Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.0applied to the IE/EI trip records for this station to preserve its two-way total of 15,970.76 external trucktripends. ODOT also determined that the automatic traffic recorders at this station had counted asuspiciously low number of vehicles in the survey direction; therefore, the memo recommended using thetotals for the non-survey direction as the counts for both directions. These corrective measures wereimplemented to create EE, IE and EI truck trip tables.As a final step, the expanded data were classified into SU and MU categories by applying the proportionsshown in Table 2-3 (Section 2.1.4) based on the average distribution of SU versus MU trucks forroadways of certain functional classes. The final expanded data were aggregated across origindestinationpairs to produce EE, IE and EI estimates for separate SU and MU vehicle classes.2.3 Seed Matrix DevelopmentThe seed matrix is an initial estimate of the FAZ-to-FAZ truck O-D tables. The SME process modifies theseed matrix interchange values by factoring them up or down, such that when the table is assigned tothe truck network the flows come as close as possible to the truck counts on the network links. The seedmatrix thus plays the important role of establishing a general spatial flow pattern for the region.The seed matrix was constructed by applying standard trip generation and distribution techniques. First,the expected numbers of SU and MU truck productions and attractions were generated for each FAZbased on employment and household totals. Next, a gravity model was used to distribute theseproductions and attractions from FAZ to FAZ. Congested travel time skims were used to calculate frictionfactors in the gravity model. Both of these steps are described in detail below.2.3.1 Truck trip generationDaily truck trip generation equations were applied to 1995 employment and household totals to generateSU and MU truck trip productions and attractions for each FAZ. The coefficients shown in Table 2-5,below, are based on a Phoenix, Arizona, study and were recommended for use as default values in theQuick Response Freight Manual (QRFM) because they provide an internally consistent set of rates, asopposed to combining rates from different studies with potentially misleading or unclear definitions(USDOT 1996, pp. 4-3 to 4-4). The QRFM rates are assumed to apply to either productions orattractions. There is a coefficient for households and four employment-based coefficients for groupingsof industries with similar truck trip generation characteristics.It was noted in the QRFM that these rates were increased to account for under-reporting and for trips notsurveyed because one end lay outside the study region (USDOT 1996, p. 4-4). The rates for MU truckwere increased disproportionately, because they are used for inter-city shipments to a much higherdegree than SU trucks. The expanded ODOT external station survey was to be used for the externaltruck trip ends; therefore, it was necessary to adjust the trip generation coefficients to reflect ratesappropriate for the generation of internal-internal trips only.To correct for potential over-prediction bias, correction factors were calculated as follows:CorrectionQRFMFactor =TripEnds − External StationQRFM Trip EndsIE,EITripEndsTruck Model - Base-Year Truck Model Development 14