Emissions of N₂O and CH₄ emissions from mobile sources from a particular vehicle depend not only on fuel type and activity level (e.g., vehicle miles traveled), but also operating characteristics such as speed and temperature, emission controls, maintenance procedures, and age. The methodology called for in the Workbook simplifies the process to some extent. Calculations are based on multiplying emission factors by activity levels for vehicles categorized by vehicle type, fuel type, and emission control technology. Here, we estimate N₂O and CH₄ emissions in 1990 and 1996 from highway vehicles only. Because fuel consumption for non-highway uses accounted for less than 0.3% of total consumption in 1996 Motor-fuel use (Federal Highway Administration, 1996 Highway Statistics, Table MF-21), we ignore this minor source.

Data on vehicle miles traveled (VMT) for highway vehicles broken down by vehicle type were obtained from the Federal Highway Administration. Workbook guidelines were used to convert from the FHWA vehicle type categories to the categories of vehicle/fuel type shown in Tables. Data on the distribution of emission control technology for each vehicle/fuel type has not yet been obtained. Since the Workbook gives default emission factors for each vehicle/fuel type based on control technology, conservative assumptions were made about the actual technology in use. For each vehicle/fuel type, the technology with the largest emission factor was selected as shown in the Tables. The potential impact of these assumptions can best be judged by considering N₂O emissions from Light Duty Gasoline Vehicles, which make up the great majority of total carbon equivalent emissions. The control technology with the largest emission factor is Tier-0 3-way catalyst, which applies to all model years between 1984 and 1993. Thus if the average age of the fleet is 7-16 years, this assumption will produce a reasonably accurate total. A younger fleet with more recent technology could reduce the emissions factor, and therefore total emissions, by a maximum of roughly 40%.  The details of the methodology and data
used are available in the Mobile Sources Appendix.

Results show that in 1996, N₂O emissions from mobile sources were 111,130 MTCE; the contribution of CH₄ was much smaller, at 3,192 MTCE, for a combined total of 114,322 MTCE. This represents a decline of 6.5% from emissions in 1990, due to a decrease in vehicle miles traveled.