Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.0In the simple nested model structure shown in Figure 2.1, the formulation employs threemultinomial logit models, one for the primary choice of mode among auto and transit, a secondlevel choice among auto submodes (drive-alone and shared-ride) and another second levelchoice among transit access modes (walk and drive access). In application, the modelindependently addresses auto submode and transit access choice first. This is expressed as:P DA =P w =eeUDAUwU DAe+ eU we+ eU DUSRA composite of the utilities of the auto submode and transit access choices then represent autoand transit respectively in the upper tier of the model structure. This composite measure is thenatural logarithm of the denominator of the logit model, often termed the "logsum". The logsumterm is effectively the total utility provided by the submodes of a particular primary mode. Alogsum value is calculated for each of the second level nests as:lnLogSumU UA=-DA[e +SRe ]LogSum U UT= - ln [we +De ]The logsum terms for the auto submodes and transit access choice then appear in the utilityexpression for the primary mode level as:PT=eTeTθ ∗Logsumθ ∗LogsumT+ eATθ ∗LogsumAThe value of the logsum coefficients θ A and θ T in the upper tier of the model (i.e., auto versustransit), is an indicator of the degree to which the lower level choices form a subchoice that isdistinct from the primary mode alternatives. A value of 1.0 indicates that the lower level modesare not a subchoice but rather are full options equally competitive with the primary modes. Inthis instance, these lower level choices can be simplified or included directly in the upper level. Avalue of 0.0 would indicate that the lower level choices are perfect substitutes for each other.Values between 0.0 and 1.0 indicate the extent to which the lower level choices represent asubchoice.2.3 Model StructureThe proposed structure for the OKI/MVRPC mode choice model is depicted in Figure 2.2. In thisstructure, a choice is first made between auto and transit. Under the transit side, the first levelnest distinguishes between local bus, express bus, light rail and commuter rail. The second leveltransit nest models the choice between walk access, park and ride access and kiss and rideaccess to each transit mode. The highway side is divided into drive alone and shared ride, withshared ride further subdivided into 2-person and 3+ person carpools.The ability to estimate all the coefficients and mode-specific constants required by this modelstructure depends, to a large extent, on the on-board survey sample size (by transit submode)and the quality of the data. Clearly, the light rail and commuter rail branches cannot beMode Choice - Mode Choice Model Specification 5