The Federal Highway Administration (FHWA) analyzes and presents vehicle-miles of travel data in its annual report, Highway Statistics, using the Highway Performance Monitoring System (HPMS). In HPMS, FHWA compiles state-provided data into a national-level database, combining "sample data on the condition, use, performance and physical characteristics of facilities functionally classified as arterials and collectors (except rural minor collectors) and system-type data for all public road facilities within each State." States report data annually. However, in some years, estimates may be made for states with incomplete data.
U.S. Department of Transportation, Federal Highway Administration, Highway Statistics 1998 (Washington, DC: 1999), p. V-1.