Estimating the Functional Form of Road Traffic Maturity

It has been observed that older high traffic motorways experience lower traffic growth than newer ones (ceteris paribus). This phenomenon is known as traffic maturity; however, it is not captured through traditional time-series long-term forecasts, due to constant elasticity to GDP these models assume. In this paper we argue that traffic maturity results from decreasing marginal utility of transport. The elasticity of individual mobility with respect to the revenue tends to decrease when the level of mobility increases. In order to find evidences of decreasing elasticity we analyse a cross-section time-series sample including 40 French motorways' sections. This analysis shows that decreasing elasticity can be observed in the long term. We then propose a decreasing function for the traffic elasticity with respect to the economic growth, which depends on the traffic level on the road. This model provides a good explanation for the observed traffic evolution and gives a rigorous econometric approach to time-series traffic forecasts.