Innovations in Travel Demand Modeling, Volume 2: Papers (2008)

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toll rates are fed into the toll optimizer, which uses them to select an initial estimate of a set of tolls that meet the con- straints, while also optimizing the policy goals. These toll levels are then fed back into the travel model; the outputs from this run are examined by the toll optimizer and given a score, based on how well they meet objectives and con- straints. The toll optimizer uses these results to create a new estimate of optimal tolls, which are fed back into the travel model. This process continues as the scores of the resultant toll scenarios increase by a threshold amount. Once the deltas between scores drop below this threshold amount, the tolls are considered optimized. ADDITIONAL AREAS OF RESEARCH There are additional limitations of existing models that should be addressed, as follows: • Lack of representation of modal options in distri - bution models; • Lack of representation of reliability in evaluating travel choices; • Inability of static demand models to represent dynamic pricing options; • Need to evaluate fairness as important in imple- mentation; • Need to represent overall societal benefits for road pricing strategies; • Need to represent safety as a performance mea- sure; and • Need to better understand and communicate risk and uncertainty. The authors believe that innovative approaches can be developed and integrated with existing models to address these issues and that this will significantly improve forecasting of the impacts of pricing strategies. For example, the lack of representation of modal options in trip distribution models means that for pricing strate- gies that allow carpools or transit users to travel toll- free, the impact of tolls on trip distribution patterns needs to be performed for toll users and toll-free users separately. Simultaneous trip distribution and mode choice models would address this particular issue, but there are few of these available and they have not been used in pricing studies (to the authors’ knowledge). Another issue is the lack of reliability in evaluating pricing strategies. Previous research indicates that travel time reliability is as important as value of time, if not more so. At the same time, there has been less research on how reliability affects travelers’ route choices. Although great strides have been made in measuring reli- ability, there is less progress in considering reliability in forecasting models. Another consideration for any pricing study is that an important driver of travel demand is growth in house- hold, employment, and income levels. It is common to use the socioeconomic data approved by the planning agencies within a region, and while these forecasts may work for the purpose for which they were intended, they have not been evaluated for their suitability for use in traffic forecasts intended to provide conservative assumptions for purpose of revenue estimates. Indeed, planning forecasts for typical projects may be conserva- tive in the other direction, trying to anticipate worst-case scenarios for future highway needs. REFERENCES 1. Expert Forum on Road Pricing. Sponsored by Federal Highway Administration, Arlington, Va., November 14–15, 2005. 2. Cambridge Systematics. Washington State Comprehensive Toll Study Interim Report. Background Paper No. 6: Limi- tations of Studies Used to Advance Toll Projects, prepared for the Washington State Transportation Commission, November 2005. 3. Cambridge Systematics. Washington State Comprehensive Toll Study Interim Report. Background Paper No. 2: Ascer- tainment Interviews: Opinion of Washington’s Community Leaders, prepared for the Washington State Transportation Commission, November 2005. 4. Bhat, C. R., and J. L. Steed. A Continuous-Time Model of Departure Time Choice for Urban Shopping Trips. Trans- portation Research Part B: Methodological, Vol. 36, No. 3, 2002, pp. 207–224. 5. Kuppam, A. R., M. L. Outwater, M. Bradley, L. Blain, R. Tung, and S. Yan. Application of Time-of-Day Choice Models Using EMME/2 – Washington State DOT Conges- tion Relief Analysis. Presented at 19th International EMME/2 User’s Group Conference, Seattle, Wash., Octo- ber 19–21, 2005. 149INNOVATIVE METHODS FOR PRICING STUDIES