4.1. Vehicle Mileage Accrual Update#
The mileage accrual rate is an estimate of the miles per year traveled per vehicle. Bureau of Automotive Repair (BAR) Smog Check Data was used to derive regional mileage accrual rates by vehicle age and class for light-duty vehicles using methods similar to those employed since EMFAC2007. Due to insufficient Smog Check program data for diesel vehicles, it was assumed that their mileage accrual rates are the same as those of gasoline-powered vehicles in the same class.
BAR Smog Check data for calendar years 2001 through 2022 were used to develop updated mileage accrual rates. Vehicle records were matched across biennial review years (such as 2001 with 2003, 2002 with 2004, etc., up to 2021 with 2022). The first record from the earlier year was matched to the first record from the later year. For each matched pair, the difference in odometer readings and test dates was computed. To avoid errors from five-digit odometer displays, only positive mileage differences were used. Based on the differences in dates and odometer readings between biennial review tests, miles per day were computed. These values were then converted to annual miles of accrual using 365 days per year. To eliminate potential data entry errors, outliers above 200,000 annual miles were excluded, following the National Highway Administration National Household Travel Survey methodology (ORNL, 2011).
For each region and vehicle class, the average mileage accrual by age was computed and used to develop regression equations. Where possible, updated regression equations for mileage accrual were determined by grouping similar Sub-Areas (See Table 3 for definitions) together as a Region. The regional grouping is illustrated in Figure 4.1. For LHD1, LHD2, and Motor Homes, there was only sufficient data to establish statewide average mileage accrual rates. In addition, where insufficient data were available for individual vehicle classes, some classes were combined into a single grouping (such as LHD1 and LHD2). Table 4.1 summarizes the groupings used to compute the mileage accrual rates.
Figure 4.1: Regional Groupings for Deriving Accrual Rates#
Vehicle Category |
Grouping |
|---|---|
LDA (Passenger Cars) |
Regional (groups of similar Sub-Areas) |
LDT1 (Light-Duty Trucks; GVWR \(<\)6,000 lbs. and ETW \(\le\)3,750 lbs.) |
Regional (groups of similar Sub-Areas) |
LDT2 (Light-Duty Trucks; GVWR \(<\)6,000 lbs. and ETW 3,751–5,750 lbs.) |
Regional (groups of similar Sub-Areas) |
MDV (Medium-Duty Trucks; GVWR 6,000–8,500 lbs.) |
Regional (groups of similar Sub-Areas) |
LHD1 & LHD2 (Light-Heavy Duty Trucks; GVWR 8,501–14,000 lbs.) |
Statewide |
MH (Motor Homes) |
Statewide |
In EMFAC2021, confidential data provided by vehicle manufacturers was analyzed to estimate the average annual mileage for electric vehicles. Based on this data, the average annual accrual rate for the base year 2015 was set at 70% of the statewide gasoline average annual mileage by vehicle age. As battery range is expected to improve over time, this 70% baseline is gradually increased each year, reaching 100% by calendar year 2025. The same methodology is incorporated in EMFAC2025. However, in EMFAC2021, electric accrual rates were uniform across all light-duty vehicle classes and regions. In contrast, EMFAC2025 updates these rates to vary by vehicle class and region while still adhering to the 70%–100% phase-in schedule.
Examples of accrual rate curves developed through this analysis are illustrated in Figures 4.2 to 4.4. Figure 4.2 compares EMFAC2021 and EMFAC2025 annual mileage accrual for LDA in Los Angeles County (South Coast Air Basin), with EMFAC2025 values based on the South California coastal region. Figure 4.3 presents the same comparison for LDA in Kern County (San Joaquin Valley Air Basin), using EMFAC2025 mileage accrual from the Central Valley South Region. Figure 4.4 compares statewide mileage accrual for the Light Heavy-Duty Trucks (LHD1, LHD2) category between EMFAC2021 and EMFAC2025. Annual mileage accrual rates for Age45+ light-duty vehicles are described in Section 2.1.2.3.
Figure 4.2: Annual Mileage Accrual for Passenger Cars (LDA) for Los Angeles County in the South Coast Air Basin#
Figure 4.3: Annual Mileage Accrual for Passenger Car (LDA) for Kern County in the San Joaquin Valley Air Basin#
Figure 4.4: Statewide Annual Mileage Accrual Curves for Light Heavy-Duty Trucks (LHD1, LHD2)#