The traditional data used to estimate the transportation impact of urban development are not appropriate for smart growth or infill development, according to a study conducted by the Texas A&M Transportation Institute (TTI) for the California Department of Transportation (Caltrans). (Infill development refers to building on vacant land within a developed area.) As a result of the study, Caltrans is now urging smart-growth developers to use TTI’s new vehicle trip-generation tool instead of the traditional suburban rates from the Institute of Transportation Engineers (ITE).
“The ITE rates were developed mainly for suburban sites in single-land-use areas, where most travel is by motor vehicles,” explains TTI Senior Research Engineer Brian Bochner. “With these relatively new and growing-in-popularity smart- growth developments in urban areas, there are usually nearby access to transit, more and better pedestrian facilities, and more bike lanes and facilities. So, it makes sense that the number of vehicle trips would be fewer than in traditional suburban areas. Our study showed that there were considerably fewer vehicle trips in smart-growth areas than what would be estimated using traditional vehicle trip-generation rates.”
Most agencies that review the transportation impacts of proposed developments require developers to conduct a transportation impact analysis, with a major element being a trip-generation estimate. After all, it’s the additional motor vehicle trips to and from the site that contribute to air pollution, traffic congestion, the need for surrounding road improvements (like additional signals or traffic lanes), and the right number of parking spaces for the project.
If impacts are overestimated, state and local agencies might require excessive mitigation measures. Developers are usually required to provide road improvements or charge fees to offset the problems their projects are expected to create. In worst-case scenarios, a planned development might be reduced in size or scope or even scrapped because of the extra costs or anticipated consequences.
“For some time, we’ve had a pretty good idea that trip-generation estimates were not accurate for our infill areas,” says Marc Birnbaum, Caltrans project manager on the trip-generation study. “Transportation experts and developers speculated that the estimates were too high. Others thought smart-growth areas would somehow generate as little as zero vehicle travel. From an equity perspective, we need to get this right.”
The goal of the project was to produce a validated and improved estimation method and, as a result, create a user-friendly tool to more accurately estimate trip generation. TTI researchers collected actual trip-generation data from 30 smart-growth sites (e.g., apartments and office complexes) in California and then compared the findings with the estimates generated from traditional suburban data.
“On average, we found that there were 44 percent fewer actual vehicle trips for smart-growth apartments and 49 percent fewer actual trips for office buildings,” TTI Associate Transportation Researcher Michael Martin explains.
Using the data derived from the project, TTI developed a spreadsheet estimator tool that automatically computes the trip-generation estimation for smart-growth developments.
“First, the tool allows users to determine whether the proposed development qualifies as a smart-growth site so that the generated trip estimates are appropriate,” Martin says. “After about 20 questions are answered, a one-page report is created that covers site information, eligibility criteria, input data and vehicle trip-generation estimates.”
Since the TTI project was completed, word is getting out that there’s a better way to estimate trip generation for apartment and office smart-growth developments. The tool is available on the project website at https://tti.tamu.edu/featured-project/smart-growth-trip-generation-study-in-california/, and the data have been incorporated into ITE’s Trip Generation Manual. A possible future next phase of the TTI study could focus on trip-generation estimates for retail and hotel developments.
“We always felt that this project had national implications for smart growth, not only in California,” Birnbaum says. “We are extremely pleased with the results of the study and the simple way developers can now determine accurate trip- generation estimates for apartment and office developments in smart-growth areas.”
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