Thresholds of Transit Effectiveness
A simple and very reliable way of determining the suitability of an urban area for transit, and the likelihood of residents to opt for transit over autos, is to measure the residential density of an area. As Pushkarev and Zupan summarize in the conclusion to Public Transportation and Land Use Policy,
Higher density of urban development acts both to restrain auto use and to encourage the use of public transit…Average figures from a number of urban areas in the United States suggest that: At densities between 1 and 7 dwellings per acre, transit use is minimal…A density of 7 dwellings per acre appears to be a threshold above which transit use increases sharply…At densities above 60 dwellings per acre, more than half the trips tend to be made by public transportation.
Several of the indicators of transit effectiveness arrived at by Pushkarev and Zupan, in addition to those above, have become standard in the transportation planning literature. The most important underlying factor supporting transit use, according to Pushkarev and Zupan, is reduced auto ownership. Increasing residential density by a factor of ten, for example, is found to drop the level of auto ownership by 0.4 percent.
In fact, density correlates extremely closely with auto ownership, such that residential density offers a basis for predicting household auto ownership with 86 to 99 percent accuracy. Still more important, they argue, is the density of nonresidential floor space in a downtown area served by transit.
High-densities of nonresidential, downtown floor space have the effect of suppressing auto use, and allowing the economy of scale for effective transit service to residential areas. As Pushkarev and Zupan conclude: “The land use policies which will do most for public transportation are those which will help cluster nonresidential floor space in downtowns and other compact development patterns.”
Rutgers University transportation researcher Reid Ewing remarks that Australia and Canada, with comparable levels of auto ownership and gross densities, nonetheless sustain transit ridership more than three times the U.S. level. The difference, Reid points out, is that “Canadian and Australian cities…have managed to create conditions favorable to transit,” primarily by clustering uses in central areas and linking development to transit infrastructure.