Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.02. Mode Choice Model SpecificationMode choice models are mathematical expressions used to estimate the modal shares of thetravel market given the time and cost characteristics of the various competing modes and thedemographic and socio-economic characteristics of the urban residents. Mode choice models aredesigned to be an integral link in the travel demand chain, and may have direct feedbackmechanisms to a number of related model components – auto ownership, trip generation, tripdistribution, and (modal) trip assignment.2.1 Logit StructuresThe mode choice model structure recommended for the OKI/MVRPC regional model is a nestedlogit mode choice model, as opposed to a hierarchical or multinomial logit model. Figure 2.1illustrates the differences between the various mode choice model structures.The multinomial logit model (shown on the top of Figure 2.1) assumes that there is equalcompetition among alternatives. This allows for the “shifting” of trips to and from other modes inproportion to the initial estimates of these modes. A common problem typically associated withthe multinomial structure is the potential for violation of the Independence from IrrelevantAlternatives (IIA) property.The hierarchical logit model (shown in the middle of Figure 2.1) is a variation of the multinomialmodel that allows for the subsequent splitting (or allocation) of trips to a set of submodes. Inmost structures of this type a logsum variable (or the denominator of the lower level choice) isused in the upper level choice together with other (typically socio-economic) explanatoryvariables. In this manner, the lower level submodes are reflected in the upper level choice, but asif they were equally competing modes with the other primary mode(s) (i.e., with a logsumcoefficient of 1.0).A nested logit model (shown at the bottom of Figure 2.1) recognizes the potential for somethingother than equal competition among modes. This structure assumes that modes, submodes, andaccess modes are distinctly different types of alternatives that present distinct choices totravelers. Its most important departure from the multinomial structure is that the lower levelchoices are more elastic than they would be in the multinomial or hierarchical structures. Thus,an improvement in walk access to transit would alter the existing diversions between walk anddrive access to transit the most. This same improvement in walk access would also shift travelersfrom auto to transit, but with elasticities that are equal to the elasticities found in the multinomiallogit models; therefore, the elasticities for access choice are higher. This increased sensitivity isreasonable if the modes included in a single level of the nest are reasonably related. It seemsintuitive that a person who has already decided to use transit would be more sensitive, withrespect to accessing the transit system, to a change in transit travel time or cost, than would bea person who is deciding to use transit or not.Mode Choice - Mode Choice Model Specification 2