A 2008 Transit Cooperative Research Program study found that housing in transit-oriented development (TOD) produces fewer vehicle trips than suggested by standards from the Institute for Traffic Engineers (ITE), and the authors conclude that regulators should lower parking ratios, as well as transportation impact fees, for residential TODs, in general.
Authors G. B. Arrington of the consulting firm PB PlaceMaking and Robert Cervero of the University of California at Berkeley studied 17 TOD residential buildings around the United States, including two in Arlington, VA at the Virginia Square Metro as well as two in Montgomery County, MD and one in Alexandria, VA.
On average, these projects generated 44 percent fewer vehicle trips per day than estimated by ITE, while AM-peak and PM-peak trips were 49 percent and 48 percent less than ITE estimates. After analyzing the relationship between various characteristics of the developments and trip generation, the authors found that overall density was the single most significant factor for explaining trip rates. The authors believe that this is because density is actually a proxy for other variables (“walking quality, parking supply, socio-demographic characteristics of the surrounding neighborhood,” p. 45).
Through multiple-regression analysis, a model that predicted trip generation as a function of the distance from the central business district, residential density, and household size provided the most predictive power.
Based on their work, the authors strongly advocate for a reduction in off-street parking requirements, asserting that most TODs are “over-parked” based “on the assumption that there is little difference betwee4n TOD and conventional development with respect to the traffic they generate” (p. 54). The authors estimate that lowering residential parking ratios by 50 percent in TODs could allow for a 20-to-33 percent increases in residential density and development cost savings of between 5 percent and 36 percent. These outcomes could provide greater profits for TOD developers, “and/or increased housing affordability” (p. 54).
The project team used pneumatic tube counts at the buildings to record vehicle trips. The ACCS Residential Building Transportation Performance Monitoring Study completed in 2013 used the same methodology, but added a resident travel survey component. Results from the ACCS study roughly matched those from the Arrington and Cervero study, though it is not possible to make direct comparisons between the numbers published in the reports from the two studies. This is because Arrington and Cervero used an earlier version of the ITE Trip Generation Manual and because they reported trips per dwelling unit, while the ACCS report reported trips per occupied dwelling unit.