Evaluating Travel and Air Quality Cost-Effectiveness of Transportation Demand Management Projects

Many transportation planners and those implementing transportation demand management (TDM) programs have been frustrated by the lack of quantitative information on what types of TDM strategies work best and where. This underscores the need for sound evaluation of TDM programs and demonstration projects. However, many evaluations to date have used a variety of methods and assumptions when quantifying the travel and air quality impacts of TDM projects. A study funded under the AB 2766 vehicle registration fee program in southern California resulted in the development of a standardized methodology and then applied the method to 15 TDM demonstration projects. The method differed from most of the self-evaluations in that it discounted vehicle trip reduction to account for those who switched from one high-occupancy vehicle mode to another and for those who accessed the new commute alternative by driving alone to a pickup point; factored out the emission of shuttle and transit vehicles used in providing new service; and used standardized emission factors to determine reductions in reactive organic gases, carbon monoxide, nitrogen oxide, and fine particulate matter. Results of the application of the method to various TDM projects reveal a range of impacts and point to the inaccuracies of self-reported results, particularly in the area of total emission reductions. More standardization of TDM evaluation methods is called for so that a large data base of consistent and reliable information can be assembled across agencies with the goal of generalizing the effectiveness and transferability of various TDM strategies and programs.