Content Type: Blog entry

By Kristina M. Currans

In August 2014, the Institute of Transportation Engineers released the 3rd edition of the Trip Generation Handbook, a 352-page text that has traditionally, until only recently, provided guidance on estimating vehicle trips generated from new development. Among other updates, this new edition includes new chapters summarizing the most recent research and methods developed allowing users to account for people (not just vehicles) in trip generation estimation practices. This industry’s transition to estimating and understanding the “people” traveling to development has been in high demand from communities looking to accommodate multimodal travel, but there still remain a number of limitations in the guidelines presented.

One such limitation is that we currently have few data that allow us to directly estimate person trip rates. Instead we are often required to: (1) estimate vehicle trips using suburban vehicle trip rate data, defined as a “base rate”; (2) convert the vehicle trips into an estimated person trips using an assumed mode share and vehicle occupancy rate from the ITE’s suburban vehicle-oriented data, and; (3) reallocate the estimated person trips into different modes (bike, walk, drive, transit, etc.) based on the urban context of the development. This “direct mode share adjustment” forces the user, first, to assume that person trip rates do not vary across urban contexts and, second, to...

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Content Type: News Item

Planners looking to develop an dense mix of urban land uses often face a dilemma: they’re using trip-generation models that undercount the very trips on bicycle and foot that the planners encourage while paving the way for more driving.

Portland State University Associate Professor Kelly Clifton dove into the topic Monday, presenting a paper at the Transportation Research Board annual meeting in Washington D.C. Kristina Currans of PSU, April Cutter of Metro, and Robert Schneider of the University of California Berkeley are coauthors.

Clifton and other panelists agreed that the Institute of Transportation Engineers’ trip generation rates don’t adequately reflect actual trips in an urban area with multiple land uses and transportation modes. They differed on the remedy, however.

Presenting another paper, Associate Professor Kevan Shafizadeh of California State University Sacramento, evaluated and tested several complex methods, finding that none truly suits smart-growth development projects. However, Shafizadeh and his team found that every method tested does a better job at predicting the number of trips generated than the ITE rates.

We need to predict trips better, Clifton said, but perhaps a simpler solution exists. She acknowledged Shafizadeh’s conclusions that each alternative also had deficiencies in certain applications, and suggested that the ITE rates could be tweaked instead of scrapped entirely.

“It’s a Band-Aid to temporarily...

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