Urban rail transit continues to expand in Los Angeles, Sacramento, San Diego, and the San Francisco Bay Area as these areas seek to improve regional mobility, reduce air pollution, and restructure urban growth. Transit, of course, only produces mobility and environmental benefits if people switch from cars to trains and buses. Many factors, however, are eroding transit's ridership base, including rapid suburbanization, much of it focused on highway corridors. Nationwide, transit ridership fell from 6.4 percent of all commute trips in 1980 to 5.3 percent in 1990 (Pisarski, 1992). While transit trips rose in absolute numbers in California between 1980 and 1990 (one of the few states where this was the case), transit's share of commute trips dropped in all metropolitan areas: greater Los Angeles -- 5.4 percent to 4.8 percent; San Francisco -- 11.9 percent to 10 percent; San Diego -- 3.7 percent to 3.6 percent; and Sacramento -- 3.7 percent to 2.5 percent.