High-density, infill developments may cut auto trips by 90 percent
Automobile trips generated by high-density, infill developments served by transit may be up to 90 percent lower than industry standards would suggest, according to experts at Nelson/Nygaard in San Francisco. Nelson/Nygaard developed software that generates far more accurate estimates of traffic impact in urban locations than can be generated through conventional tools. Created for the California Air Resources Board, the software called Urbemis is designed to be applied anywhere in the US, Patrick Siegman of Nelson/Nygaard told New Urban News. It is available for free at www.urbemis.com and is designed for Windows operating systems.
The Institute for Traffic Engineers’ Trip Generation report and companion Trip Generation handbook are the best sources for estimating automobile traffic, but are best applied to stand-alone suburban sites, write Siegman and Adam Millard-Ball, also of Nelson/Nygaard. The use of these standards — the only ones available prior to Urbemis — were often wrongly applied to walkable urban locations, despite cautions against doing so by ITE, Siegman and Millard-Ball report.
The software prompts the user to fill in site-specific data, such as density, transit service, mixed-use, and characteristics of development within walking distance. It then offers trip reduction credits of up to 55 percent for high density, up to 9 percent for a mix of uses, up to 2 percent for neighborhood retail, up to 15 percent for transit service, and up to 9 percent for “pedestrian/bicycle friendliness.” Further credits are available for demand management and other policy measures. The reductions were based on available data and studies. “The data developed for the location-efficient mortgage program was very useful,” Siegman says. It was peer reviewed by Dr. Richard Lee of Fehr & Peers Associates and John Holtzclaw of the Sierra Club. Commenting on the inordinate impact of density on trip generation, Siegman notes that increases in density bring destinations significantly closer and is also associated with smaller household sizes. “We’ve been waiting a long time for such a guide,” says Holtzclaw.
