Temporal Variability of Car Usage as an Input to the Design of Before & After Surveys

Before & After surveys are a common method of measuring the effect of specific policies and projects designed to cause changes in travel behaviour. The purpose of this paper is to consider some issues involved in the design of Before & After surveys required for the evaluation of projects designed to change travel behaviour. To do this, it was important to obtain a quantitative understanding of the underlying variability of the parameters to be measured (in particular, the variation over time in travel by car). This was obtained by a detailed analysis of the MobiDrive data from Germany, and the estimation of Coefficients of Variation in key travel parameters for the Melbourne situation. Several features emerged from this analysis. Firstly, larger sample sizes are generally required to detect changes in either distance travelled or travel time than in trips undertaken. Secondly, larger sample sizes are required to detect changes from repeated cross-sectional surveys than from a panel survey. Thirdly, larger sample sizes are required to detect changes when using a daily travel diary compared to using a weekly travel diary (although this difference can be substantially reduced in a panel survey by maintaining the same day of the week for each household in later waves of the panel). Finally, larger sample sizes are required to detect changes from person travel data than from household travel data. Traded off against these sample size advantages, however, is the fact that some of the design parameters enabling smaller sample sizes also make the survey more difficult to conduct.