The General Accounting Office (GAO) examined the reliability of the Environmental Protection Agency's (EPA's) mobile source emission factor model (MOBILE4) and the effect of budget constraints on its reliability. Briefly, GAO found that the model has undergone continuous revisions to more accurately represent prevailing conditions and the model year vehicles or fleet in use. According to EPA, certain MOBILE4 assumptions need to be refined and revised to enhance the model's reliability. However, although the overall reliability of MOBILE4 has not been determined, EPA believes its statistical reliability should surpass that of the previous version, MOBILE3. Meanwhile, EPA continues developmental work to reflect changing conditions in the vehicle fleet, address known limitations, and update assumptions used in the model. These actions should improve the reliability of the next version of the model. For now, however, MOBILE4 remains an important tool to understanding efforts to limit and decrease atmospheric ozone and carbon monoxide pollution. Funding constraints have limited EPA's ability to perform emission testing of later model vehicles. Therefore, MOBILE4's estimates for later model vehicles are statistically less reliable than its projections for vehicles of earlier model years, because of the smaller sample size for later model years. However, EPA has begun addressing past funding inadequacies by providing increased funds in fiscal years 1989 and 1990 to increase the sample size of later model vehicles.

  • Record URL:
  • Supplemental Notes:
    • Report to the Chairman, Subcommittee on Oversight and Investigations, Committee on Energy and Commerce, House of Representatives.
  • Corporate Authors:

    U.S. General Accounting Office

    441 G Street, NW
    Washington, DC  United States  20548
  • Publication Date: 1990-5

Media Info

  • Features: Appendices;
  • Pagination: 8 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00497337
  • Record Type: Publication
  • Report/Paper Numbers: GAO/RCED-90-138
  • Files: TRIS
  • Created Date: Sep 30 1990 12:00AM