Non-linearities in the Valuations of Time Estimates
A de Palma, C Fontan, University of Cergy-Pontoise;
The valuations of prices and costs of travel times appear to be key parameters in the calibration of any demand traffic simulation systems. For example in the UK, Wardman (2000) propose a complete review of the empirical estimates. However, it seems that practical requirements are fogging the real nature of these concepts, which should be strictly related to the individual tastes and behaviors, but are also depending on the travel alternatives and their corresponding effective levels of supplied attributes. Thus, we should find individual specific values of travel times, changing with the attributes levels and with transportation mode specific tastes and behavioural considerations. First, theoretical approaches on the topic have considered the time component as a scarce resource. Thus, it is crucial to avoid its irrational and senseless spending (Becker 1965), but also to search for a minimal time allocation, all other things being equal, in boring but necessary activities, particularly in travel, hence it is possible time has an implicit price, at least a resource price. De Serpa (1971) noticed that it is not exactly the case when we consider that the consumer is not always the producer of his/her activities, particularly in travelactivity. In this case, the user is a time-taker and rationally knows that he/she has to account for a minimal amount to be spent. Then, time resources are different from travel time, and de Serpa defines the value of travel time as the implicit transfer value from travel time to resource times. Truong and Hensher (1985) have generalised this approach, by splitting the travel time in different components: access/egress times, waiting times, commuting times and riding/on-board times and have found significant values of travel times savings. It has become a usual approach in practical modelling (Bhat 1998-a and 1998-b, Horowitz and al 1980, Quinet 1998). Another general important standpoint is to differentiate the several reasons why the travel time has to be allocated. The basic idea is that the activity to be realised at destination does not improve the same way the well-being of the individual.
Thus, the corresponding needed travel time to access it, does not take the same importance in the mind of the individual. This implies different preferences for different situations, and has led to activity-based travel demand systems (Domenich and McFadden 1975, Train and McFadden 1978, Small 1992, BenAkiva and dePalma 1996). We focus in this paper on the regular Journey-To-Work (JTW) framework, particularly the transportation mode choice issue. MVA (1987) and Mcfadden (2000) have noticed in practical discrete choice modelling, that several possible econometric specifications correspond to different theoretical approaches (McFadden 1973, Ben-Akiva and Lerman 1985). We have also noticed that most of the empirical applications have used functional forms with pre-specified properties for the implicit valuations of travel portfolios attributes, such as income effects (see for instance dePalma and Kilani 1999, de Palma and Fontan 2001). MVA (1987) and Mcfadden (2000) have also highlighted the fact that the travel time may be unreasoningly quantified. It is possible to account for these effects using simple transformations of the corresponding variables, but we argue that it is a misleading approach, since it is based on a priori behavioural considerations, that are simply leading to pre-specified results. Thus, we develop in a first section a regular journey-to-work transportation mode choice model, within the traveller is faced to mutually exclusive alternatives, each supplying distinct travel portfolios. We describe the decision process of a rational traveller. Hence, we derive definitions for the concepts of prices and values of travel times. We analyse their response functions under general assumptions. We show that there are different situations emerging according to the capacity of endogenising leisure patterns during the OD trip, defining behavioural profiles for the traveller, so that the effective travel time may be twisted by the individual which often does not strictly quantify it. We also allow for an income effect entering the willingness to pay for saving the time attributes. Each price of times is specific to the individual and to the alternative. We develop a binomial Box-Cox Logit model (Gaudry 1978, Gaudry 1981, Gaudry and al. 1996), particularly adapted for joint estimation of tastes and behaviour parameters, as it will be motivated. We propose some estimatesof the parameters of the theoretical VOT functions in French Parisian region using an updated 1998 sub-sample of the large regional travel survey.
Association for European Transport