VMT Mix Estimation Method Refinement for MOBILE6

Vehicle classification data is at the heart of the emissions estimation process. The on-road mobile source emissions estimation process (whether using emissions rates generated by MOBILES or MOBILE6) requires vehicle classification data as a surrogate for vehicle miles of travel (VMT) mix. This information is a direct input into the inventory estimation process. The Texas Department of Transportation (TxDOT) collects vehicle classification counts periodically throughout the state. Vehicle classification data is compiled by hour, typically for a 48-hour count period. Count station location, roadway functional classification, and date are also recorded. Weekend counts are typically not collected. Vehicle classification counts classify vehicles into the standard Federal Highway Administration (FHWA) vehicle classifications. Procedures developed by the Texas Transportation Institute (TTI) are available to convert these counts into the standard eight EPA vehicle classifications, or the new 28-vehicle classifications used by MOBILE6. In this way, VMT mix is available by county and by roadway functional classification, as well as by time-of-day and year. MOBILES vehicle classification analyses used eight vehicle types. These are light-duty gas vehicles (LDGVs), light-duty gas trucks 1 (LDGTIs, up to 6,000 pounds), LDGT2s (6,001-8,500 pounds), heavy-duty gas vehicles (HGDVs over 8,500 pounds), light-duty diesel vehicles (LDDVs), light-duty diesel trucks (LDDTs no distinction at 6,000 pounds since emissions standards are the same), heavy-duty diesel vehicles (HDDVs, over 8,500 pounds), and motorcycles (MC).