Testing for the significance of induced highway travel demand in metropolitan areas

The theory of induced growth in vehicle travel hypothesizes that highway improvements which add capacity to a specific corridor or regional transportation network will attract increased levels of vehicle traffic. This relationship of highway capacity to travel demand is an important consideration when evaluating how effective highway expansion alternatives will be in solving transportation problems. Two different but complementary empirical studies were conducted to quantify the effect of highway system improvements on travel behavior. In the first study, I apply ordinary least squares regression models to estimate travel demand elasticities with respect to travel time using travel survey data from the 1995 Nationwide Personal Transportation Survey. This is one of a very few research studies to use disaggregate household-level travel data. Travel time elasticities of -0.3 to -0.5 were found, after accounting for the effects of household size, income, population density, and household employment. These results suggest that capacity additions that reduce travel time by 10 percent will increase vehicle-miles of travel by 3 to 5 percent. My second study investigates geographic differences in travel behavior before and after highway capacity was expanded in the Dallas-Fort Worth metropolitan area. Calibrated travel model data from 1984 and 1995 were used. This analysis is unique in that it applies three statistical research designs to quantify the effect of capacity expansion on induced demand for travel using a before-and-after case study approach. The three techniques, namely analysis of covariance, difference-indifferences, and OLS regression on travel changes from 1984 to 1995, produce similar results. After controlling for changes in number of households, income, and population density, total weekday vehicle-miles of travel and daily trip productions were found to increase by approximately 20 percent and 7 percent, respectively, in survey zones that had undergone significant capacity additions versus zones that remained largely unchanged over the 11-year period. Overall, the results of this research provide evidence that highway capacity improvements generate additional demand for travel. These induced demand effects should not be ignored by transportation planners and policy makers when evaluating highway system investment alternatives. Transportation