Road supply and traffic in California urban areas

The authors estimate relationships between the supply of state highways, measured in lane-miles, and the quantity of traffic, measured in vehicle-miles traveled, for urban counties and metropolitan areas in the state of California. The analysis employs a panel data set of annual observations for the years 1973 to 1990. The authors estimate several versions of a long-linear model including fixed regional and time period effects. The present main concern is with models of state highway (as opposed to total) vehicle-miles traveled. By using two types of models designed to capture long-term effects, the authors estimate that state highway vehicle-miles traveled has a lane-mile elasticity of 0.6-0.7 at the county level and 0.9 at the metropolitan level, and that the full impact of vehicle-miles traveled materializes within five years of the change in road supply. They also consider limited data on off-state highway vehicle-miles traveled, and find no conclusive evidence that increases in state highway lane-miles have affected traffic on other roads. Population, income, and gasoline price elasticities are also discussed. The authors find that, even when all these factors are accounted for, there has been a sharp increase in the propensity towards vehicle travel over the period of study, particularly during the late 1980s.