We get our default VMT values from the Highway Performance Monitoring System (HPMS) of the Federal Highway Administration, which collects VMT information from every state. Although this is a good source of consistent total VMT information, the system is designed to obtain data at a state-wide level.
We have a process that uses surrogates such as human population (from the U.S. Census), roadway miles and economic modeling to develop allocation factors for distributing the state-wide total VMT to individual counties. We have done studies to compare our method to develop default VMT for counties to state developed estimates (usually based on additional information not provided to FHWA) and have found that there can be significant differences between our default values and state estimates.
EPA expects users to to develop local VMT estimates for SIPs and regional conformity analyses. The MOVES technical guidance (available at http://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100NN9L.txt) provides more information on the development of local VMT estimates.