Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.03. Highway Assignment MethodologyThe OKI/MVRPC consolidated model uses multi-class equilibrium assignment to load trips ontothe highway networks. Five trip classes or modes are considered, three for passenger traffic(single-occupant cars, two-occupant cars, three or more occupant cars) and two for truck traffic(single unit trucks and multiple unit trucks). This class separation allows the modeling ofrestrictions in network usage, to represent for example HOV lanes, which should be used only bymultiple occupant vehicles, or truck prohibitions, to keep multiple unit trucks from using certainstreets.Highway assignments are performed in three separate steps of the model: initial loop, feedbackloops and final loop. The purpose of the initial and feedback loops is to calculate congested peakperiod speeds to feed back into trip distribution and mode choice. Thus, only the AM trips areassigned during these loops. The full set of highway networks (AM, MD, PM and NT) are loadedat the end of the feedback process, once the AM assignment has converged.The consolidated model uses the volume-delay functions developed for OKI Model v54 1 . Thereare five functions, one for each facility type. Figure 3.1 shows the functions developed forfreeways and major roads. The volume-delay functions are used to calculate the degradation infree-flow speed (i.e. the congested speed) that results from non-zero traffic volumes.Figure 3-1 OKI Model v54 Volume-Delay Functions1.0Congested/FreeFlowSpeed0.80.60.40.2BPRFreewaysMajor Road0.00.0 0.5 1.0 1.5 2.0 2.5Volume/CapacityIn this multi-class assignment, the link volume input to the volume-delay function is the sum ofall vehicle modes using the link. In particular, truck vehicles are counted as one unit each,instead of factoring them to passenger car equivalents. The effect of trucks on traffic flow speedis considered instead by a reduction in link capacity. These capacity reductions follow Highway1 See Development of Classified Speed/Capacity Table and Speed-Volume Relationships. Prepared by theOhio-Kentucky-Indiana Regional Council of Governments. June 2000.Assignment and Validation - Highway Assignment Methodology 5