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ANNEXE 3 NOTE ON DATA REQUIREMENTS FOR COST CALCULATION
Extract from Nash, C.A and Samson, T. (1999) Calculating Transport Congestion and Scarcity Costs. Final Report of the Expert Advisors to the High Level Group on Infrastructure Charging (Working Group 2). ITS, University of Leeds.
Wherever possible, external road congestion costs should be estimated from a model which simulates the interaction of demand and supply on the road network. The model can then be used to approximate the marginal external costs of congestion by rerunning it with small changes in traffic volumes, and examining the effects on journey time for existing traffic. This model would ideally incorporate a detailed network description, with both speed/flow relationships and junction delays, and allow for user behaviour in terms of rerouting, retiming, changing destination or mode or changing frequency of travel, in order to obtain a new set of flows and journey times following imposition of a charge. Data is therefore required on the base O/D matrix, base generalised costs and responses to changes in these values. The calculation of generalised cost requires knowledge of operating costs, values of time and vehicle occupancy rates. Only when the charge is equal to the marginal external cost in this new position has the optimal level of charge and traffic been found.
Where this is not possible, we recommend that calculations are undertaken for typical inter urban or rural roads at alternative traffic levels and mixes of types of vehicle using link speed/flow relationships. Separate calculations will be needed according to the type of road (number of lanes; motorway or conventional road). Again, data on base traffic flows and generalised costs are needed, and traffic volumes should again be adjusted for the introduction of charges, if necessary by means of a simple price elasticity of demand, in order to obtain an equilibrium value.
For urban areas, the degree of interaction between roads means that such an approximation will be particularly crude. If a full network model is not available, the use of area speed/flow relationships relating to the entire network for central, inner and outer urban areas is likely to be preferable to link based speed/flow relationships.
Forecasting the impact of increased traffic on unreliability is more difficult, but given the importance of the issue it should be attempted wherever possible. A variety of approaches exists, including the use of micro-simulation models which model individual vehicles and can thus estimate the spread of journey times, and purely empirical approaches, which require data on unreliability and on traffic flows for a set of roads over time.
All the above relationships should relate to local conditions in the area concerned, and relate to conditions such as driving styles and typical speeds in that location. It would be counter-productive therefore to attempt to specify Europe-wide relationships, although results may with care be transferred from comparable situations elsewhere in Europe if local information is not available.
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