Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.02. Gravity Model Calibration2.1 BackgroundThe OKI/MVRPC consolidated trip distribution model departs in several ways from the originalOKI (Model 5.4) trip distribution model. The original Model 5.4 trip distribution model usesgravity models to distribute daily HBW, HBO, HBU and NHB trip production and attractions. Theimpedance measure used is a composite highway travel time, constructed as the weighted sumof the peak travel time and the off peak travel time. The weights are defined as the proportionof trips occurring at each time period, and they vary by trip purpose.While this method obviates the need to maintain separate peak and off peak friction factors, itrelies on the assumption that peak and off-peak trip patterns are identical. There is of course noevidence that this is the case. Instead of relying on this assumption, different peak and off-peakfriction factors were developed for the OKI/MVRPC model.The use of highway travel time as the impedance measure ignores the effects that cost andtransit service may have on trip distribution. It is in fact possible to formulate an approach inwhich all modes and all components of travel (cost, time and distance) affect the distribution oftravel. This requires the use of a composite measure of accessibility, the logsum, which isobtained from the denominator of the mode choice model.The logsum measures the spatial separation between zones giving adequate consideration totravel time, travel cost and other variables included in the mode choice model. This impedancealso gives weight to household characteristics of the traveler, such as income or auto ownership,through the use of these characteristics in the stratification of mode-specific constants. Thelogsum has all the desirable characteristics of impedance measures, including:• Its value decreases as any mode improves, that is, as the time or cost decreases.• Its value increases if any mode is unavailable.For use in trip distribution, the logsum is calculated at the upper level of the mode choice nestand its value scaled by the in-vehicle time coefficient, to obtain equivalent minutes of travel.Trip Distribution - Gravity Model Calibration 2