Is there a limit to car ownership growth? : an exploration of household saturation levels using two novel approaches.

Household car ownership models, by and large, follow a theory of product diffusion whereby the demand for new products is initially slow, then as the product becomes more established demand increases, and finally, as the market becomes close to saturation, the rate of increased demand reduces. The saturation level therefore drives ownership forecasts in areas of high car ownership. In this paper the authors present and compare the findings of two separate studies of car ownership. The first uses aggregated data for the UK to derive "Tanner style" ownership models in which saturation levels are estimated directly using non-linear-least-squares estimation techniques. The second uses a new procedure by which saturation levels in logit models can be estimated directly using a nested logit structure. This approach uses disaggregate data from the UK Family Expenditure Survey to estimate a global saturation level for car ownership that can be directly compared with that derived from the aggregate data set. In addition, the authors propose to allow saturation levels to vary across different household and area types to identify which have saturation levels below 100%.