Is average daily travel time expenditure constant? In search of explanations for an increase in average travel time
Introduction
For the past three decades a discussion has been going on in the literature on the issue of whether people, on average, have a fairly constant travel time budget. Researchers concluding this to be the case include Szalai, 1972, Zahavi, 1979 and Schafer and Victor (1997), though in later work Zahavi does not reach the conclusion that such a time budget exists (Golob et al., 1981). In the past few years, the discussion has re-entered the Dutch transportation research area, with several Dutch authors doing research on this subject and discussing the hypotheses of constant travel time budgets (see, for example, Goudappel, 2001, Kraan, 1996, Muconsult, 1997, Muconsult, 2001, Peters et al., 2001, Rietveld, 1999, SCP, various years; van Goeverden, 1999). The theme of constant travel time budgets is important because, if present, it implies that increases in travel speeds due to such long-term developments as technological change or transportation policies result in an increase in total transport volumes. In particular, the constancy of travel time might imply that the development of faster modes would lead to longer travel distances. In addition, depending on the underlying mechanisms, constant travel time budgets may jeopardise basic assumptions in conventional travel behaviour modelling (Mokhtarian and Chen, 2004).
In this paper we will first summarise the discussions on constant travel time budgets as found in the literature (Section 2). We will follow this with an analysis of Dutch data on travel times (Section 3), suggesting that the average time spent travelling by the Dutch population might have increased in the past decades. This will be followed by an overview of possible categories of theories that may explain the constancy of time spent on travel, and the possible increase that has appeared in the Netherlands (Section 4). Our conclusions with respect to these theories are then used to categorise possible explanations for this increase (Section 5). Finally, we will discuss the results and address strategies for further research (Section 6).
This paper attempts to make a contribution to a renewed discussion on travel time budgets in the context of categorising theories that might explain constancy in travel time budgets, and as an item in the overview of possible explanations for increased average travel times, and related strategies for further research.
Section snippets
A literature overview
As early as 1961 the possibility of constant travel time budgets was hypothesised by Tanner in his paper ‘Factors affecting the amount of travel’ (Tanner, 1961). In the 1970s empirical research was carried out by researchers such as Szalai (1972) and Zahavi (1979), suggesting that at an aggregate level (e.g. all people living in one country) the average time spent on travel is more or less the same for that group. Since then, the constant travel time budget has become a subject for discussion
An overview and analysis of Dutch data
There are two data sources in the Netherlands that investigate trends in time use for transport: The National Travel Survey (OVG) and the Time Use Survey (TBO).
The OVG is a cross-sectional survey which has been conducted every year since 1978 by Statistics Netherlands (CBS). The sampling unit is the household. Sample size increased from 10,000 in the 1978–1995 period to 60,000 households from 1995 onwards. Data on travel are collected using travel diaries. The design of this survey was changed
Theoretical explanations for constant travel time budgets
We now continue by focusing on explanations for the likely increase in the average travel time in the past decades as found in the literature.
Based on an extensive literature review, including sources from economics, psychology, biology, sociology and other disciplines, Peters et al. (2001) presented three categories of explanations for constant travel time budgets.
Possible causes for an increase in average travel time
In this section we present possible causes for the increase in travel times. Further research is needed to find out if these causes really play a role (see Section 6). After presenting the possible causes, Section 5.4 presents a simple model to visualise the impact of changes in costs and utility on average travel time and links the causes as presented below to this model.
Discussion and suggestions for further research
The literature ‘findings’ above suggest the possible increase in travel time to be mainly the result of the increased utility trips in general and of longer trips in particular in terms of travel time, as well as of changes in the transport system. Changes in the population probably play an insignificant role. We suggest that the benefits of more and longer trips have increased in the last few decades and the costs of travel have decreased, the result being an increase in the average travel
Acknowledgement
The authors thank Cees van Goeverden for processing and providing the OVG data, and two anonymous referees for their comments on the draft of this article.
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