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).