Current trip generation models are insensitive to the effects of travel demand management (TDM) strategies. To evaluate the potential effectiveness of TDM solutions, transportation professionals must rely largely on the results of case studies, which can not be generalized for areas other than the one in which the study was performed. To evaluate TDM strategies in a context that is sensitive to the unique characteristics of each urban area, TDM strategies should be incorporated into regional travel demand models.
Five TDM strategies affecting trip generation rates were examined: land-use strategies, pricing strategies, telecommunications, alternative work schedules, and on-site facilities. To analyze these strategies, household, person, and trip data from the Puget Sound Transportation Panel (PSTP) were used. Variables derived from the PSTP data that may help explain the impacts of these TDM strategies were evaluated for significance in trip generation models for the following purposes: home-based work, home-based shopping, home-based other, work-other, and other-other. The trip generation models were specified with Poisson and negative binomial regression techniques. After the models had been estimated, the significance of the variables representing the impacts of TDM strategies was analyzed and justified.
Many of the "TDM variables" were indeed significant in the trip generation models; however, in some cases, the significance of the variables can be attributed to factors that are not related to the effects of TDM strategies. For example, the effects of trip chaining appeared to have played a major role in the significance of certain variables. However, some variables appeared to explain the effects of certain TDM strategies quite well. With further research, the four-step modeling process may provide a viable mechanism for evaluating the impacts of TDM strategies on trip generation rates.