Time Allocation and Valuation of Travel Time Savings in the Presence of Simultaneous Activities
J Polak, J Pawlak, Imperial College London, UK
This paper presents a new time allocation model accounting for simultaneous activities
The valuation of travel time savings plays a fundamental role on transport modelling and appraisal. The theoretical foundation for placing a value on travel time savings rests on models of time allocation. Although many alternative time allocation models exist, for many years, this area of theoretical debate was regarded as relatively uncontroversial, with several well-established theories providing a justification for conventional demand modelling practices.
In recent years however, the adequacy of these frameworks has begun to be questioned. For example, a number of authors have reported both qualitative and quantitative results which they interpret as indicating that in certain circumstances there can be a positive utility associated with increasing travel time. Other authors have noted that developments in ICT have blurred the classical distinction between activities and travel, enabling some types of activities to be conducted during the course of travel.
In the light of these developments, this paper argues that there is an urgent need to revisit some of the fundamental assumptions underlying classical time allocation models. In particular, we argue that all existing models share a critical weakness in that they assume that a given quantum of time can be used for one and only one activity (or for travel). Drawing on evidence from time use surveys from a number of contexts, we argue that this is a highly unrealistic and limiting assumption, and that its importance is by no means restricted only to the context of ICT enabled activities. Episodes of simultaneous activity engagement, in which the same quantum of time is used for multiple distinct activities, are deeply engrained and pervasive in daily life, both inside and outside the home ? e.g., childcare combined with leisure, shopping combined with personal business, work combined with commuting etc. We argue that failure to explicitly accommodate this type of simultaneous activity engagement underlies a number of the apparently anomalously results recently reported in the literature. Building of this conceptual position, the paper presents a new framework for modelling time allocation which explicitly accommodates multiple simultaneous activities. This framework is based on an extension of the Goods-Leisure framework of Train and McFadden and the more recent work of Jara-Dias and colleagues (itself drawing on ideas from the earlier work do Evans and De Serpa).
The paper is divided into a number of sections. In the first section, we provide a brief overview of the relevant time allocation model literature, highlighting the common features and differences of different approaches. The second section discusses the available evidence regarding the frequency and nature of simultaneous activities touching both on the substantive empirical results and on methodological issues such as the definition of the term activity and granularity of temporal measurement. The third section presents the new modelling framework and discusses a number of alternative mechanisms for representing simultaneous activities. The fourth section presents the results of an empirical application of this framework using data from a survey of rail passengers? use of mobile technologies for work and leisure activities undertaken in the UK. The fifth and final section discusses the implications of these results, and the wider modelling framework, for modelling and appraisal, and highlights directions for future research.
Association for European Transport