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Journal articleAit 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 articleBansal P, Krueger R, Graham DJ, 2021,
Fast Bayesian estimation of spatial count data models
, Computational Statistics & Data Analysis, Vol: 157, Pages: 1-19, ISSN: 0167-9473Spatial 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.
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Journal articleBansal P, Dua R, Krueger R, et al., 2021,
Fuel economy valuation and preferences of Indian two-wheeler buyers
, JOURNAL OF CLEANER PRODUCTION, Vol: 294, ISSN: 0959-6526- Author Web Link
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- Citations: 9
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Journal articleHörcher D, Tirachini A, 2021,
A review of public transport economics
, Economics of Transportation, Vol: 25, Pages: 1-34, ISSN: 2212-0122Public 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.
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Journal articleLi H, Zhu M, Graham DJ, et al., 2021,
Evaluating the speed camera sites selection criteria in the UK
, Journal of Safety Research, Vol: 76, Pages: 90-100, ISSN: 0022-4375Introduction: 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.
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Journal articleGraham D, Carbo J, 2020,
Quantifying the impacts of air transportation on economic productivity: a quasi-experimental causal analysis
, Economics of Transportation, Vol: 24, ISSN: 2212-0122Air 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
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Book chapterGraham DJ, 2021,
Causal Inference for Ex Post Evaluation of Transport Interventions
, International Encyclopedia of Transportation: Volume 1-7, Pages: 283-290This 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.
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Journal articleMa 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: 1352-2310Public 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.
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Journal articleSingh 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: 2632-6736In 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.
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Journal articleAnupriya, Graham DJ, Bansal P, et 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.
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Journal articleAnupriya A, Graham DJ, Horcher D, et 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: 0191-2607This 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.
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Journal articleLi H, Wu D, Graham DJ, et 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- Author Web Link
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- Citations: 11
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Journal articleHorcher D, Graham D, 2020,
The Gini index of demand imbalances in public transport
, Transportation, Vol: 48, Pages: 2521-2544, ISSN: 0049-4488The 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.
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Journal articleLu Q, Tettamanti T, Hörcher D, et 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 -
Journal articleAnupriya, Graham DJ, Carbo JM, et al., 2020,
Understanding the costs of urban rail transport operations
, Transportation Research Part B: Methodological, Vol: 138, Pages: 292-316, ISSN: 0191-2615There 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.
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Journal articleHörcher D, De Borger B, Graham DJ, 2020,
Benefit Spillovers and Subsidy Exporting in Inter-Regional Public Transport Provision
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Journal articleAit 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: 1757-7802This 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%.
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Journal articleAit Bihi Ouali L, Graham D, Trompet M, et 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: 0964-1998Concerns 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.
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Journal articleOuali LAB, Graham DJ, Barron A, et 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>
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Journal articleHorcher D, Graham DJ, 2020,
MaaS economics: Should we fight car ownership with subscriptions to alternative modes?
, Economics of Transportation, Vol: 22, ISSN: 2212-0122Proponents 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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Journal articleLi H, Zhu M, Graham DJ, et al., 2020,
Are multiple speed cameras more effective than a single one? Causal analysis of the safety impacts of multiple speed cameras
, Accident Analysis and Prevention, Vol: 139, ISSN: 0001-4575Most previous studies investigate the safety effects of a single speed camera, ignoring the potential impacts from adjacent speed cameras. The mutual influence between two or even more adjacent speed cameras is a relevant attribute worth taking into account when evaluating the safety impacts of speed cameras. This paper investigates the safety effects of two or more speed cameras observed within a specific radius which are defined as multiple speed cameras. A total of 464 speed cameras at treated sites and 3119 control sites are observed and related to road traffic accident data from 1999 to 2007. The effects of multiple speed cameras are evaluated using pairwise comparisons between treatment units with different doses based on the propensity score methods. The spatial effect of multiple speed cameras is investigated by testing various radii. There are two major findings in this study. First, sites with multiple speed cameras perform better in reducing the absolute number of road accidents than those with a single camera. Second, speed camera sites are found to be most effective with a radius of 200 m. For a radius of 200 m and 300 m, the reduction in the personal injury collisions by multiple speed cameras are 21.4 % and 13.2 % more than a single camera. Our results also suggest that multiple speed cameras are effective within a small radius (200 m and 300 m).
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Journal articleSingh R, Graham DJ, Horcher D, et al., 2020,
Decomposing journey time variance on urban metro systems via semiparametric mixed methods
, Transportation Research Part C: Emerging Technologies, Vol: 114, Pages: 140-163, ISSN: 0968-090XThe availability of automated data for urban metro systems allows operators to accurately measure journey time reliability. However, there remains limited understanding of the causes of journey time variance and how journey time performance can be improved. In this paper, we present a semiparametric regression modelling framework to determine the underlying drivers of journey time variance in urban metro systems, using the London Underground as a case study. We merge train location and passenger trip data to decompose total journey times into three constituent parts: access times as passengers enter the system, on-train times, and egress times as passengers exit at their destinations. For each journey time component, we estimate non-linear functional relationships which we then use to derive elasticity estimates of journey times with respect to service supply and demand factors, including operational and physical characteristics of metros as well as passenger demand and passenger-specific travel characteristics. We find that the static fixed physical characteristics of stations and routes have the greatest influence on journey time, followed by train speeds, and headways, for which the average elasticities of total journey time are −0.54 and 0.05, respectively. The results of our analysis could inform operators about where potential interventions should be targeted in order to improve journey time performance.
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Journal articleZhang F, Graham DJ, 2020,
Air transport and economic growth: a review of the impact mechanism and causal relationships
, Transport Reviews, Vol: 40, Pages: 506-528, ISSN: 0144-1647The impacts of air transport on the economy arise both directly, via activity in the aviation sector; and indirectly, via increased spending and wider economic benefits associated with improved access to resources, markets, technology and economic mass. Economic activity, in turn, supports and generates demand for air transport. Despite its potential importance, the reciprocal nature of the causal relationship between air transport and economic performance has remained somewhat understudied. This paper provides a synthesis review of the channels the aviation sector interacts with regional economy. The review focuses on quantitative studies that contribute to the state-of-the-art understandings of the causality. We find that the reciprocal causal relationship is more likely to prevail in less developed economies. For more developed economies, only one direction of the causality is recognised, which runs from air transport to economic growth. Especially substantial is the effect of airline enplanement on service-related employment. The reverse direction of the relationship is, however, not as significant as believed in a causal sense within the developed world. Therefore, cautions need to be taken when applying income elasticities (such as the elasticity of air passenger demand with respect to GDP) in air travel demand forecasting, which implicitly assumes that economic growth causally leads to air traffic increment. Based on the fundamental links between air transport and economic growth, some typical imperfections and inefficiencies in aviation markets are discussed and promising avenues for future research are proposed.
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Journal articleMorse L, Trompet M, Barron A, et al., 2020,
A benchmarking framework for understanding bus performance in the U.S.
, Benchmarking: an international journal, Vol: 27, Pages: 1533-1550, ISSN: 1463-5771Purpose This paper describes a benchmarking framework applied to medium-sized urban public bus agencies in the United States which has overcome the challenges of data quality, comparability and understanding.Design/methodology/approach The benchmarking methodology described in this paper is based on lessons learned through seven years of development of a fixed route key performance indicator (KPI) system for the American Bus Benchmarking Group (ABBG). Founded in 2011, the ABBG is a group of public medium-sized urban bus agencies that compare performance and share best practices with peers throughout the United States. The methodology is adapted from the process used within international benchmarking groups facilitated by Imperial College and consists of four main elements: peer selection, KPI system development, processes to achieve high-quality data, and processes to understand relative performance and change.Findings The four main elements of the ABBG benchmarking methodology consist of eighteen sub-elements, which when applied overcome three main benchmarking challenges; comparability, data quality, and understanding. While serving as examples for the methodology elements, the paper provides specific insights into service characteristics and performance among ABBG agencies.Research limitations/implications The benchmarking approach described in this paper requires time and commitment and thus is most suitably applied to a concise group of agencies. Practical implications This methodology provides transit agencies, authorities and benchmarking practitioners a framework for effective benchmarking. It will lead to high-quality comparable data and a strong understanding of the performance context to serve as a basis for organizational changes, whether for policy, planning, operations, stakeholder communication, or program development. Originality/value The methodology, while consistent with recommendations from literature, is unique in its scale, in-depth validation
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Journal articleHörcher D, De Borger B, Seifu W, et al., 2020,
Public transport provision under agglomeration economies
, Regional Science and Urban Economics, Vol: 81, Pages: 1-19, ISSN: 0166-0462The purpose of this paper is to investigate, using both theoretical and numerical analysis, the impact of agglomeration externalities on short-run policy decisions in public transport, i.e. socially optimal pricing, frequency setting, and subsidisation. We develop a simple two-mode model in which commuters can opt for car or public transport use; car use leads to congestion, and public transport is subject to crowding. Allowing for agglomeration externalities, we show the following results. First, if car use is correctly priced for congestion, agglomeration benefits imply substantially lower public transport fares and higher frequencies. They neutralise to some extent the pressure to increase fares to correct the crowding externality. Second, as a consequence, agglomeration benefits justify low cost recovery ratios in public transport. Assuming an agglomeration elasticity of 1.04, a value well within the range of reported elasticities, numerical implementation of the model finds that cost-recovery ratios are 35.8% lower than in the absence of the productivity externality. Third, interestingly, the effect of agglomeration benefits on fares and frequency is much smaller if road use is exogenously under-priced. In this case, any modal shift induced by lower public transport fares has opposing agglomeration effects on the two modes, since agglomeration benefits are not mode-specific.
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Journal articleGraham D, Niak C, McCoy EJ, et al., 2019,
Do speed cameras reduce road traffic collisions?
, PLoS One, Vol: 14, ISSN: 1932-6203This paper quantifies the effect of speed cameras on road trafficcollisions using anapproximate Bayesian doubly-robust (DR) causal inference estimation method.Previous empirical work on this topic, which shows a diverse range ofestimatedeffects, is based largely on outcome regression (OR) models using the Empirical Bayesapproach or on simple before and after comparisons. Issues of causality andconfounding have received little formal attention. A causal DR approach combinespropensity score (PS) and OR models to give an average treatmenteffect (ATE)estimator that is consistent and asymptotically normal under correct specification ofeither of the two component models. We develop this approach withina novelapproximate Bayesian framework to derive posterior predictive distributions for theATE of speed cameras on road traffic collisions. Our results for England indicatesignificant reductions in the number of collisions at speed cameras sites (mean ATE =-15%). Our proposed method offers a promising approach for evaluation of transportsafety interventions.
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Conference paperAit Bihi Ouali L, Graham DJ, 2019,
Gender Differences in the Perception of Safety in Transport: The case of subways
, 8th hEART Conference -
Journal articleGraham D, Gibbons S, 2019,
Quantifying wider economic impacts of agglomeration for transport appraisal: existing evidence and future directions
, Economics of Transportation, Vol: 19, ISSN: 2212-0122This paper is concerned with the Wider Economic Impacts (WEIs) of transport improvements that arise via scale economies of agglomeration. It reviews the background theory and empirical evidence on agglomeration, explains the link between transport and agglomeration, and describes a three step procedure to appraise agglomeration impacts in a number of different settings. It includes new analytical work on measures of agglomeration and reports agglomeration-productivity elasticity estimates for the UK not previously published in the academic literature. The paper concludes with a set of recommendations for future empirical work on agglomeration and transport appraisal.
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Journal articleLi H, Graham DJ, Ding H, et al., 2019,
Comparison of empirical Bayes and propensity score methods for road safety evaluation: a simulation study
, Accident Analysis and Prevention, Vol: 129, Pages: 148-155, ISSN: 0001-4575Statistical evaluation of road safety interventions can be undertaken using a variety of different approaches, typically requiring different assumptions to obtain causal identification. In this paper, we conduct a simulation study to compare the performance of empirical Bayes (EB) and propensity score (PS) based methods, which have featured prominently in the recent literature, in settings with and without violation of key assumptions. The estimators considered include EB, inverse probability weighting (IPW), and Doubly Robust (DR) estimation. We find that while the EB approach has good finite sample properties when model assumptions are met, the consistency of this estimator is substantially diminished when the reference and treated sites follow different functions. The IPW estimator performs well in large samples, but requires a correctly specified PS model with sufficient overlap in covariate distributions between treated and control units. The DR estimator allows for violation of assumptions in either the regression or PS model, but not both. We find that this added level of robustness affords overall better performance than attained via EB or IPW estimation.
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Journal articleSingh R, Graham DJ, Anderson RJ, 2019,
Characterizing journey time performance on urban metro systems under varying operating conditions
, Transportation Research Record, Vol: 2673, Pages: 516-528, ISSN: 0361-1981Automated fare collection (AFC) data provide opportunities for improved measurement of public transport service quality from the passenger perspective. In this paper, AFC data from the London Underground are used to measure service quality through an analysis of journey time performance under regular and incident-affected operating conditions. The analysis involves two parts: (i) parametrically defining the shape of journey time distributions, and (ii) defining three performance metrics based on the moments of the distributions to measure the mean and variance of journey times. The metrics show that mean journey times are longest during the afternoon peak across all lines analyzed, and are more variable during the afternoon and off-peak periods depending on the line. Under incident conditions, mean journey times range from 8% to 39% longer compared with regular conditions, depending on the line. Overall, the main application of this work is that the metrics presented here can be directly applied by operators to quantify customer journey time performance, and can be further extended for industry-wide application to compare performance across metro networks.There has been increasing recognition in the transport industry of the need for performance metrics that capture journey time reliability from a passenger perspective as opposed to the traditional operator-oriented indicators. In a report for the Organisation for Economic Co-operation and Development (OECD) on service quality metrics used by metro operators, it is noted that the three most commonly reported metrics relating to journey time are train delay, wait times, and passenger journeys on-time (1). The first two metrics capture train performance from a schedule and headway adherence point of view. The third attempts to capture the experience of the user; however, it is recognized that operator-oriented indicators are rarely able to measure the true impact of passenger delay (2).The journey time distribution on
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