2024-05 Induced Calculations

Distribution of Induced Trips with New Modes and Mode Shifts

TREDIS already offered methods to model new travel that was induced specifically for the case where improved performance in one or more modes that existed in both the Base and Project alternatives. With this release, TREDIS can now consider induced travel for modes that are entirely new to the Project alternative (that is, for modes where the Base alternative has 0 trips).

Additionally, TREDIS now has a more logical methodology for estimating the share of all induced trips among numerous modes. When multiple modes take on additional passengers or freight in the Project alternative, TREDIS now uses a weighted distribution of all travel to determine the share of induced trips for that mode. (As always, the induced trips are measured separately for each purpose, year, travel segment, and region.)

TREDIS does not assume that any trips are induced for modes that lose travelers. Modes with additional travelers gain them proportionately from other modes and latent demand. We believe this logic will generally give reasonable results, despite the ambiguity involved in multi-modal analysis.

Induced Benefit Calculation Clarification

Now that induced trips can be estimated for new modes, TREDIS can calculate the induced benefit of these trips using the "rule of half" to estimate consumer surplus for travelers induced from latent demand. For induced travel, TREDIS compares costs in the Project alternative to costs in the Base alternative and multiplies them by the number of induced trips.

However, since new modes have no cost equivalent in the Base alternative to compare to, TREDIS now handles this ambiguity by comparing the new Project costs per trip to an average cost among all modes (for the given purpose) per trip in the Base alternative, using the following equation:

InducedBenefit = ((AvgCostPerTripBase − CostPerTripProj) / 2) × InducedTrips

This estimate of average cost provides a means to include more scenarios for new technology (both innovative solutions and also just new options to the region or segment) in TREDIS analysis. Because TREDIS deals with aggregate travel flows, the use of the average cost addresses ambiguities about new travelers' true costs of travel, while at the same time maintaining significant flexibility for different use cases.

When the cost per trip for a mode in the Project is greater than the average cost per trip in the Base alternative, the induced benefit is simply set to zero. A mode should not gain travelers unless it is lower cost than the options available to the traveler in the base case. If the benefit is not found in the analysis, it is likely that there are benefits not accounted for in TREDIS for the improved mode (specifically in the Project alternative) or costs not accounted for in the other modes (specifically in the Base alternative) that should be included in the analysis. In reality, different travelers may have different preference sets and constraints that also shape mode choice. Because of this heterogeneity, not all modes may have the same demand function.

This will change how consumer surplus was estimated for previous projects when rerun, as the previous method assumed all new travelers for a mode faced that mode's Base alternative cost. This assumption does not provide an opportunity to consider new offerings in scenarios.