It has been demonstrated how the modeling technique provides a consistent approach to predict travel behavior change brought about by both hard and soft TDM programs. The next step is to link behavioral change to impacts within an evaluation framework that provides cost effectiveness benchmarking and allows TDM to be compared to traditional infrastructure investments.
The estimated number of trips reduced must be linked to a set of benefits or impacts. It is therefore relevant to determine what should constitute the set of appropriate benefits to be used for final evaluation. The literature review uncovered two main factors that lead to a proper evaluation of TDM strategies:
TDM evaluation must be comprehensive, embracing benefits that directly relate to the users and indirectly to all other individuals, and it should
Lead to a framework of consumer demand conducive to a benefit cost analysis approach for comparative assessment.
The literature on TDM benefits shows how these are directly related to cost generated by SOV use. Generally, costs that are borne directly by the users are defined as internal costs and those costs that are not directly borne by the users are defined as external costs. The latter societal costs belong to what economists describe as negative externalities. Negative externalities arise whenever costs associated with SOV use, such as added congestion delay, air pollution, and increased accident risk, are not directly sustained by auto users but are rather imposed on the society as a whole.
Although a comprehensive approach should capture all impacts whether internal or external, the proper set of impacts should be defined based on the nature of the decision- maker. If the evaluation has an employer focus, then the set of impacts might include benefits mainly representing actual savings to employers. If the evaluation is carried out by a public decision making agent, such as a transit agency or a government body, then the set of impacts should be more inclusive of negative externalities. If the evaluation has an employer focus, then the set of impacts might include benefits mainly representing actual savings to employers.
Table 5 shows the range of transportation costs categories or TDM benefits that comprise the set of evaluation criteria to be used within the proposed modeling framework. It consists of a subset of all possible impacts that might be generated by TDM policies, focusing on those that can be readily monetized in a consistent and robust fashion. The list of impacts is also consistent with what is currently employed in evaluating traditional infrastructure investments. 15
15 Although, in most transportation infrastructure investment decisions a subset of impacts is usually considered, such as travel time, vehicle operating, accident, and pollution costs.