Direct and indirect influences in the use of travel time: insights from uk national travel survey 2002-2010
Kaveh Jahanshahi, Ying Jin and Ian Williams (University of Cambridge, UK)
A well-established strand of literature has examined how urban land use and the built form affect travel behaviour, particularly the distances and frequencies of travel. Owing to the complex, often endogenous relationship between urban land use, traveller profiles and the purposes of travel, the data requirements are extremely demanding. As a result of the lack of data, there are few studies that examine all the above influences in one model, and most existing studies were unable to examine the influences upon the use of travel time, which is just as important as the distances and frequencies of travel. Insights into the possible influences upon travel time use are crucial for making a step change in travel behaviour towards city-scale sustainable transport solutions.
This paper aims to investigate in one comprehensive model all the main influences upon the variations in personal travel time use, thus filling an apparent gap in the literature. The analysis takes into account the interactions and endogeneities among all the influences. This will measure more precisely the effectiveness of specific interventions, such as the impacts of the built form arising from urban planning and urban design.
We have developed a structural equation model that enables us to account for not only the direct influences, but also the indirect ones, many of which have not been examined in existing literature. The empirical analysis is supported by a large and extensive UK National Travel Survey (NTS) dataset, which after appropriate processing has a suitable sample size and range of variables to support the conceptual model. We use multi-level analysis to control for possible heterogeneity among household members. Using multi group analysis, we have also examined changes in travel behaviour before and after the year 2007 (i.e. economic crisis).
The indirect influences that are mediated through the interactions among urban land use, traveller profiles and the purposes of travel are found to be very important. In particular, the spatial sorting of residents and their car ownership appear to be at the centre of the endogenous interactions. Ignoring such indirect influences would therefore seriously bias our assessment of the effects on travel time use of urban form and related interventions. Notably, from analysing the total direct and indirect effects, we can conclude that an average person who resides in a denser, more urbanized area with no access to car within their family tend to spend longer on commuting when everything else is constant. The indirect effect of land use factor on commuting time through car ownership accounts for 45% of this influence. Therefore, exogenous modelling of car ownership, at the very least result in underestimating commuting time for those who reside in a denser, more urbanized area. We did not detect any significant behavioural change after 2007 compared with the earlier period, except that the influence of urban form on car ownership became 23% stronger after 2007.
Research support: UK EPSRC Energy Efficient Cities; UK EPSRC Doctor Training Grant.
Keywords: land use transport interaction, built form, travel time use, structural equation modelling, multi-group analysis, multi-level analysis