REF] Macneilage | The motor core of speech: A comparison of serial organization patterns in infants and languages[END_REF] assert that the maintenance of the CV preferences in adult language "probably means that the patterns have been present

since the origin of speech in hominids" (p. 158). Similarly, [START_REF] Davis | Acquisition of serial complexity in speech production. A comparison of phonetic and phonological approaches to first word production[END_REF] state, "The common occurrence of these results in infants, languages and the proto-word corpus suggests a fundamental status for these CV co-occurrence constraints in languages" (p. 94). Both of these explanations rely on continuity between babbling and language, whereas the emergence into adult speech is claimed to occur when the infant "escapes frame dominance" [START_REF] Davis | The articulatory basis of babbling[END_REF]:1208). The persistence of the CV preferences in adult language is, then, unexpected. Even the more successful biomechanical account, AP, does not necessarily predict that CV synergy will be represented in the dictionary. Overall usage of words is clearly not dictated by these synergies, as seen in the English spoken corpora. Adults have greater control over their articulators than infants do, and thus they are not as subject to the preferences that arise biomechanically from the combinations of certain gestures. We might expect, however, that there would be circumstances in which such small preferences could have an effect, such as in speeded production of nonsense words or judgments of which string of Cs and Vs would make a better word.

Such small effect may be more powerful in rare words, leading to an overall preference for the synergistic CV combinations in the dictionary as a whole. Still, if a language's lexicon did not exhibit these preferences, we would not take that as evidence that the synergies were different in that language, only that they did not happen to influence the lexicon.

The relationship between the type and token counts for the adult language is weakly positive: The Mandarin corpus was positively correlated, while the French and English corpora were uncorrelated. The English results, in particular, seem to be due to a sizable number of very frequent words that contain non-favored combinations.

The Mandarin correlation may also be due to a design decision that was made to maintain comparability with the babbling results. For babbling, fricatives and affricates were excluded from the C counts due to their low frequency. Given that Mandarin has a phonological process of affrication of alveolars before high front vowels (Chao, 1968:21), this resulted in a great many exclusions from what would have been the preferred coronal-front combination. It may be that including those would change the pattern of type and token frequencies so that they, like the French and English, would be uncorrelated.

In summary, we have found evidence that the preferences found for certain CV combinations emerge in babbling even if the adult language does not show the same pattern. This indicates that the previously proposed biomechanical explanations are necessary, because the infants are not always imitating the input. The persistence of these CV preferences in adult lexicons remains to be explained, given that adult spoken frequencies often depart from those patterns.

Table 1 .

 1 Ratios of observed to expected frequencies generated for spoken English (Switchboard database).

	C \\ V	Front	Central	Back
	Coronal	0.83	0.87	1.51
	Labial	1.25	1.12	0.41
	Velar	0.90	1.11	0.94

Table 2 .

 2 Ratios of observed to expected frequencies generated for spoken English (Buckeye database).

Table 3 .

 3 Ratios of observed to expected frequencies generated for a dictionary of English (SOED database).

	C \\ V	Front	Central	Back
	Coronal	1.07	0.90	1.04
	Labial	1.04	0.99	0.89
	Velar	0.74	1.28	1.08
		English--CMU
	C \\ V	Front	Central	Back
	Coronal	1.08	0.92	1.02
	Labial	1.01	1.03	0.88
	Velar	0.80	1.12	1.17

Table 4 .

 4 Ratios of observed to expected frequencies generated for a dictionary

Table 5 .

 5 Ratios of observed to expected frequencies generated for a corpus of French lectures and conversations, token count.Table6shows the results for the French subtitles. Similarly to Table5, the diagonals are noticeably larger than those found in babbling.

	C \\ V	Front	Central	Back
	Coronal	1.21	0.70	0.79
	Labial	0.74	1.61	0.85
	Velar	0.96	0.57	1.86
		French-Subtitles, Token counts
	C \\ V	Front	Central	Back
	Coronal	1.27	0.42	0.91
	Labial	0.61	1.59	1.17
	Velar	0.57	1.00	1.75

Table 6 .

 6 Ratios

of observed to expected frequencies generated for a corpus of French subtitles in films (Lexique 3 database), token count.

Table 7 .

 7 Ratios

	, Type counts

of observed to expected frequencies generated for a corpus of French subtitles in films (Lexique 3 database), type count.

Table 8 .

 8 Ratios of observed to expected frequencies generated for a corpus of spoken Mandarin (CASS database).

	C \\ V	Front	Central	Back
	Coronal	1.30	0.96	0.79
	Labial	1.02	0.86	1.28
	Velar	0.09	1.35	1.15

Table 9 .

 9 Ratios of observed to expected frequencies generated for a dictionary of Mandarin (CEDICT database).

	C \\ V	Front	Central	Back
	Coronal	1.17	1.03	0.81
	Labial	1.25	0.94	0.83
	Velar	0.01	1.05	1.87

This work was supported by NIH grants DC-000403 and DC-002717 to Haskins Laboratories. We thank Aude Noiray, Stephen Frost, Barbara L. Davis and an anonymous reviewer for helpful comments.