Is it Necessary to Relax the IID Assumptions in the Logsum-Based Accessibility Analysis?

Abstract

Accessibility is an important link between transportation and land use. As a typical measure of accessibility, logsum or a utility-based measure has been widely used in project appraisal, urban transit accessibility evaluation, destination choice, and network vulnerability analysis. Since the logsum term is the log of the denominator of the choice probability expression, it inherits the independently and identically distributed (IID) assumptions of the classical multinomial logit (MNL) route choice model. This paper aims to explore whether the IID assumptions have a significant effect on the logsum-based accessibility analysis, given that accessibility analysis focuses at the origin-destination (O-D) level and zonal level (aggregate analysis) rather than at the route level (disaggregate analysis). We derive two new logsum terms for two representative extended logit stochastic user equilibrium (SUE) models, that is, the C-logit model for relaxing the independence assumption and the MNL model with scaling effect (MNLs) for relaxing the identically distributed assumption. The case analysis of a real network in Winnipeg, Canada shows that: (1) there does exist a difference in accessibility evaluation among the three logsum terms using the three route choice models; (2) relaxing the identically distributed assumption is more important than the independence assumption since the difference in accessibility evaluation between MNLs-logsum and MNL-logsum is larger than that between C-logit-logsum and MNL-logsum; (3) the difference in accessibility evaluation at the zonal level is smaller than that at the O-D level; and (4) the difference increases with the dispersion parameter.