VMT Forecasting Alternatives for Air Quality Analysis

Traffic growth forecasting plays a pivotal role in the air quality analyses conducted in nonattainment and maintenance areas throughout the country. However, traffic forecasting is a difficult and often inaccurate process. For planning purposes, many agencies have begun producing estimates of future development and traffic with ranges. This approach demonstrates the disparity of forecasts based on different sources of data and assumptions. There are opportunities for using alternative forecasts of vehicle miles of travel (VMT) within the air quality planning process. Within the transportation conformity process, alternative VMT forecasts can be used to evaluate conformity assumptions or to adjust travel demand model forecasts. Within the statewide implementation plan (SIP) process, alternative VMT forecasts can play an even greater role. They may be used to develop consistent statewide forecasting methodologies for emission inventories or may be used to develop and assess risks when preparing conformity emission budgets.