Source: adapted from American Public Transportation Association, Public Transportation Fact Book.
The impacts of individual mobility and motorization on urban transportation have been significant. The outcome was a substantial decline in the share of public transit in urban mobility to less than 2% of the passenger miles in the early 21st century. The 1920s saw a dramatic drop in the share of public transit in urban motorized mobility as the automobile reached mass production. The only reversal of this trend was during World War II when austerity measures (e.g. rationing gasoline) forced urban residents back to urban transit. As soon as the war ended, the trend resumed. In the first half of the 20th century, the number of passenger miles increased in spite of a declining share of public transit due to the large growth of the urban population. However, since the 1970s there have been virtually no changes in the level of public transit ridership in the United States.
After a significant decline from the 1950s to the 1970s, transit ridership has shown limited growth in the United States, despite continuous investments and improvements. Variations in ridership are linked with economic cycles of growth and recession. It is mainly the poorest segment of the population that relies the most on public transit and this segment is also the most vulnerable to economic downturns. Light rail systems, from being the dominant form of urban transit before World War II, have experienced a comeback as many metropolitan areas tried to implement those systems as less costly transit solutions for lower densities. This is an indication that most of the urban development occurred in suburban areas, which are poorly serviced by transit systems. Still the overall impact of light rail is negligible at best and ridership figures are comparatively very small and are likely to remain so. Heavy rail systems (subways) have fared better with a ridership that remained constant form the 1960s to the 1980s and that have increased since then. The question remains about what will be the share of public transit in future urban mobility.