Abstract

This report provides a review of the California Air Resources Board’s current method for adjusting estimated emissions from on-road motor vehicles. These estimates are a critical feature of CARB’s air quality modeling efforts, which are an important tool for maintaining and improving California’s air quality. The central approach of the method adjusts “average weekday” emissions estimates using fac- tors derived from measurements of diurnal and day-of-week variations in multi-class vehicle ac- tivity provided by Caltrans from automated vehicle classifer (AVC) data. The diurnal factors are used to adjust the inputs to the Direct Travel Improvment Model (DTIM), which provides gridded, hourly estimates that are assumed to represent general spatio-temporal emissions patterns on an average weekday. The day-of-week factors are used to adjust the average weekday county-wide emissions estimates produced by EMFAC, CARB’s tool for estimating emissions from on-road ve- hicles. The day specific spatio-temporal DTIM patterns are then used to produce hourly, gridded emissions from the county-wide EMFAC totals. The body of this report begins with a review the general methodology in the context of the broader model. It continues with a review of the SAS programs that compute the diurnal and day-of-week factors from Caltrans AVC data. We conclude with some recommendations for how the method might be improved over the near-term and long term.