Apples, Oranges & Transit Oriented Development

A new TCRP report (#158, Effects of TOD on Housing, Parking and Travel) concludes that automobile trips are approximately one-half as frequent per dwelling unit in transit oriented development (TOD) apartments as in all other apartments. The report, however, raises more questions than it answers.

Based upon a quick review, it appears that the study suffers from three important biases.

    1. Usage of a "trips per dwelling unit" measure prejudices the results in favor of transit oriented development. The authors note that TOD residents tend to be single or couples, with no children. In contrast, the average number of persons per household in all apartments is 2.1 (American Community Survey, 2006). With fewer people per household in TOD developments, it is not surprising that trip generation rates per dwelling unit should be less. A good portion of the observed difference appears to be simply the result of differing types of households.

    2. Usage of an overall apartment trip generation rate also prejudices the results in favor of TODs. That is because TODs, by definition, are generally close to rail transit lines that provide good access to downtown areas. On the other hand, non-TOD apartments are spread throughout urban areas, with more than 40 percent being in the suburbs. A good portion of the observed difference appears to be simply geographical location.

    3. TODs tend to attract people who are more inclined than others to ride transit. This “self-selection” has been documented in research by one of the authors of the TCRP report (Robert Cervero) and is acknowledged in the report. Yet the report does not attempt to correct its conclusions for this bias.

What this report does not demonstrate (contrary to its claims) is that people in TODs drive less than their neighbors. Examining that question would require much more focused methodology. Rather, the report merely provides evidence transit is used more where it better connects people with downtowns. In the larger urban fabric, with its dispersed trip patterns and dispersed employment, sufficient levels of transit service simply cannot be afforded, whether in American or Western European urban areas.

The conclusion: TOD apartments and apartments in general are like apples and oranges with respect to trip generation.