Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.0Capacity Manual procedures, and are important primarily in links with long steep uphill grades.The reductions apply only to a subset of freeway and expressway links in the OKI portion of thehighway network. The OKI Regional Council was responsible for calculating the capacityreductions.In addition to these customized speed-delay functions, two other sets of parameters were takenfrom the Model v54 assignment process: the capacity factor and the time and distance factors.The capacity factors are necessary to factor up hourly capacities to values representative of thetime period being considered in the assignment. The capacity factor indicates, for eachassignment period, what proportion of trips occurs in the most congested hour of each timeperiod. These values were initially derived from the time-in-motion data described in Section 2,and may be adjusted during model validation. The consolidated model uses the same factorsderived for Model v54 (see Table 3.1), except for the night time factor. The latter wasrecalculated from the time-in-motion data because the prior factor (0.10) is unreasonable.Table 3-1 Capacity FactorsTrip Assignment PeriodCapacity FactorMorning (AM) 6:00 AM - 8:30 AM 0.53Midday (MD) 8:30 AM - 3:00 PM 0.23Evening (PM) 3:00 PM - 6:30 PM 0.35Night (NT) 6:30 PM - 6:00 AM 0.36Time and distance factors are used to calculate a composite impedance, which in turn is used tofind the minimum path between each origin and destination during assignment. The standardmethodology is to use a time factor of 1.0 and a distance factor of 0.0, which results in minimumtravel time paths and a travel time user equilibrium. The OKI Model v54 uses a time factor of0.414 and a distance factor of 0.46. The effect of the distance factor is to favor less circuitous,but slower, paths. The consolidated model uses the factors originally developed for Model v54.An important new feature was added to the OKI/MVRPC model that did not exist in version 5.4 ofthe OKI model: feedback iteration until model convergence. The previous version of the OKImodel applied only one feedback loop, but no check was made at the end of this loop to ensurethat the estimated AM speeds approximated well the initial, assumed AM speeds. Theconvergence algorithm added to the OKI/MVRPC model checks, at the end of each feedbackloop, whether the model has converged. If it has not, then the estimated speeds are fed back tothe highway network build step and the full model run is repeated until convergence is reached.The following two criteria need to be met for the model to converge:• Link convergence: at least 95% of all links have an assigned ADT volume that is within 10%of the volume assigned in the previous model iteration.• Trip table convergence: at least 95% of the OD interchanges have a number of trips that iswithin 10% of the trips estimated for the OD interchange in the previous model iteration.The trip table convergence is applied at the district level (i.e., 300x300 trip table instead of a2531x2531 trip table). Districts with less than 10 trips are not included in the convergencecheck, because oftentimes the convergence criteria are exceeded simply due to bucket rounding.Assignment and Validation - Highway Assignment Methodology 6