MOVES gets default national 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. To allocate this total VMT to county in MOVES2014, we use values from Version 1 of the 2011 National Emission Inventory (NEI) analysis; the national VMT by roadtype is allocated to county based on State-level VMT from the United States Highway Performance Monitoring System (HPMS) as distributed to the individual counties in each state as part of the NEI analysis. For more information, see the "Population and Activity of On-road Vehicles in MOVES2014," EPA-420-R-16-003 (January 2016) available at https://www.epa.gov/moves/moves-onroad-technical-reports
For SIPs and regional conformity analyses, EPA expects users to develop local VMT estimates. 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.