Preserving the symmetry of estimated commuter travel elasticities

Travel price and time elasticities are increasingly being derived from disc rete choice models of the multinomial or nested legit form. These elasticit ies are then applied to obtain predictions of changes in travel demand cons equent on a policy change in prices and travel times. The majority of the c hoice elasticities are estimated within the behavioural setting of modal ch oice, holding total travel fixed. A few mode choice models have recently re laxed the multinomial legit model assumption of equal variance in all the r andom components of the indirect utility function to permit unconstrained v ariances' across all-alternatives (subject to identification for one altern ative). This enables the derivation of behaviourally meaningful and unique cross choice elasticities for each pair of alternatives. Under constant var iance, only the direct choice elasticities have behavioural meaning. While this is an important advance in discrete choice modelling, the derivation o f share elasticities is conditional on a fixed total demand, and the proced ure cannot be relied on to carry through two important properties of the-mo del into the demand elasticity matrix-namely symmetry and zero share weight ed column sums. This paper takes a set of empirically derived choice elasti cities and presents a second stage procedure to adjust these elasticities t o arrive at an internally consistent matrix of demand elasticities. We draw on a recent data set collected in Sydney which utilises revealed preferenc e and stated choice data to estimate a joint model of ticket choice conditi onal on mode and choice of mode for commuter travel. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.