Using odometer readings to assess VKT changes associated with a voluntary travel behaviour change program

In order to detect changes in daily average vehicle kilometres travelled (VKT) induced by a large-scale TravelSmart intervention in Melbourne, a panel of households was asked to complete before and after surveys, which included week-long odometer readings. In contrast to results reported from previous TravelSmart applications, the Melbourne program did not induce a statistically significant change in the average daily VKT when measured 1 year after the intervention. Multiple regressions revealed that the variability in change in VKT was better explained by socio-demographic variables than by the TravelSmart treatment. The change in VKT was also found to be strongly negatively correlated with the average daily vehicle kilometres recorded in the before survey--indicating the possibility of the 'regression-to-the-mean' effect well known in the road safety literature. The conditions under which the regression-to-the-mean effect may create the illusion of a positive TravelSmart program impact on the reduction in daily average VKT are examined. It is concluded that, in the context of voluntary travel behaviour change evaluations, greater attention should be paid to instrument reactivity arising from the impact of the before travel survey on TravelSmart uptake and/or on change in VKT, and to regression-to-the-mean effects.