Estimation and Effects of Vehicle Mix on On-road Emissions Estimates

Heavy-duty vehicles have been considered a large fraction of emissions from on-road vehicles, but have become of greater interest, as the light-duty emissions have been controlled to a greater extent than heavy-duty vehicles. Because a heavy-duty vehicle can produce 10 to 100 times the emissions (of NOx and PM emissions especially) of a light-duty vehicle, heavy-duty vehicle activity needs to be better understood and characterized. One of the key uncertainties with the use of MOBILE6 includes the fraction of heavy-duty vehicles on all types of roadways at all times of day. This paper describes the analysis of extensive automatic traffic recorder (ATR) data collected by State Departments of Transportation to investigate overall and temporal (hour of day, day of week, and month of year) patterns of vehicle mix by roadway type. A method is described and used to cross-reference the vehicle types identified by ATR data into MOBILE6 vehicle types and used to estimate fleet emissions. Comparisons will be provided that show the differences in the spatial and temporal emissions estimates by roadway type compared with estimates using the national average fleet mix.