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  • Journal article
    Buddhavarapu P, Bansal P, Prozzi JA, 2021,

    A new spatial count data model with time-varying parameters

    , TRANSPORTATION RESEARCH PART B-METHODOLOGICAL, Vol: 150, Pages: 566-586, ISSN: 0191-2615
  • Journal article
    Ait Bihi Ouali L, Musuuga D, Graham D, 2021,

    Quantifying responses to changes in the jurisdiction of a congestion charge: a study of the London western extension

    , PLoS One, Vol: 16, ISSN: 1932-6203

    This paper quantifies behavioural responses to changes in the jurisdiction of a congestion charge, with a successive focus on (i) an extension and (ii) a reduction in the size of the charging zone. We exploit the unanticipated nature of both the implementation and removal of London’s Western Expansion Zone (WEZ) as quasi-natural experiments to test whether individual responses to policies are asymmetric. We use the UK Department of Transport Annual Average Daily Flow (AADF) data, which records traffic flows for seven transport modes (including cars, buses, bicycles, heavy and light goods vehicles). Using a difference-in-differences approach, we find that the introduction of the WEZ led to a 4.9% decline in road traffic flows in the new congestion charge area. These results are robust to different model specifications. HGVs traffic did not significantly change post-WEZ, which indicates that their road demand is price inelastic. The removal of the WEZ led to no significant variations in traffic. This result indicates asymmetry in behaviour with persistent changes in post-intervention traffic demand levels.

  • Journal article
    Kutela B, Langa N, Mwende S, Kidando E, Kitali AE, Bansal Pet al., 2021,

    A text mining approach to elicit public perception of bike-sharing systems

    , TRAVEL BEHAVIOUR AND SOCIETY, Vol: 24, Pages: 113-123, ISSN: 2214-367X
  • Journal article
    Kazemzadeh K, Bansal P, 2021,

    Electric bike navigation comfort in pedestrian crowds

  • Journal article
    Xuto P, Anderson R, Graham D, Horcher Det al., 2021,

    Optimal infrastructure reinvestment in urban rail systems: A dynamic supply optimisation approach

    , Transportation Research Part A: Policy and Practice, Vol: 147, Pages: 251-268, ISSN: 0191-2607

    The state of infrastructure in many developed countries around the world is an increasingly pressing issue, with mounting costs the longer repairs are deferred. In today’s rapidly urbanising world, the urban rail network is particularly critical, since infrastructure failures can have severe economic consequences for both the operator’s finances and user time costs. This paper thus provides a system-level model of welfare-oriented supply optimisation that integrates asset management with the literature on optimal pricing and capacity provision. Using a simulation approach and calibrating with London Underground data, this paper delivers three key contributions. First, the economic efficiency of long-term capital planning is highlighted, with up to an 87% welfare gain when comparing a 40- versus 5-year planning horizon. Second, in general, the longer the planning horizon, the higher the annual welfare, demand, asset condition, fare and supply, in the steady-state. Third, the analysis explores why policies in reality diverge from the welfare optimum: we show that election cycles can have a detrimental effect, with increased asset neglect and volatility in spending. Economic efficiency improves in the short-term at the expense of the long-term; significant intervention is needed to break this downwards trend, as also reflected in various rail systems’ histories.

  • Journal article
    Ait Bihi Ouali L, Graham D, 2021,

    The impact of the MeToo scandal on women’s perceptions of security

    , Transportation Research Part A: Policy and Practice, Vol: 147, Pages: 269-269
  • Journal article
    Bansal P, Krueger R, Graham DJ, 2021,

    Fast Bayesian estimation of spatial count data models

    , Computational Statistics & Data Analysis, Vol: 157, Pages: 1-19, ISSN: 0167-9473

    Spatial count data models are used to explain and predict the frequency of phenomena such as traffic accidents in geographically distinct entities such as census tracts or road segments. These models are typically estimated using Bayesian Markov chain Monte Carlo (MCMC) simulation methods, which, however, are computationally expensive and do not scale well to large datasets. Variational Bayes (VB), a method from machine learning, addresses the shortcomings of MCMC by casting Bayesian estimation as an optimisation problem instead of a simulation problem. Considering all these advantages of VB, a VB method is derived for posterior inference in negative binomial models with unobserved parameter heterogeneity and spatial dependence. Pólya-Gamma augmentation is used to deal with the non-conjugacy of the negative binomial likelihood and an integrated non-factorised specification of the variational distribution is adopted to capture posterior dependencies. The benefits of the proposed approach are demonstrated in a Monte Carlo study and an empirical application on estimating youth pedestrian injury counts in census tracts of New York City. The VB approach is around 45 to 50 times faster than MCMC on a regular eight-core processor in a simulation and an empirical study, while offering similar estimation and predictive accuracy. Conditional on the availability of computational resources, the embarrassingly parallel architecture of the proposed VB method can be exploited to further accelerate its estimation by up to 20 times.

  • Journal article
    Bansal P, Dua R, Krueger R, Graham DJet al., 2021,

    Fuel economy valuation and preferences of Indian two-wheeler buyers

    , JOURNAL OF CLEANER PRODUCTION, Vol: 294, ISSN: 0959-6526
  • Journal article
    Vickerman R, 2021,

    Will Covid-19 put the public back in public transport? A UK perspective

    , TRANSPORT POLICY, Vol: 103, Pages: 95-102, ISSN: 0967-070X
  • Journal article
    Hörcher D, Tirachini A, 2021,

    A review of public transport economics

    , Economics of Transportation, Vol: 25, Pages: 1-34, ISSN: 2212-0122

    Public transport provision requires substantial organisational efforts, careful planning, financial contributions from the public, and coordination between millions of passengers and staff members in large systems. Efficient resource allocation is critical in its daily operations. Therefore, public transport has been among the most popular subjects in transport economics since the infancy of this discipline. This paper presents an overview of the literature developed over the past half century, including more than 300 important contributions. With a strong methodological orientation, it collects, classifies, and compares the frequently used analytical modelling techniques, thus providing a cookbook for future research and learning efforts. We discuss key findings on optimal capacity provision, pricing, cost recovery and subsidies, externalities, private operations, public service regulation, and cross-cutting subjects, such as interlinks with urban economics, political economy, and emerging mobility technologies.

  • Journal article
    Chauhan V, Gupta A, Parida M, Gupta Aet al., 2021,

    Demystifying service quality of Multimodal Transportation Hub (MMTH) through measuring users’ satisfaction of public transport

    , Transport Policy, Vol: 102, Pages: 47-60, ISSN: 0967-070X
  • Journal article
    Li H, Zhu M, Graham DJ, Ren Get al., 2021,

    Evaluating the speed camera sites selection criteria in the UK

    , Journal of Safety Research, Vol: 76, Pages: 90-100, ISSN: 0022-4375

    Introduction: Speed cameras have been implemented to improve road safety over recent decades in the UK. Although the safety impacts of the speed camera have been estimated thoroughly, the criteria for selecting camera sites have rarely been studied. This paper evaluates the current speed camera sites selection criteria in the UK based on safety performance. Method: A total of 332 speed cameras and 2,513 control sites with road traffic accident data are observed from 2002 to 2010. Propensity score matching method and empirical Bayes method are employed and compared to estimate the safety effects of speed cameras under different scenarios. Results: First, the main characteristics of speed cameras meeting and not meeting the selection criteria are identified. The results indicate that the proximity to school zones and residential neighborhoods, as well as population density, are the main considerations when selecting speed camera sites. Then the official criteria used for selecting camera sites are evaluated, including site length (a stretch of road that has a fixed speed camera or has had one in the past), previous accident history, and risk value (a numerical scale of the risk level). The results suggest that a site length of 500 m should be used to achieve the optimum safety effects of speed cameras. Furthermore, speed cameras are most effective in reducing crashes when the requirement of minimum number of historical killed and seriously injured collisions (KSIs) is met. In terms of the risk value, it is found that the speed cameras can obtain optimal effectiveness with a risk value greater than or equal to 30, rather than the recommended risk value of 22.

  • Journal article
    Graham D, Carbo J, 2020,

    Quantifying the impacts of air transportation on economic productivity: a quasi-experimental causal analysis

    , Economics of Transportation, Vol: 24, ISSN: 2212-0122

    Air transport capacity expansions are often justified on the grounds that they will improve economic perfor-mance and induce growth. Such causal impacts are hard to identify empirically due to the fundamentally endogenous nature of the relationship between air transport and the economy. This paper contributes to the empirical literature on aviation-economy effects by conducting a case study of the impacts of air transportation activity on productivity in Chinese provinces. For exogenous variation we exploit a policy scenario created by the 2003 deregulation of the Chinese aviation sector, which was applied in all provinces of China except Beijing and Tibet. We find that this policy intervention resulted in substantial growth in air transport passengers and cargo. We estimate the causal effect of air transport on productivity by comparing GDP per employee in Tibet relative to a synthetic control region affected by the deregulation policy. We find a significant positive productivity effect from aviation expansion following the 2003 deregulation. Use of a differences-in-differences specification con-firms this result

  • Book chapter
    Graham DJ, 2021,

    Causal Inference for Ex Post Evaluation of Transport Interventions

    , International Encyclopedia of Transportation: Volume 1-7, Pages: 283-290, ISBN: 9780081026717

    This chapter reviews statistical methods for ex post evaluation that can be used to quantify the causal effects of transport interventions. It introduces the potential outcome model for causal inference as a framework within which to conceptualize ex post evaluation, defines its key elements, and explains the theoretical principles underpinning techniques for treatment effect estimation. The aim of these techniques is to quantify changes that have occurred due to explicit intervention (or “treatment”) net of other effects, in nonexperimental settings with nonrandomly assigned treatments. The chapter argues that these conditions tend to characterize ex post analyses of transport interventions and that the key issue for successful evaluation involve identifying and adjusting for consequent sources of bias in estimation. Contemporary causal inference approaches achieve this either through model-based adjustment or by exploiting sources of exogenous variance that are orthogonal to the treatment. The chapter reviews a number of such approaches that have featured in the recent literature and that hold potential for practical application.

  • Journal article
    Ma L, Graham DJ, Stettler MEJ, 2021,

    Air quality impacts of new public transport provision: A causal analysis of the Jubilee Line Extension in London

    , Atmospheric Environment, Vol: 245, Pages: 118025-118025, ISSN: 1352-2310

    Public transport is commonly associated with benefits such as reducing road traffic congestion and improving air quality. This paper focuses on evaluating the causal impact of a new public transport provision in London, the Jubilee Line Extension (JLE) in 1999, on air quality. Using meteorological normalisation and a regression discontinuity design with time as the forcing variable, we show that the JLE led to only small changes in air pollution at some specific locations; detectable changes in NOx, NO2, and O3 concentrations were found at 63%, 43% and 29% of air pollution monitoring sites, respectively. For those sites where a change in pollution was detected, the responses ranged from −2% to +1% for NO2 and -1% to 0% for O3. We calculate that the long-run effects are greater, ranging from −11% to +3% for NO2 and from −2% to +2% for O3 at sites that showed a response to the JLE. Aggregating across all sites in London for a city-wide effect, both short and long-run effects were less than 1% or insignificant. We find statistically significant increases in NO2 and O3 concentrations at some background sites, but the magnitude of effect is within +1% in the short-run and +3% in the long-run. Our analysis shows that the effect of the JLE on air pollution in some areas was greater than others, however across London the effect was small and this indicates that public transport provision on its own is not an effective strategy to improve air quality.

  • Journal article
    Singh R, Graham DJ, Anderson RJ, 2020,

    Quantifying the effects of passenger-level heterogeneity on transit journey times

    , Data-Centric Engineering, Vol: 1, Pages: e15-1-e15-28, ISSN: 2632-6736

    In this paper we apply flexible data-driven analysis methods on large scale mass transit data to identify areas for improvement in the engineering and operation of urban rail systems. Specifically, we use data from automated fare collection (AFC) and automated vehicle location (AVL) systems to obtain a more precise characterisation of the drivers of journey time variance on the London Underground, and thus an improved understanding of delay. Total journey times are decomposed via a probabilistic assignment algorithm and semiparametric regression is undertaken to disentangle the effects of passenger-specific travel characteristics from network related factors. For total journey times, we find that network characteristics, primarily train speeds and headways, represent the majority of journey time variance. However, within the typically twice as onerous access and egress time components, passenger-level heterogeneity is more influential. On average, we find that intra-passenger heterogeneity represents 6% and 19% of variance in access and egress times, respectively, and that inter-passenger effects have a similar or greater degree of influence than static network characteristics. The analysis shows that while network-specific characteristics are the primary drivers journey time variance in absolute terms, a non-trivial proportion of passenger-perceived variance would be influenced by passenger-specific characteristics. The findings have potential applications related to improving the understanding of passenger movements within stations, for example, the analysis can be used to assess the relative way-finding complexity of stations, which can in turn guide transit operators in the targeting of potential interventions.

  • Journal article
    Anupriya, Graham DJ, Bansal P, Hörcher D, Anderson Ret al., 2020,

    Congestion in near capacity metro operations: optimum boardings and alightings at bottleneck stations

    During peak hours, metro systems often operate at high service frequencies totransport large volumes of passengers. However, the punctuality of suchoperations can be severely impacted by a vicious circle of passenger congestionand train delays. In particular, high volumes of passenger boardings andalightings may lead to increased dwell times at stations, that may eventuallycause queuing of trains in upstream. Such stations act as active bottlenecks inthe metro network and congestion may propagate from these bottlenecks to theentire network. Thus, understanding the mechanism that drives passengercongestion at these bottleneck stations is crucial to develop informed controlstrategies, such as control of inflow of passengers entering these stations. Tothis end, we conduct the first station-level econometric analysis to estimate acausal relationship between boarding-alighting movements and train flow usingdata from entry/exit gates and train movement data of the Mass Transit Railway,Hong Kong. We adopt a Bayesian non-parametric spline-based regression approachand apply instrumental variables estimation to control for confounding biasthat may occur due to unobserved characteristics of metro operations. Throughthe results of the empirical study, we identify bottleneck stations and provideestimates of optimum passenger movements per train and service frequencies atthe bottleneck stations. These estimates, along with real data on daily demand,could assist metro operators in devising station-level control strategies.

  • Journal article
    Bansal P, Liu Y, Daziano R, Samaranayake Set al., 2020,

    Impact of discerning reliability preferences of riders on the demand for mobility-on-demand services

    , Transportation Letters, Vol: 12, Pages: 677-681, ISSN: 1942-7867
  • Journal article
    Anupriya A, Graham DJ, Horcher D, Anderson R, Bansal Pet al., 2020,

    Quantifying the ex-post causal impact of differential pricing on commuter trip scheduling in Hong Kong

    , Transportation Research Part A: Policy and Practice, Vol: 141, Pages: 16-34, ISSN: 0191-2607

    This paper quantifies the causal impact of differential pricing on the trip-scheduling of regular commuters using the Mass Transit Railway (MTR) in Hong Kong. It does so by applying a difference-in-difference (DID) method to large scale smart card data before and after the introduction of the Early Bird Discount (EBD) pricing intervention. We find statistically significant but small effects of the EBD in the form of earlier departure times. Leveraging the granularity of the data, we also allow for the treatment effect to vary over observed travel characteristics. Our empirical results suggest that fares and crowding are the key determinants of commuter responsiveness to the EBD policy.

  • Journal article
    Li H, Wu D, Graham DJ, Sze NNet al., 2020,

    Comparison of exposure in pedestrian crash analyses: A study based on zonal origin-destination survey data

    , SAFETY SCIENCE, Vol: 131, ISSN: 0925-7535
  • Journal article
    Horcher D, Graham D, 2020,

    The Gini index of demand imbalances in public transport

    , Transportation, Vol: 48, Pages: 2521-2544, ISSN: 0049-4488

    The paper studies a general bidirectional public transport line along which demand varies by line section. The length of line sections also varies, and therefore their contribution to aggregate (line-level) user and operational costs might be different, even if demand levels were uniform. The paper proposes the Gini index as a measure of demand imbalances in public transport. We run a series of numerical simulations with randomised demand patterns, and derive the socially optimal fare, frequency and vehicle size variables in each case. We show that the Gini coefficient is a surprisingly good predictor of all three attributes of optimal supply. These results remain robust with inelastic as well as elastic demand, at various levels of aggregate demand intensity. In addition, we find that lines facing severe demand imbalances generate higher operational cost and require more public subsidies under socially optimal supply, controlling for the scale of operations. The results shed light on the bias introduced by the assumption of homogeneous demand in several existing public transport models.

  • Journal article
    Lu Q, Tettamanti T, Hörcher D, Varga Iet al., 2020,

    The impact of autonomous vehicles on urban traffic network capacity: an experimental analysis by microscopic traffic simulation

    , Transportation Letters, Vol: 12, Pages: 540-549, ISSN: 1942-7867
  • Conference paper
    Chen K, Wang S, Mao J, 2020,

    Travel Time Prediction for Multi-Airport Systems Via Multiclass Queuing Networks

    , 2020 Integrated Communications Navigation and Surveillance Conference (ICNS), Publisher: IEEE
  • Journal article
    Krueger R, Bansal P, Buddhavarapu P, 2020,

    A new spatial count data model with Bayesian additive regression trees for accident hot spot identification

  • Journal article
    Anupriya, Graham DJ, Carbo JM, Anderson RJ, Bansal Pet al., 2020,

    Understanding the costs of urban rail transport operations

    , Transportation Research Part B: Methodological, Vol: 138, Pages: 292-316, ISSN: 0191-2615

    There is considerable variation in the average cost of operations across urban rail transport (or metro) systems. Since metros are typically owned and operated by public authorities, there is a public interest case in understanding the key drivers of their operational costs. This paper estimates short-run cost functions for metro operations using a unique panel dataset from twenty-four metro systems around the world. We use a flexible translog specification and apply dynamic panel generalised method of moments (DPGMM) estimation to control for confounding from observed and unobserved characteristics of metro operations. Our empirical results show that metro systems with a high density of usage are the most cost-efficient. We also find that operational costs fall as metro size increases. These results have important implications for the economic appraisal of metro systems.

  • Journal article
    Hörcher D, De Borger B, Graham DJ, 2020,

    Benefit Spillovers and Subsidy Exporting in Inter-Regional Public Transport Provision

  • Journal article
    Ait Bihi Ouali L, Carbo JM, Graham D, 2020,

    Do changes in air transportation affect productivity? A cross country panel approach

    , Regional Science Policy and Practice, Vol: 12, Pages: 493-505, ISSN: 1757-7802

    This paper quantifies the economic impact of air transportation worldwide using two panel data methods to assess the effect of air cargo and air passenger volumes on GDP per employee (aggregate labour productivity). Fixed effects methods and instrumental variables allow us to tackle endogeneity concerns and simultaneity biases. We first use a generalized method of moments specification (GMM) on a World Bank panel dataset containing information for all countries worldwide, separated into 264 areas over the period 1990‐2017. Results show that a 10% increase in air passengers is associated with a 0.6% increase in GDP per employee. Complementary instrumental variables estimates indicate a slight negative bias in this result, yielding an effect of 0.86%. Results are very similar for different parts of the world, with elasticity estimates ranging between 0.01 and 0.04, except in North Africa and Middle Eastern countries, where effects on labour productivity are found to be insignificant. Overall, air passenger traffic has a stronger and more positive effect on GDP per employee than air cargo. We conduct a complementary analysis at the European level using Eurostat data (NUTS2) and perform an analysis on over 300 European sub‐regions. Results indicate that air transport has a positive, stronger and more significant effect on GDP per employee than air cargo, with a 10% increase in air passengers being associated with a labour productivity increase of 3.2%.

  • Journal article
    Ouali LAB, Graham DJ, Barron A, Trompet Met al., 2020,

    Gender Differences in the Perception of Safety in Public Transport

    , Journal of the Royal Statistical Society Series A: Statistics in Society, Vol: 183, Pages: 737-769, ISSN: 0964-1998

    <jats:title>Summary</jats:title><jats:p>Concerns over women's safety on public transport systems are commonly reported in the media. We develop statistical models to test for gender differences in the perception of safety and satisfaction on urban metros and buses by using large-scale unique customer satisfaction data for 28 world cities over the period 2009–2018. Results indicate a significant gender gap in the perception of safety, with women being 10% more likely than men to feel unsafe in metros (6% for buses). This gender gap is larger for safety than for overall satisfaction (3% in metros and 2.5% in buses), which is consistent with safety being one dimension of overall satisfaction. Results are stable across specifications and robust to inclusion of city level and time controls. We find heterogeneous responses by sociodemographic characteristics. Data indicate that 45% of women feel secure in trains and metro stations (and 55% in buses). Thus the gender gap encompasses more differences in transport perception between men and women rather than an intrinsic network fear. Additional models test for the influence of metro characteristics on perceived safety levels and find that more acts of violence, larger carriages and emptier vehicles decrease women's feeling of safety.</jats:p>

  • Journal article
    Ait Bihi Ouali L, Graham D, Trompet M, Barron Aet al., 2020,

    Gender differences in the perception of safety in public transport

    , Journal of the Royal Statistical Society Series A: Statistics in Society, Vol: 183, Pages: 737-769, ISSN: 0964-1998

    Concerns over women's safety on public transport systems are commonly reported in the media. In this paper we develop statistical models to test for gender differences in the perception of safety and satisfaction on urban metros and buses using large-scale unique customer satisfaction data for 28 world cities over the period 2009 to 2018. Results indicate a significant gender gap in the perception of safety, with women being 10\% more likely than men to feel unsafe in metros (6% for buses). This gender gap is larger for safety than for overall satisfaction (3% in metros and 2.5% in buses), which is consistent with safety being one dimension of overall satisfaction. Results are stable across specifications and robust to inclusion of city-level and time controls. We find heterogeneous responses by sociodemographic characteristics. Data indicates 45% of women feel secure in trains and metro stations (respectively 55% in buses). Thus the gender gap encompasses more differences in transport perception between men and women rather than an intrinsic network fear. Additional models test for the influence of metro characteristics on perceived safety levels and find that that more acts of violence, larger carriages, and emptier vehicles decrease women's feeling of safety.

  • Journal article
    Horcher D, Graham DJ, 2020,

    MaaS economics: Should we fight car ownership with subscriptions to alternative modes?

    , Economics of Transportation, Vol: 22, ISSN: 2212-0122

    Proponents of the Mobility as a Service concept claim that subscriptions to alternative modes can effectively reduce car ownership and the adverse effects of underpriced car use. We test this hypothesis in a microeconomic model with endogenous mode choice as well as car and subscription ownership. The model contains congestible urban rail and car sharing options as substitutes of underpriced private car use. We find that aggregate car ownership is not a reliable proxy for road congestion: subscriptions may reduce car ownership while increasing the vehicle miles travelled by remaining car owners. Subscriptions induce welfare losses for two reasons. First, pass holders overconsume the alternative modes, as the marginal fare they face drops to zero. Second, non-pass holders tend to shift to car use due to the crowding induced by pass holders, causing additional distortions. We illustrate numerically that differentiated pricing is more efficient in achieving the goals of MaaS.

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