There is strong interest from policy-makers in the measures that might be applied to reduce or rationalise the use of private cars, in order to alleviate such problems as congestion and poor urban air quality. However, this requires some prior assessment of the likely impact on car travel demand associated with each policy option. For example, what is the likely effect of road pricing on the number and distance of trips? Such effects can be quantified as 'elasticities', defining the percentage change in a response variable (such as vehicle kilometres driven) to a 1% change in an independent variable (such as travel cost per kilometre).
The aim of the TRACE project was to provide elasticities that can be used for a first order assessment of the effects of changes in car travel time and car travel cost (including parking charges) on car travel demand and on demand for other modes, for a range of contexts.
TRACE reviewed the available evidence concerning elasticities of private car travel demand, and supplemented this with calculations using national and regional traffic models. The findings were summarised in an Elasticity Handbook, aimed at national and regional authorities in Europe. In addition, a software tool (called TRACER) was developed to provide elasticities for new contexts not covered by existing traffic models. This contains a databank with many thousands of elasticity values (impacts on trips and kilometres, with segmentation by mode, trip purpose, distance class, urbanisation, parking class and public transport quality).
Elasticities are provided for both the short and long term. The long term is defined as including the option to change the choice of destination, and not just the mode or distance travelled. Elasticities are defined in terms of three policy variables: the fuel price for cars, travel time by car and car parking charges. Consequently, all other polices to be evaluated have first to be translated in terms of one of these three variables. For example, speed limits can be simulated in terms of their effect on average travel time.
TRACE found that almost all elasticities have been derived using some kind of modelling - there are few values that result from direct before and after observation.
A 10% change in car travel time was found to have a bigger impact on trips and kilometres travelled than a 10% change in the cost of car travel. The short-term elasticities of distance travelled by car are around 50% of the values for the long run.
The elasticities presented by TRACE were derived from a finite number of simulations. Assessments based on these elasticities must therefore be considered as first order approximations in discussions on policy options. When a potentially successful policy has been identified using the elasticity approach, it will often be necessary to conduct further studies to take account of specific local circumstances. Nevertheless, the outputs from TRACE provide an effective and user-friendly screening tool.