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I N N O VAT I V E M E T H O D S F O R P R I C I N G S T U D I E S 149
toll rates are fed into the toll optimizer, which uses them to Another issue is the lack of reliability in evaluating
select an initial estimate of a set of tolls that meet the con- pricing strategies. Previous research indicates that travel
straints, while also optimizing the policy goals. These toll time reliability is as important as value of time, if not
levels are then fed back into the travel model; the outputs more so. At the same time, there has been less research
from this run are examined by the toll optimizer and given on how reliability affects travelers' route choices.
a score, based on how well they meet objectives and con- Although great strides have been made in measuring reli-
straints. The toll optimizer uses these results to create a ability, there is less progress in considering reliability in
new estimate of optimal tolls, which are fed back into the forecasting models.
travel model. This process continues as the scores of the Another consideration for any pricing study is that an
resultant toll scenarios increase by a threshold amount. important driver of travel demand is growth in house-
Once the deltas between scores drop below this threshold hold, employment, and income levels. It is common to
amount, the tolls are considered optimized. 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
ADDITIONAL AREAS OF RESEARCH have not been evaluated for their suitability for use in
traffic forecasts intended to provide conservative
There are additional limitations of existing models that assumptions for purpose of revenue estimates. Indeed,
should be addressed, as follows: planning forecasts for typical projects may be conserva-
tive in the other direction, trying to anticipate worst-case
· Lack of representation of modal options in distri- scenarios for future highway needs.
bution models;
· Lack of representation of reliability in evaluating
travel choices; REFERENCES
· Inability of static demand models to represent
dynamic pricing options; 1. Expert Forum on Road Pricing. Sponsored by Federal
· Need to evaluate fairness as important in imple- Highway Administration, Arlington, Va., November
mentation; 1415, 2005.
· Need to represent overall societal benefits for road 2. Cambridge Systematics. Washington State Comprehensive
pricing strategies; Toll Study Interim Report. Background Paper No. 6: Limi-
· Need to represent safety as a performance mea- tations of Studies Used to Advance Toll Projects, prepared
sure; and for the Washington State Transportation Commission,
· Need to better understand and communicate risk November 2005.
and uncertainty. 3. Cambridge Systematics. Washington State Comprehensive
Toll Study Interim Report. Background Paper No. 2: Ascer-
The authors believe that innovative approaches can tainment Interviews: Opinion of Washington's Community
be developed and integrated with existing models to Leaders, prepared for the Washington State Transportation
address these issues and that this will significantly Commission, November 2005.
improve forecasting of the impacts of pricing strategies. 4. Bhat, C. R., and J. L. Steed. A Continuous-Time Model of
For example, the lack of representation of modal options Departure Time Choice for Urban Shopping Trips. Trans-
in trip distribution models means that for pricing strate- portation Research Part B: Methodological, Vol. 36, No. 3,
gies that allow carpools or transit users to travel toll- 2002, pp. 207224.
free, the impact of tolls on trip distribution patterns 5. Kuppam, A. R., M. L. Outwater, M. Bradley, L. Blain, R.
needs to be performed for toll users and toll-free users Tung, and S. Yan. Application of Time-of-Day Choice
separately. Simultaneous trip distribution and mode Models Using EMME/2 Washington State DOT Conges-
choice models would address this particular issue, but tion Relief Analysis. Presented at 19th International
there are few of these available and they have not been EMME/2 User's Group Conference, Seattle, Wash., Octo-
used in pricing studies (to the authors' knowledge). ber 1921, 2005.
