Tuesday, February 17, 2009

Transit oriented developments may create fewer car trips than thought

Opponents of high-density, transit-oriented developments (TODs), often claim they create more of a traffic problem than they solve. But new research shows the number of trips they create is often exaggerated by as much as 50%.

First, a little background. Transportation planners struggle with how to reliably predict the number of net vehicle and person trips generated by new or infill mixed-use development such as TODs. Since the old model has relied primarily on single-use, free-standing sites, it is very difficult to make predictions with any certainty. Add to the mix internal capture rates, and the level of uncertainty rises.

The Institute of Traffic Engineers (ITE) is the only one to suggest a framework for establishing a data collection procedure for estimating multi-use trip generation to include internal capture rates. However, this framework is based on even more limited information than the old model. Currently, "...so little information is available about internal capture rates that traffic impact studies for mixed-use developments become little more than exercises in speculation." (Ewing, R., M. Deanna, and S.C. Li, Transportation Research Record 1518, pp. 1-6).

New research recently completed for the Transit Cooperative Program seems to confirm the disconnect between the modeling and the actual demand. The research confirms that the ITE trip generation and parking generation rates over estimates automobile trips for TOD developments by as much as 50%.

To view the actual study go to the TCRP website or check out this synopsis from on Planetizen.

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