One of the key deficiencies of most traffic impact studies is their use of standard vehicle trip generation rates, which take no account of a development’s context, the availability of transit, and other key factors that significantly affect vehicle trip rates. As a result, traffic studies often overestimate the traffic impacts of compact, mixed-use, and transit-oriented projects, while underestimating the impacts of single-use suburban developments.
To address this problem, California's Air Quality Management Districts, working in partnership with the California Department of Transportation, commissioned Nelson\Nygaard to develop an approach that would more fairly assess the traffic and air pollution impacts of new development. Patrick Siegman and his colleague Adam Millard-Ball directed the project.
The project team developed the “Mobile Source Mitigation Component” of the URBEMIS 2007 model, which was California’s most widely used model for estimating air pollution emissions from land development projects. The peer-reviewed model assesses the impact of numerous factors on vehicle trip rates. These include the design and mix of land uses; the proximity and frequency of transit service; bicycle and pedestrian facilities, parking pricing and supply, the development’s transportation demand management measures, and other factors, making it one of the most advanced tools available to assess the traffic impacts of development.
The URBEMIS 2007 model has since been used in jurisdictions throughout California, as well as in cities outside California, such as Albuquerque, Chicago, and New Haven. In the San Joaquin Valley, the Air District’s first-in-the-nation air quality impact fees are levied on the basis of the model’s predictions of future emissions. This approach gives developers an incentive to build compact, walkable, transit-oriented neighborhoods, and to adopt other vehicle trip reduction measures whose benefits are recognized by the URBEMIS model.
Chico photos courtesy of R. John Anderson