Crediting Low-Traffic Developments: Adjusting Site-Level Vehicle Trip Generation Using URBEMIS, Nelson\Nygaard Consulting Associates, August 2005

The authors of this report describe the URBEMIS model as a useful tool in correcting some of the shortcomings of the ITE Trip Generation model in assessing the environmental impact of development.  This comes from the introduction:

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. Now in its seventh edition, Trip Generation provides a wealth of data on the average number of vehicle trips generated at different times of day by hundreds of land uses, from office buildings to funeral parlors.

Trip Generation is an invaluable reference for traffic studies and environmental assessments, as it is by far the most comprehensive source of empirical data on the traffic impacts of different land uses. 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. . . .

. . . The URBEMIS mitigation component 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. . . .

 The most amazing feature of the URBEMIS mitigation model is the very high mitigation effect it gives to residential development.  Overall, URBEMIS indicates a 55% mitigation in ITE environmental (& traffic) effects from residential development.  It also shows in the following table how high-density residential has the greatest impact in reducing environmental effects of development.


Fairfax County uses the ITE Trip Generation Report as its basic tool for calculating traffic, but it too has adjustments.  It is not clear that it mitigates the impact on traffic & environmental effects that intense residential development generates.  We need to be sure that it does.

Here's the full report:

Crediting Low-Traffic Developments, Nelson/Nygaard Consulting Assoc., August 2005