Forecasts of Future Vehicle Miles of Travel in the United States

Abstract

Transportation infrastructure demand is driven by a need to replace existing infrastructure and to provide capacity to meet future travel demands. Thus, long-term transportation policy and financial planning benefit from an understanding of future investment needs. The hypothesis that the United States has reached a critical juncture in underlying sociodemographic conditions and travel behavior that will result in more moderate rates of future vehicle miles of travel (VMT) growth is explored. Two simple model formulations for predicting VMT are proposed, and historical trends for establishing inputs for scenario forecasts of future VMT are examined. An understanding of future VMT demand is critical for policy decision making and infrastructure investment planning. The VMT predicting formulas enable National Household Travel Survey information to be used to predict input components in forecasting future VMT. Two VMT predictions are provided and compared with a forecast reported in 2002 Status of the Nation's Highways, Bridges, and Transit: Conditions and Performance. While this study builds a case for slowing VMT growth, it hypothesizes that there may continue to be declining system performance despite slower VMT growth because more of the roadway system is at or near critical congestion levels and hence more susceptible to performance deterioration with modest increases in travel demand. It also suggests that land use pattern effects on travel behavior and person travel time budget growth are the least understood factors and possibly weak links in reaching conclusions about the ultimate pace of VMT growth.