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The Role of Trip Generation On Right-Sizing Road Infrastructure


By Alejandro Angel, PhD, PE, PTOE, ENV SP and Darlene Danehy, PE, TE, PTOE

The Role of Trip Generation On Right-Sizing Road Infrastructure

What typically comes to mind in discussions about sizing roadways are congestion and the need to widen intersections. But what about  the opposite issue, one we also come across often, but tend to think about less. We are referring to over-built facilities, which are most common in suburban areas. Have you been unnecessarily stopped at an intersection and thought, “who decided a signal was needed here?” Or have you wondered why a little used roadway was built as a four-lane facility?

For decades, calculating how many trips a project would be expected to generate has been done through Traffic Impact Studies based on trip generation rates provided in the Institute of Transportation Engineers (ITE) Trip Generation Manual.  The resulting volumes then dictate infrastructure needs, such as the need for new or widened roadways, turn lanes, access points, and/or traffic signals.

Because single-family subdivisions are the predominant type of development in suburban contexts, that land use is arguably the most commonly used category in the manual. Unfortunately, as shown below, while the Trip Generation Manual provides trip rates based on more than 300 studies, over 90% of those studies are more than 20 years old.

Needless to say there have been many societal changes over the last 20 years that affect travel behavior. An example is the growth of Travel Demand Management strategies such as telecommuting, flexible work hours to encourage off-peak travel, and emphasis on alternative transportation modes. Likewise, e-commerce has gone from non-existent to representing 25% of all retail sales (excluding food services) by the end of 2016. These changes have also been paired with trends towards more infill projects, flexible zoning to support mixed-uses, and other land use changes.

As a result, it’s appropriate to ask whether we should be weighing “old” data so heavily when making infrastructure decisions. Currently, we often forecast traffic for the year 2040 based on data that at that point will be more than 50 years old.

While preparing a Traffic Impact Study for a suburban, large-scale residential master plan in the Tucson area, Psomas decided to study trip generation rates for similar large subdivisions in Tucson to verify whether the information in the Trip Generation Manual was valid for those projects.  The developments studied ranged from 700 to 2,000 single-family homes. Additional independent counts from Pima County, in which Tucson is located, were also used to verify the findings.

The comparison of the residential trip generation rates from the manual and those collected in the field is shown below. As indicated, the measured trip rate during the evening peak hour is one-half the rate predicted by ITE, while the daily rate is 40% lower than indicated in the manual. In other words, a 1,000 home subdivision would produce 500 trips in the evening peak period instead of the 1,000 trips predicted by the manual.

What does this mean? It means that where a traffic signal would typically be recommended, a stop-controlled intersection may work. Large multi-lane arterials would sometimes be replaced with less costly roads with fewer lanes and smaller footprints. Furthermore, the decision to oversize roadways may keep some projects from being financially viable, while potentially reducing investment in other infrastructure such as parks and utilities for projects that are still viable.

 

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