How Better Traffic Models Can Lead to More Mixed-Use Development

Here’s another obscure but significant obstacle to building walkable places in America: the Institute of Transportation Engineers’ shoddy traffic generation models for mixed-use development.

The model used by traffic engineers around the country to measure “trip generation” at new developments consistently overestimates the amount of motor vehicle traffic produced by mixed-use projects, according to the Environmental Protection Agency. This often increases the cost of building mixed-use projects, because the developers are asked to take steps to compensate for the added traffic. To address this problem, the EPA worked with transportation researchers around the United States to develop a better traffic prediction model.

Current traffic modeling overestimates the traffic caused by mixed-use development by about 35 percent, on average. Image: ## Gradinger on Twitter##

Reid Ewing, a transportation engineering professor at the University of Utah, helped develop the new model to forecast the traffic generation of walkable development. I caught up with him at the Congress for New Urbanism conference last week in Salt Lake City.

Angie Schmitt: Can you explain the new method?

Reid Ewing: There’s a current methodology, which is the Institute of Transportation Engineers’, and it overestimates the number of external vehicle trips generated by a development if the development has mixed uses. If it’s got residential, retail, and office, those uses interact and a lot of trips stay within the development. And if it’s a development downtown, a lot of those trips that leave the development are walk and transit trips. ITE doesn’t account for that, it doesn’t account for the full number of trips that will stay within a development or the use of alternative modes for those that leave the development. So we developed a methodology.

AS: How many trips are reduced by mixed-use development?

RE: It varies from almost zero to over 50 percent. In a master planned community, it’s huge — a lot of the trips are going to stay within the community. If you have a stand-alone, freeway-oriented community that happens to have mixed-used, a much smaller percentage will stay within the community. But on average, about 35 percent. So the ITE method seems to overestimate by, on average, about 35 percent.

AS: So, how does the ITE method cause problems?

RE: If the developer proposes a mixed-use development, it doesn’t get credit for trip reduction. So that means the developer will pay more in impact fees and would be required to do more offsite improvements, that are required by the development to mitigate its impact. More parking is required to be provided than a mixed-use development really needs. The roads are scaled and sized according to numbers that inflate the actual capacity needs of the road. So there are all sorts of direct and indirect impacts.

So our effort, our original effort, was funded by the Environmental Protection Agency. They were doing it as a way of promoting smart growth. They wanted to do what they could to promote smart growth, and one of the ways was to offer an alternative to [the method used by] ITE.

AS: How many places have adopted the new model?

RE: I have no idea. I know that the state of Virginia has adopted it. The Utah Transit authority has. The Wasatch Front Regional Council of Governments [Salt Lake City’s metropolitan planning organization] has also.

Until the methodology finds its way into the ITE trip generation handbook, it still won’t be widely used.

5 thoughts on How Better Traffic Models Can Lead to More Mixed-Use Development

  1. The ITE outdated model also presents obstacles to striping bike lanes because the mixed use development requires roads to be over built more than needed, with no room left for bike lanes. A better model would be nice, but honestly, we shouldn’t have to model traffic-it’s such a stupid thing to worry about when we are trying to build better cities. It’s better to let the traffic flow where it may while we build walkable, bikeable neighborhoods.

  2. I did a health impact assessment on the mixed-use redevelopment proposal for an abandoned industrial site here in Atlanta… The trip generation estimates were not only based on the decades-old, suburban California studies, they also accounted for anticipated 2% annual regional traffic increases for the next 30 years. This in spite of the fact that traffic volume has actually been flat or declining in the region for about a decade now. But as a result of the estimate, they had planned for triple turn lanes and road widening, with some vestigial sidewalks and crosswalks and 0 bicycle facilities.

  3. I’m an ITE member and a Traffic Engineer, and I have to respond to your article. You make some broad statements that paint ITE and all the work the organization has done to document and study trip generation in a negative light. Yes, some users with a basic understanding of the three volume ITE trip generation manual may simply take the suburban style trip generation rates and apply them. But your article fails to recognize that the vast majority of savvy transportation professionals know the intricacies of trip generation and are doing a lot to develop better estimates of traffic for mixed-use developments. Particularly in Southern California and the Bay Area, most in the transportation field recognize the concept of chained trips and the inherent trip reduction of mixed-use developments. Unfortunately the tone of your article is such that a reader may just say, “well, the Institute of Transportation Engineers doesn’t understand mixed-use development or the principles of new urbanism.” I take offense to that, as it belittles my profession, my training, and my personal goals of ensuring safe and efficient movement for all users of our transportation system. Please recognize that the Institute of Transportation Engineers “gets it.”

  4. Contrary to statements made in the blog, the current ITE Trip Generation Handbook contains a recommended method for reducing overall ITE vehicle trip estimates based on internal capture between land uses within a mixed-use development. The current method does not explicitly take into account walking and transit because of limitations in the data on which the method was developed. This limitation is clearly explained in the ITE handbook and it recommends that the user make adjustments if the development is not in a suburban, stand-alone setting.

    ITE is currently in the process of updating the trip generation guidance. Several new methodologies from various sources, including EPA, are under consideration. The new ITE procedures for estimating trip generation for mixed-use development (as well as for urban infill and transit-oriented development) are planned to be published for public comment in the fall of 2013.

    As a note, ITE is always receptive to receiving new data sources, especially for mixed-use, urban infill, or transit-oriented development. Better data will lead to better methods which in turn will lead to better applications.

  5. As coauthor of the MXD research Reid Ewing describes and as a member of ITE, I’d like to clarify a few points. ITE is aware that the Handbook methods are insufficient and is attempting to improve them. We only wish they could move faster, as it’s been over four years since we submitted the MXD research to ITE for review. So, as new urbanists and multi-modal transportation planners are working toward the same goals, I suggest we focus on achieving those goals rather than questioning ITE motives.

    While the current ITE Handbook methods account for some trip reduction, they still overestimate traffic generation by 35%. The MXD method reduces that overestimate to 4%, essentially removing the bias against mixed-use while retaining a reasonable safety factor. The newest MXD version combines the EPA research with research from the Transportation Research Board (NCHRP 684), and it explains 97% of the variation in traffic among different types of mixed-use sites. We feel it’s a long-overdue means of removing the traffic study bias against mixed-use and other forms of smart growth development.

    In addition to the approvals Reid mentions, the MXD method has been tested and approved by the San Diego Association of Governments, and my firm, Fehr & Peers, has used MXD for over a dozen approved environmental documents in the western US. The method’s been peer reviewed and has been published by the American Society of Civil Engineers, the National Association of Environmental Professionals, and the American Planning Association. The APA Planning Advisory Service memo was published in May for PAS subscribers: . In mid-June, I’ll be able to send the latest information to non-subscribers who contact me at .

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