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Freight Movement & Air Quality

Appendix B: Estimation of Future Truck Emissions

To estimate future trucking emissions, we first estimated current (2002) truck VMT by truck type and facility type. Table B-1 shows the distribution of truck VMT by these categories. This information is derived from the VM1 and VM2 tables in Highway Statistics. We distributed truck VMT across the four MOBILE6 roadway types, assuming the same distribution for each truck/fuel type. VMT for single-unit vehicles was split between gas and diesel based on the distribution in the 1997 Vehicle Inventory and Use Survey (VIUS).

Table B-1: Truck VMT by Functional Class and Vehicle Type, 2002 (millions)

We estimated future trucking VMT by applying the estimated annual growth in trucking ton-miles (2.5 percent, as presented in Table 2-5) to current trucking VMT figures (by truck type and facility type). We assume that growth is uniform across all truck and facility types. We then developed truck emission factors using MOBILE6.2. We developed emission factors for single-unit heavy-duty gasoline trucks, single-unit heavy-duty diesel trucks, and combination diesel trucks. Emission factors differ by the four facility types in MOBILE6 (local streets, arterial/collector, urban freeway, and rural freeway). These emission factors are presented in Tables B-2 through B-5.

Table B-2: Local Road Truck Emission Factors

Table B-3: Arterial Truck Emission Factors

Table B-4: Urban Freeway Truck Emission Factors

Table B-5: Rural Freeway Truck Emission Factors

Applying the emission factors to the VMT forecasts, we estimate national truck emissions in 2002, 2010, and 2020, shown in Table B-6. These results show truck emissions are expected to drop steeply over the next two decades, despite more than 50 percent growth in VMT. Total NOx emissions from freight trucks in 2020 will be one-tenth the level in 2002. PM-10 emissions in 2020 will be one-quarter current levels.

Note that our 2002 estimate of truck emissions is significantly different than the EPA National Emission Inventory (NEI) heavy-duty truck emissions for 2002 (presented in Table 2-9). Our estimate is 33 percent higher in the case of NOx and 23 percent lower in the case of PM-10. There are likely a variety reasons for this discrepancy. The NEI is developed in part using data submitted by state and regional air quality agencies, and these agencies use local VMT estimates that most likely do not sum to the national total reported in Highway Statistics. EPA, state, and local emissions estimates may also have disaggregated VMT into more vehicle classes than we did, since MOBILE6 allows up to 16 vehicle classes. Finally, the NEI reflects some local differences in vehicle speeds, whereas our estimate uses a single average speed by facility type.

Our future year estimates of truck emissions are also different than the estimates reported in EPA's regulatory impact analysis documents for the 2007/10 heavy-duty truck emission standards. Our 2010 NOx estimate is 2 percent higher than EPA's estimate for that year, and our 2020 NOx estimate is 26 percent lower than EPA's estimate. Again, a variety of factors probably contribute to this discrepancy. For example, our analysis used MOBILE6.2 while the EPA analysis used MOBILE5 (although EPA made adjustments to account for the changes to the MOBILE model). Our analysis uses a 2.5 percent annual growth rate for all VMT. EPA's estimate is based on the VMT forecasts inherent in the MOBILE model, which are determined from the forecast growth in the heavy-duty truck population and assumptions about vehicle mileage accumulation rates. And, as described above, our emissions forecasts are calculated using different values for the base year (2002) emissions.

Table B-6: Estimated Current and Future National Truck Emissions