One of the most important resources for determining the transportation impacts of new land development is the Institute of Transportation Engineers (ITE) Trip Generation Manual.
When a new neighborhood, shopping mall, or office complex is proposed, transportation engineers conduct a study that forecasts “trip generation” – or how much traffic will come in and out of it, and many of these studies rely on the Trip Generation Manual.
The current edition of the Trip Generation Manual includes trip-generation rates that are primarily based on suburban sites with little or no transit service, nearby pedestrian amenities, or travel demand management programs. This limitation is based primarily on the lack of data available at the time this report was last published.
ITE continuously seeks new trip- and parking-generation data from the transportation profession in order to provide comprehensive and up-to-date guidelines. Many agencies, organizations, companies, including Arlington County Commuter Services, have assisted ITE by providing this valuable information. ITE appreciates these submittals and strongly encourages more of them in order to update this important transportation resource.
As additional data become available, they are distributed through the periodic updates of the Trip Generation Manual. ITE is currently working on a new, third edition of this handbook that will cover transit-oriented developments, urban infill developments, and person-trip (not just vehicle-trip) generation.
Why is updating this information so important? Simply because the information influences how our cities are built.
Let’s say a major chain would like to build a store on the corner of a street in a largely residential area. Professionals frequently rely on ITE’s trip-generation estimates to give the developers, neighbors, and transportation officials an estimate of new trips that the development will attract and produce. Based on this information, the city or county might decide that a new traffic signal will be necessary at the end of the street in order to keep vehicles moving efficiently or avoid safety problems caused by the new traffic. Perhaps an additional turn lane will be needed to accommodate vehicles turning left into the site. Trip-generation estimates might also influence traffic-signal timings, or the length of each green and red phase.
Trip-generation estimates are also sometimes used for calculating fees (called impact fees) charged to the project’s developers to help pay for transportation improvements elsewhere in the jurisdiction. For example, a developer may be charged a set dollar amount for every new trip generated by the proposed site.
Having more data, from a wider variety of sources that document more types of travel situations, helps engineers, planners, developers, and the community make more informed decisions.
Photo by Washington State DOT