Changing speed-VMT distributions: The effects on emissions inventories andconformity

The emissions factor modeling component of the motor vehicle emissions inve ntory (MVEI) modeling suite is currently being revised by the California Ai r Resources Board (CARB). One of the proposed changes in modeling philosoph y is a shift from using link-based travel activity data to trip-based trave l data for preparing mobile emissions inventories. Also as part of the revi sions, new speed correction factors (SCFs) will be developed by CARE for th e revised model. The new SCFs will be derived from vehicle emissions on 15 new driving cycles, each constructed to represent a typical trip at a speci fic average speed. This paper discusses how the new SCFs will affect transp ortation conformity and emissions inventory development, and evaluates the differences in total emissions produced by trip-based and link-based distri butions of speed and vehicle miles of travel (VMT). We simulated both link-based and trip-based speed-VMT distributions using t ravel data from the Sacramento and San Diego travel demand models. On the b asis of the simulation results, there is reason to expect that mobile emiss ions inventories constructed using the proposed trip-based philosophy will differ markedly from those constructed in the current manner. Noting that r esults may vary by region, increases are expected in the CO and HC inventor y levels, with concomitant decreases in the NOx mobile emissions inventorie s.