Deficiencies of Travel‐Forecasting Methods Relative to Mobile Emissions

This paper examines the ability of conventional travel‐forecasting models to respond to forecasting needs created by the Clean Air Act Amendments (CAAA) of 1990 and the air‐quality lawsuit brought against the Metropolitian Transportation Commission, San Francisco. Initially, the probable response of travel behavior to the various transportation control measures of the CAAA are reviewed. The ability of the travel‐forecasting models to reflect each of the various responses is examined, concluding that only those transportation‐control measures (TCMs) resulting in a change in travel mode, auto occupancy, or destination can be modeled. Most remaining TCMs require significant model changes. Similarly, with capacity changes, three of the seven potential changes in travel behavior can be modeled, if the models are run through a series of feedback loops. For emissions modeling, the inputs cannot be obtained with the required specificity. Short‐run and longer‐run changes are identified that should be made to current modeling procedures to add further to the abilities of the models to produce good travel forecasts.