TDM study suggests we are overestimating vehicle trip generation rates

By Robbie Webber

In a recent study done in Melbourne, Australia, researchers compared transportation demand management plans (called “travel plans” in Australia) at four new residential developments with control sites with similar characteristics. The study targeted new developments because studies of TDM plans at existing sites were relatively common—reporting a 10-15 percent reduction of car use—but there was limited evidence of the application at residential sites. The results showed lower car mode share and trip generation in the sites with TDM plans, but also significantly lower rates of vehicle trip generation than those published in commonly-used sources.

In addition to studying the effectiveness of TDM plans, they compared the results to three published trip generation rates: The Institute of Transportation Engineers (ITE) 2008 rates for high-rise apartments, based on data from the United States; Roads and Traffic Authority (RTA) 2002 rate for high-density residential flats, based on data from the Australian state of New South Wales; and Roads and Maritime Services (RMS) 2013 rates for high-density residential flats based on data from Sydney, Australia.

All case study and control locations were high-density apartments. Each of the four study sites had a different set of strategies, including memberships in bike- or car-sharing services, free transit passes, limited car parking availability, transportation information for tenants, umbrellas in the lobby to encourage walking, and online forums to organize carpooling. Control sites had no TDM plans for tenants.

Assessment of mode use was done weekdays during AM peak and Saturdays between 10 AM and 1 PM. Car and bike parking supply and demand counts were also done for weekday AM peaks. The results of the study showed that all study sites had lower car usage than the control sites during the AM peak, and three had lower car usage than the controls on Saturday. On average, the case study sites had 14 percent lower car usage than the control sites. In all case study and control sites, the car mode share was lower than the average for the local government area.

More surprisingly, not only were the case studies lower than the control sites, but in almost all cases even the control sites were far lower than the published trip generation rates, leading the researchers to conclude, “Therefore, in the absence of any control sites, the difference in the car driver mode share would have been grossly overestimated.”

Trip generation chart

The authors go on to speculate about why the published trips generation rates may have been so much higher than what was observed. They conclude that while the surveys on which the rates are based accounted for the type of development—in this case a high-density apartment—all the surveys were for a limited number of sites and possibly locations with differing levels and types of transportation networks from the study sites. Finally, “with the exception of the RMS rates, the published rates are based on surveys conducted more than 20 years ago, which are likely to have observed different travel patterns from those of today.”

The researchers suggest that some tenants may be self-selecting the case study locations because of their existing car use and travel habits, but they also feel that more studies are needed to study the effectiveness of TDM plans in residential locations.

Robbie Webber is a Senior Associate at SSTI.