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E M E R G I N G M O D E L I N G C O N S I D E R AT I O N S 49 less than $25,000 income group, the value of time was The model specification is a multinomial logit $10.64 and the equivalent minutes for a $3.00 toll were structure with 32 alternatives. The trip assignment uses 16.9. The value of time for a heavy-duty truck was $40.00 four feedback loops to equilibrate congested times based and the equivalent minutes for a $3.00 toll were 4.4. on lower values of time. Final iteration is based on higher The impact of pricing on model components--trip values of time by market segment. The iterative assign- distribution, mode choice, time-of-day choice, and trip ments are based on the five time periods. The objective assignment--was examined. The trip distribution com- of the toll optimization tool is throughput or revenue ponent incorporates generalized cost in minutes for travel maximization with the constraints of achieving a target time. The cost was converted to time based on the value level of service. of time by market segment. Four feedback loops are used A number of conclusions emerged from the study. to equilibrate congested times. The modal representation First, the use of values of time by market segment enables in the distribution are work trips based on log-sum or better evaluation of pricing scenarios. Second, time-of- weighted average of times and costs by mode. Nonwork day choice models can be estimated with 30+ time peri- carpool and transit trips are distributed based on no-toll ods with existing data. Third, models are sensitive to travel patterns. In mode choice, travel time and cost are time and cost trade-offs, as well as demographic factors considered separately by mode and trip purpose. Lower and bridge constraints. Fourth, calibration by mode, trip values of time were used in the final assignments. purpose, and direction, as well as for volumes, provides A logit time-of-day choice model was applied after more behavioral understanding of results. mode choice to automobile trips. There are 32 time peri- A number of areas for additional research were ods. The time periods are in half-hour increments, except identified. These areas include examining the lack of rep- for the first and last periods. Variables include demo- resentation of modal options in distribution models and graphics, trip characteristics, and delay. The model also the lack of representation of reliability in evaluating includes costs measured in units of time and the use of a travel choices. The inability of static-demand models to nonlinear shift variable within three larger time periods. represent dynamic pricing options represents another The time periods included the morning peak, mid- area for further research. The study identified the need to day, the afternoon peak, evening, and night. The morn- evaluate fairness as an important factor in implementa- ing peak includes 10 time periods in 30-minute tion and to represent overall societal benefits for road increments from 5:00 a.m. to 10:00 a.m. The midday pricing strategies. It was also noted that there was a need includes 10 time periods in 30-minute increments from to represent safety as a performance measure and to bet- 10:00 a.m. to 3:00 p.m. The afternoon peak includes 10 ter understand and communicate risk and uncertainty. time periods in 30-minute increments from 3:00 p.m. to 8:00 p.m. The evening includes one 3-hour time period from 8:00 p.m. to 11:00 p.m. The night includes one 6- Rachel Gossen, Oakland Metropolitan Transportation hour time period from 11:00 p.m. to 5:00 a.m. Commission, moderated this session.