Is there a limit to car traffic growth ? Potential demand and convergence paths towards saturation

Car traffic growth has temporarily stopped during the 2000's, before coming back to growth after 2010, in relation with economic recovery and decreasing fuel prices. However, there are strong reasons to believe that car traffic growth is potentially limited, among which close completion of the diffusion process for car ownership, limited travel time budgets in relation with stable or declining travel speeds, decreasing marginal returns of additional car travel and infrastructure capacity restrictions. Representing car ownership as the result of an equilibrium between potential demand and economic constraints, and assuming additional car travel to be of decreasing marginal utility, one can implement a model to estimate saturation thresholds and describe the convergence path towards saturation. The model is disaggregated by household type and vehicle rank to account for heterogeneous choice sets and structural change in demographics and activity rates. It highlights the existence of an incomplete diffusion process for some groups and allows to break down potential demand between negotiable and non-negotiable needs. Projection results for France prove the existence of residual potential growth for car equipment among non-working adults which is nonetheless limited, while population ageing and changing cohabitation patterns will have a downwards effect on demand. By combining these results with car mileage projections, we find average car traffic per adult to increase gently until 2040, and remain almost stable afterwards. From this date, traffic trends would be essentially determined by demographic factors.