How transit scaling shapes cities

Transit accessibility to jobs (the ease of reaching a place of work by public transport) affects both residential location and commute mode choice, resulting in gradations of residential land-use intensity and transit (public transport) patronage. We propose a scaling model explaining much of the variation in transit use—the number of transit commuters per km2—and residential land-use intensity with transit accessibility. We find that locations with high transit accessibility consistently have more riders and higher residential density; transit systems that provide greater accessibility and with a larger base for patronage have proportionally greater ridership increase per unit of accessibility. All 48 metropolitan statistical areas in our sample have a scaling factor less than 1, so a 1% increase in access to jobs produces a less than 1% increase in transit riders; the largest cities therefore have higher scaling factors than smaller cities, indicating returns to scale. The models, derived from a new database of transit accessibility measured for every minute of the peak period over 11 million US census-blocks, and estimated for 48 major cities across the United States, find that the number of jobs reachable within 45 minutes of the rider’s base most affect transit rider density. The findings support the idea that transit investment should focus on mature, well-developed regions.