My space or your space? Towards a measure of joint accessibility

Abstract

Research on space–time accessibility is usually concerned with determining and assessing the opportunities for individual activity participation. However, a significant part of everyday activities is conducted jointly with others, implying that the feasibility of potential locations for activity engagement depends on the accumulation of conditions related to the spatiotemporal setting of each participant. In this paper, we put forward a method to gauge the accessibility of places to a particular group of people willing to engage in a joint activity. The method calculates locational benefits by taking into account network-based travel times, individual activity schedules, and the attractiveness and temporal availability of facilities. The applicability of the method extends towards situations with multiple facilities and participants, being available for multiple periods of time. As a possible output of our model, maps of joint accessibility are created which can be used to evaluate different rendezvous scenarios.

Introduction

One of the greatest contributions of activity-based approaches to travel demand analysis is their attention to interactions between the activities conducted by different individuals. More and more studies of travel behaviour now distinguish between independent (personal), shared or joint, and allocated activity and travel episodes (Zhang et al., 2002, Zhang et al., 2005). The term allocated activities pertains to household-related activities that are conducted by one of the adults in (nuclear) households. Shared or joint activities can be defined as activities where at least two individuals are co-present at the same spatial location for at least some time to pursue a common set of acts. The activity can be fully synchronized – meaning that all individuals start and end the activity simultaneously – or not, which is the case as one or more persons commence the activity earlier than (the) other(s) and/or continue after (the) other(s) have relinquished the activity. Participation in joint activities can be motivated by multiple factors, including efficiency (time management, scheduling), companionship and altruism (Gliebe and Koppelman, 2002, Schwanen et al., 2007).

Nonetheless, in spite of the benefits likely to accrue from joint activities, participation in shared activities tends to entail coordination costs and additional space–time constraints for individuals, because they have to negotiate and balance their own preferences and needs with those of others (Carrasco and Miller, 2006, Hägerstrand, 1973). It should be recognized that such negotiation processes need not be fully consensual. They are often driven by power differentials with one or several of the persons involved having a larger weight in the decisions about where, when and what joint (or allocated) activities are eventually undertaken. These points have long since been raised in feminist critiques of Becker’s (1965) conceptualization of household decision-making about the allocation of employment and household responsibilities (Morris, 1990, Pratt and Hanson, 1991), and have recently been accommodated in micro-economic models of group decisions about activity participation (Zhang and Fujiwara, 2006, Zhang et al., 2002, Zhang et al., 2005).

Empirical investigations into interpersonal interactions in activity participation have proliferated in the past decade but have mostly addressed interactions between adults within nuclear family-households (e.g. Gliebe and Koppelman, 2002, Golob and McNally, 1997, Schwanen et al., 2007, Scott and Kanaroglou, 2002, Srinivasan and Bhat, 2005, Zhang and Fujiwara, 2006, Zhang et al., 2002, Zhang et al., 2005). That the focus of these studies has been restricted to family-households is at least in part due to data limitations; data about travel behaviour do not routinely collect detailed information about activity participation by non-household members (Carrasco, Hogan, Wellman, & Miller, in press). Recently, however, a number of studies have sought to analyse and model interactions in the activities of persons within social networks, partly in response to the increased use of such Information and Communication Technologies (ICTs) as the Internet and mobile phone (e.g. Harvey and Taylor, 2000, Larsen et al., 2006, Páez and Scott, 2007). Most of these studies tend to concentrate on social activities (Carrasco and Miller, 2006, Ohmori et al., 2006) but some have also considered such maintenance activities as chauffeuring children to places (Schwanen, in press, Tillberg Mattson, 2002).

While empirical research about joint activity participation currently flourishes, almost all work to date has considered realized or actual behaviour of (groups of) individuals. Recent studies have paid far less attention to the ways the intention to perform activities together affects the opportunities available for activity participation. Time-geographers explored these issues already in the 1970s and 1980s but mainly at the conceptual level (Carlstein, 1980, Mårtensson, 1979). Recently, Ohmori et al. (2006) explored the extent to which the opportunities for individual activity participation are affected through dynamic rescheduling of the times and places for meeting others enabled by the mobile phone among adolescents in Japan. They did not, however, model the opportunities for joint activity participation. More insight into the factors delimiting the possibilities for joint activity participation is nonetheless required, especially in the current era where collective space–time rhythms seem to be eroding and new ICTs allow for relatively easy reorganizations of activity patterns (Ohmori et al., 2006, Schwanen and Kwan, 2008). Both trends may complicate as well as facilitate the group decision making about joint activity participation.

In light of these considerations, the current paper presents a formal model of the opportunities for joint activity participation, drawing on the extensive literature about accessibility. The approach adopted comprises three major components. First, we provide a theoretical framework for measuring the interaction possibilities based on time-geography and random utility theory. Point of departure is the work by Burns (1979) and Miller (1999) which accounts for the attractiveness of activity locations, the activity participation time, and the cost of spatial separation. We extend this framework for situations with multiple facilities and activity participants, being available for multiple periods of time to multiple activity participants. Second, based on this theoretical framework, we describe a procedure for calculating the locational benefits by taking into account network-based travel times, individual activity schedules, relative influences of participants, and the attractiveness and temporal availability of facilities. More specifically, the concept of joint accessibility is implemented as a Visual Basic module. Third, as an illustration of our conceptual model, a series of simulations has been carried out for a study area north of Brussels (Belgium) to elaborate the effects of space–time constraints, group size and power differentials between group members.

As an introduction to the proposed framework and model, we start the remainder of this paper with a brief review of the extensive literature on accessibility.