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.