GAO reviewed the trends and general effects of the substantial federal investment in urban mass transit, focusing on: (1) changes in transit service, using measures such as the amount of service provided, the cost of service, and the quality of service; (2) changes in ridership levels; and (3) the extent to which improved transit service has contributed to the broad social, economic, and environmental benefits generally associated with such improvements. GAO found that federal assistance has helped stabilize and increase the amount of transit service; however, operating costs have increased due to increasing labor costs, declining labor productivity, and rising fuel costs. Some of these operating costs, such as the demand for transit service during peak hours, are difficult to control, and some believe that the federal assistance program has not provided sufficient incentives to control costs. Although revenues have increased, they have not kept up with increasing service costs. In evaluating service quality, GAO focused on vehicle reliability and found declines in many of the nation's transit systems because of inadequate preventive maintenance. GAO also found that ridership has grown because of: (1) federal assistance, which has expanded service and stabilized fares; (2) increases in gasoline prices; and (3) shifts in population and employment. Service expansion, stabilized fares, and special transit services have benefited disadvantaged and disabled persons, but their transportation needs may be more efficiently addressed by targeting the individuals for specific subsidies, rather than subsidizing all riders. The impact on energy use, air pollution, and traffic congestion depends on the decreased use of automobiles; however, mass transit's commuting share has declined so that it represents a small portion of urban transportation and may not be the most efficient or effective means to address these problems.