The Vehicle-Miles of Travel -- Urban Highway Supply Relationship

The objective of this research was to determine whether a relationship exists between measures describing urban highway supply and VMT and, if so, to quantify the relationships. Furthermore, highway supply measures were to be related to air pollution rates and fuel consumption through the use of known VMT/air pollution/fuel consumption relationships. To be useful to practitioners, the relationships were to be generalized and in the form of tables, graphs, and monographs. The review of previous research and the analysis process carried out in this project have led to the conclusion that there is a relationship between urban highway supply and vehicle-miles of travel (VMT). Although this conclusion is not unanimous among previous analysts of the topic, many previous studies point to this relationship. The work reported here shows that when urban travel is simulated using a detailed set of empirical models, validated against observed data, VMT changes do occur as highway supply changes are specified. Even when the total VMT change in a large urban area is predicted to be insignificant, its variations by location, type of highway facility, and highway speed result in much more significant changes in a wide range of VMT-related impacts: mobility, quality of travel service, air quality, fuel consumption, and safety. (Author)