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Crediting Low-Traffic Developments

By Adam Millard-Ball and Patrick Siegman

A new tool to quantify the traffic reduction benefits of New Urbanist, transit-oriented and infill development projects

Traffic studies are at the heart of many fundamental decisions on land use, street design and urban form. By analyzing the number of trips expected from a new development, and the consequent impact on traffic congestion at neighboring intersections, the traffic study is a driving force behind roadway widths, street and intersection design, and the level of fees that a developer must pay to upgrade the transportation infrastructure.

The Institute for Traffic Engineers' (ITE's) Trip Generation report and the companion Trip Generation Handbook are the most definitive available sources for estimating the automobile traffic that different land uses will generate. However, the information is most useful for auto-oriented, stand-alone suburban sites, from where the vast majority of data were collected. For downtowns or areas with good public transportation, ITE advises that traffic engineers should collect local data, or adjust the ITE average trip generation rate to account for reduced auto use.

All too often, however, ITE's warnings are ignored and standard trip generation rates are applied in inappropriate locations - with serious impacts on the character and financial feasibility of urban development. Part of the reason is that, until now, there has been no standardized tool to allow these adjustments to trip generation rates to be made. In order to address this problem, the air quality management districts of California, along with the California State Department of Transportation, worked together in 2004 to examine all of the key variables that influence automobile trip generation. They were able to quantify the trip generation impact of key locational and programmatic factors, and inserted these formulas into URBEMIS, a national model for calculating air quality impacts of projects.

The URBEMIS mitigation component, developed by Nelson\Nygaard, is a simple yet powerful tool; it employs standard traffic engineering methodologies, but provides the opportunity to adjust ITE average rates to quantify the impact of a development's location, physical characteristics and any demand management programs. In this way, it provides an opportunity to fairly evaluate developments that minimize their transportation impact, for example, through locating close to transit or providing high densities and a mix of uses.

Download the full report (pdf less than 1 MB)