Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.0The implications of these “mixed distribution” curves are somewhat longer average trip-lengths in theseed matrix, compared with the exponential formulations. The synthetic trip matrix estimation processwill adjust the seed matrix flows to match the truck counts on certain network links, changing the shapeof the distribution to some degree. The exceptions are intra-zonal (FAZ) flows, because they are neverloaded on the network during the SME process. The final intra-zonal flows will be the same as in theseed matrix.The travel time skims used in the gravity model were based on an assignment of daily passenger-vehicletrip tables to the full CRM network, imported into TransCAD. The volume-delay function used was theBureau of Public Roads (BPR) function used in the user-equilibrium assignment of the passenger-vehiclesand is formulated asT = T +cV β[ 1 α ( ) ]0 Cwhere T c is the calculated congested travel time resulting from the volume-delay function and T 0 is thefree-flow travel time. Hourly capacities and the values of the α and β parameters in the function areconsistent with those developed by OKI for specific link functional class and area types.Since the SME procedure is set up to calibrate the truck model according to daily traffic count data,consistency dictates that the distribution of SU and MU truck trips in creating the seed matrix be basedon an average daily travel condition, rather than separate peak and off-peak travel time skims. This isfurther supported by the diurnal patterns of truck trips, shown in Section 2.5, which are spread moreevenly throughout the day than passenger vehicle trips. Accordingly, diurnal vehicle trip data wereobtained from OKI and used to develop a daily link capacity factor to convert hourly capacities to dailyequivalents, which resulted in the multiplication of hourly capacities by a factor of 13.03. A userequilibriumassignment of the daily passenger vehicle table was performed and the travel times skimmedfrom the loaded network.FAZ centroid connectors were assigned a speed of 25 miles-per-hour, with total travel time based on linklength, to reflect intra-zonal travel conditions. External station connectors were assigned a nominal traveltime of five minutes.Using the expected productions and attractions from the truck trip generation phase along with thesenetwork skim times, the gravity model was run to produce internal-internal flow estimates for both SUand MU truck types. These internal-internal flow estimates constitute the seed matrix for each trucktype. These initial estimates of SU and MU truck trip flows are then calibrated using SME.2.4 Synthetic Matrix EstimationThe SME calibration step of the truck model utilizes the single-path matrix estimation algorithmimplemented in TransCAD. SU and MU truck trip tables are calibrated separately. In order to preservethe EE/EI flows in the ODOT external station survey, a multi-stage SME implementation was developed.2.4.1 Description of single-path matrix estimation algorithmA TransCAD implementation of the single-path matrix estimation (SPME) algorithm (Nielsen 1993) wasused for this project. The algorithm differs from other synthetic matrix estimation algorithms in that ittreats traffic counts on the network links as stochastic variables. In doing so, it minimizes the differenceTruck Model - Base-Year Truck Model Development 20