The reasons why passengers and freight operators choose one form of transport over another are many and varied. There is, however, a view that the influences can be broadly classified into those that are related to time, to cost, and to service quality (the latter being additional to time-related influences). The challenge for any investigation of influences on modal choice is to identify the attributes that describe each class of influence and to place weights on each attribute to establish its relative influence. In addition, it is recognised that not all attributes are easy to measure, and hence are often ignored in an analysis even though they continue to influence modal choice. In addition to identifying attributes that are important, it is generally accepted that behavioural responses to attribute levels and their impact on modal diversion vary by market segment. For example, the commuter typically is willing to pay a higher price to save travel time than a non-commuter for reasons related to the fixity of the arrival time and because commuting is linked to income-earning whereas non-commuting is more leisure-based. The objective of this chapter is to review the key elements of the methods used in practice to evaluate the role of modal attributes and market segment in the explanation of modal split and modal diversion. The author focuses on the discrete-choice modelling approach to studying influences on modal diversion (see Louviere et al. (2001)). The chapter begins with an overview of the changing context for a study of modal diversion, followed by an overview of the methods used to establish the weights for each attribute and the types of policy outputs that analysts typically seek from such models (such as values of travel time savings, share elasticities, and market shares).
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