The Federal Highway Administration analyzes and presents vehicle-miles of travel data in their annual report, Highway Statistics, using the Highway Performance Monitoring System (HPMS). The HPMS 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.
SOURCE: U.S. Department of Transportation, Federal Highway Administration, Highway Statistics 1998 (Washington, DC: 1999), p. V-1.