4.3. Vehicle Start Adjustment

For gasoline light-duty vehicles, the number of vehicle starts is an important input for estimating vehicle start exhaust emissions (STREX). Vehicle start values were last updated in EMFAC2017 using data from the California Household Travel Survey (CHTS; NuStats, 2013). EMFAC2025 keeps these baseline vehicle start values but applies an adjustment factor to account for changes in vehicle miles traveled (VMT) per vehicle. The adjustment factor scales vehicle starts based on VMT per vehicle, as shown in Equation (4.2):

(4.2)\[\text{Vehicle Starts}_{y} = \text{Vehicle Starts} \times \frac{\left(\frac{\text{VMT}}{\text{Vehicle Population}}\right)_{y}}{\left(\frac{\text{VMT}}{\text{Vehicle Population}}\right)_{2012}}\]

where \(y\) is the year for which the vehicle starts are calculated and \(\left(\frac{\text{VMT}}{\text{Vehicle Population}}\right)_{2012}\) is the VMT per capita for the year 2012, when the vehicle start data from CHTS were collected.

Figure 4.8 shows the adjustment factors for the number of vehicle starts that EMFAC uses for light-duty vehicle classes. The adjustment factors for LHD1 and LHD2 apply to both their Public and Other categories (see Section 2.3).

Figure 4.8: Adjustment Factors for the Number of Vehicle Starts Relative to Reference Year 2012