Trip generation statistics: Mixed use, TOD and Infill

Louis Meuler writes on another list, in response to a query:

I researched, over the Internet only, the subject of adopted trip generation rate assumptions used by various jurisdictions about two years ago. At the time I found very few places that had conducted local research and had adopted and use local rates that were different than ITE's trip generation rates for mixed-use areas or infill areas that where not a part of a Central City area. Most places seem to use ITE's figures for required traffic studies and impact fee calculations.

San Diego is one place that has adopted different rates than standard ITE figures. These were generated by a detailed local study.
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I have added a link to the San Diego Trip Generation Manual. It's pretty great. Check it out. Interesting that they find that typical subdivision housing generates 12 trips/day--Virginia says 9, Jeff Tunlin of Nelson Nygaard says as many as 15--but multiunit housing generates 6 trips/day if the density is 20+ units or greater. I think that's a reasonable assumption for rowhousing...
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The current practice for most places seems to be:

1. Most places use ITE as a base because they have not conducted or thought to conduct a local study. I have found that most of the traffic (pro auto mobility) engineers I have worked with do not understand or have not thought about the relationship between the ITE numbers and its effect on urbanism. The ITE numbers that appear to be generated by only observing suburban development are now being used to review both suburban and urban projects that are outside of a central city area. Urban infill and mixed-use development has generally had less detailed national study on reductions of auto trip generation.

2. As a result of 1. most projects are required to use the sub-urban (auto dominated) standards found in ITE unless they can argue (usually through a detailed study) that they can or will achieve different trip generation rates based on other modes of travel or reduced auto travel because of good urbanism. This is often negotiated at the local level.

At my jurisdiction we often have to coach the private traffic study engineers (they must mostly work on suburban projects) to use the flexibility (reduced trip generation rates) found in ITE numbers even in the Central City areas. Many times the private engineers seem to forget that there are different rates outlined in ITE for more urban areas.

Here is a link to a local project (includes links to traffic studies) that ended up being able to justify some reduction in assumed trip generation rates because of its infill location and efforts to integrate with bus transit and a potential future trolley system.

Index Keywords: transportation; urban-design-placemaking