Model Implementation: TRIMMS
To translate the theoretical framework of the prediction model developed in the previous section, we developed a sketch-planning model in the form of a spreadsheet module. Sketch-planning models represent a class of rather straightforward models that focus on the initial or screening phases of analysis. These models are used for situations where performing a more sophisticated analysis is not possible, or requires a costly effort in terms of data gathering.
Spreadsheet modules or applications represent an ideal tool for the first phases of planning and estimation. They are designed in a way which allows someone other than the developer to perform useful work without extensive training. Spreadsheets enable the end user to perform a diverse set of analyses using efficient methods and algorithms to produce accurate results, utilizing an interface that is clear and consistent. Modules can be designed in a fashion so that they can be modified with minimum effort and maximum flexibility; they are portable and run on any system that has commonly used software.
The spreadsheet model takes the acronym of TRIMMS, Trip Reduction Impacts of Mobility Management Strategies. The model takes its name from its main characteristic, namely the estimation of the value of a passenger trip removed as a result of a TDM policy.
Figure 4 depicts the internal structure of TRIMMS, which follows from the theoretical framework developed in the previous section.
Starting from a base case scenario representative of the program or strategy being evaluated, the constant elasticity model estimates final mode share changes, resulting in hard program impacts. If the evaluation extends to soft programs, such as program promotion or any other voluntary behavioral change measure, a separate module is designed to estimate the impacts. The evaluation is carried out in an interactive fashion where the user answers a set of questions, whose results are used to call a priori estimated diversion rates.
The final impact shares are then used to conduct the social welfare analysis that leads to the production of the evaluation metric explained in the previous section, namely the value of a trip removed.