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Costs of Congestion: Literature Based Review of Methodologies and Analytical Approaches



1.1 This review lies within research associated with the Scottish Executive's high level transport objective which has a focus on promoting economic growth by enhancing the effectiveness of the transport network and reducing congestion. Congestion is seen as having significant impacts on a number of sectors including the environment and economy as a whole and therefore has an increasing prominence on the political agenda. Whilst an increasing amount of research and literature is emerging with respect to tackling congestion (including the potential for economic instruments such as road pricing and the benefits of 'packages of measures'), less evidence is available on the full costs of congestion.

1.2 The primary objective of the work has been to conduct an international literature review on the costs of traffic congestion, providing a comprehensive list of sources and reflecting evidence on how costs are distributed. This has included the follow sub tasks:

  • To describe congestion within the traffic situation in Scotland
  • To review definitions of congestion and how it has been measured in past research and practice
  • To describe and assess the different methods used to measure the costs of congestion
  • To outline the literature concerning the link between economic growth and congestion ('decoupling')

1.3 In terms of the scope of the work, the main emphasis has been on the second and third tasks, with the first and last providing context to the findings.

1.4 The review covers both the interurban and urban road contexts. The Executive is responsible for the management of the inter-urban trunk road network and Local Authorities for the remainder of the network. As 'city regions', centred on Scotland's congested urban areas, are increasingly being viewed as a mechanism for promoting economic growth, the cost of congestion in urban areas is important to the objectives of this research. As a result, the evidence on measurement and costs of urban congestion has been included within the scope of the review. The research has concentrated on literature that has been produced in the past 5 years with some key pieces of evidence extending back around 10 years. It has not been the intention to review the methods adopted to reduce levels of congestion, the literature on packaging of measures or evidence on barriers to the implementation of economic and other transport measures. These are all very relevant issues if the full costs of congestion are to be taken at some future date into marginal social cost pricing schemes, either alone or in policy packages. Their inclusion would, however, require a much more extensive piece of work than is envisaged within this project. Whilst a key aspect to the review has been the methodology used in deriving the costs, the scope has been confined to describing the methodologies and any reported advantages and disadvantages, but not to generate recommendations or guidance on which should be used.

1.5 In terms of the structure of the report, following the executive summary an overview of the evidence of congestion in areas of Scotland is given in chapter 2. This is followed in chapter 3 by a summary of the different ways in which congestion is defined in the literature and perceived by users. In chapter 3, quantified measures of congestion are given using evidence from the international literature. Following a short background to the question of measuring the costs of congestion in chapter 4, a more detailed elaboration of the three main approaches (marginal cost, total cost and 'cost of excess burden' is given in chapters 5 and 6 of the report. Finally an outline of research into the issue of decoupling is described in chapter 7 with overall conclusions in chapter 8. Appendices have been included to allow a greater degree of detail on some sections of the findings. In addition to this report, a database of literature sources has been separately produced for the Scottish Executive.