University of California Transportation Center

What explains transit ridership? The answer to this simple question is both obvious and complex. Public transit systems carry large shares of person travel in older, larger metropolitan areas around the globe, but in most places – old and new, large and small – transit is losing market share to private vehicles. Nationally, only 2.1 percent of all trips were on public transit in 2001, compared to 85.8 percent by private vehicle, 9.9 percent by foot and bicycle, and 2.2 percent by other means (2001 National Household Travel Survey).

Even the most casual observer of cities can offer informed speculation on why the share of year 2000 commuters using public transit in metropolitan San Francisco (19 percent) in nearly five times higher than in metropolitan Atlanta (4 percent). Population density, levels of private vehicle ownership, topography, freeway network extent, parking availability and cost, transit network extent and service frequency, transit fares, transit system safety and cleanliness, and so on all surely play a role. But the relatively importance of these various factors, and the interaction between them is not well understood. Yet understanding the relative influence of these factors is central to public policy debates over transportation system investments and the pricing and deployment of transit services. But the research literature on explaining transit ridership is surprisingly uneven, in some cases poorly conceived, and the results are often ambiguous or contradictory.

The goal of this paper is to review the literature on explaining transit ridership, critique the sometimes significant weaknesses in previous studies, draw conclusions from the more rigorous studies on the factors which most influence transit use, and recommendations on the steps needed to better understand and explain transit ridership. To do this, we begin by offering a taxonomy of public transit ridership research.