Summary

Pursuant to a congressional request, GAO reviewed traffic congestion in large and small metropolitan areas, focusing on: (1) the forces that affect traffic congestion, and how they shape its nature and severity; (2) how the Federal Highway Administration (FHwA) measured traffic congestion; (3) the credibility of FHwA urban freeway delay estimates; and (4) whether FHwA measured the effects of traffic congestion.

GAO found that: (1) the forces that shaped traffic congestion included trends in suburban development, the economy, the labor force, automobile use, truck traffic, and the highway infrastructure; (2) traffic congestion problems have increasingly occurred in suburban and outlying rural areas; (3) random interruptions in traffic flow may have a greater effect on traffic delays than recurring congestion during peak traffic periods; (4) federal, state, and local transportation agencies measured traffic flow conditions through traffic density, average travel speeds, maximum service flow rates, traffic flow to facility capacity ratios, average daily traffic volume, and daily vehicle travel miles; (5) FHwA used an urban freeway delay model to estimate present and future congestion levels nationally and to rank the most severely congested metropolitan areas; (6) the model's omission of capacity improvements and its sensitivity to changes in freeway capacity raised questions about its accuracy; (7) information on potential environmental, economic, and human stress effects was limited; (8) FHwA assigned dollar values to time and fuel wasted in traffic delays to quantify economic effects; and (9) laboratory tests on the health and environmental effects of motor vehicle emissions have shown that motor vehicles emit high levels of some pollutants under conditions associated with traffic congestion, while some studies have linked traffic congestion with physiological and behavioral changes.