The central basis of the activity-based approach to travel demand modeling is that individuals' activity-travel patterns are a result of their time-use decisions within a continuous time domain. This paper reviews earlier theoretical and empirical research in the time-use area, emphasizing the need to examine activities in the context or setting in which they occur. The review indicates the substantial progress made in the past five years and identifies some possible reasons for this sudden spurt and rejuvenation in the field. The paper concludes that the field of time-use and its relevance to activity-travel modeling has gone substantially past the "tip of the iceberg", though it certainly still has a good part of the "iceberg" to uncover. Important future areas of research are identified and discussed.
Keywords: Time use research, activity-based travel modeling, activity episode analysis, time allocation, hazard-based duration model, activity scheduling
Activity based travel analysis has received much attention and seen considerable progress in the past decade. It has enabled us to comprehend and appreciate the complexity and variability in the activities that an individual undertakes during any given period.
Very broadly, activity based travel analysis attempts to better understand the behavioral basis for individual decisions regarding participation in activities in certain places at given times. This behavioral basis includes all the factors that influence the how, where and why of performed activities. Among these factors are the needs, preferences, prejudices, and habits of individuals' (and households), the cultural/social norms of the community, and the travel service characteristics of the surrounding environment.
In the evolution of approaches to travel demand analysis, there has been a distinct paradigm shift in the past couple of decades from trip-based methods to activity-based methods. Trip-based methods focus on analyzing trip-related decisions without considering the time-use context in which activity participation and travel decisions are made. The consequence is that trip-based methods lose sight of the broader picture within which travel decisions are made (for a detailed review of the shortcomings of the trip-based method, the reader is referred to Kitamura, 1988, Jones et al., 1990, Axhausen and Garling, 1992, and Kurani and Kitamura, 1996). The activity-based approach (see Recker, 1995 or Kitamura and Fujii, 1996) views travel as a derived demand; derived from the need to pursue activities distributed in space. This approach adopts a holistic framework that recognizes the complex interactions in activity and travel behavior. It focuses on sequences or patterns of activity behavior, with the whole day or longer periods of time as the unit of analysis.
A fundamental difference between the trip-based approach and the activity-based approach is the way time is conceptualized and represented in the two approaches (Pas, 1996, Pas and Harvey, 1997). In the trip-based approach, time is reduced to being simply a "cost" of making a trip. The activity-based approach, on the other hand, treats time as an all-encompassing continuous entity within which individuals make activity/travel participation decisions (see Kurani and Lee-Gosselin, 1996). Thus, the central basis of the activity-based approach is that individuals' activity-travel patterns are a result of their time-use decisions. Individuals have 24 hours in a day (or multiples of 24 hours for longer periods of time) and decide how to use that time among activities and travel (and with whom) subject to their schedule, socio-demographic, locational, and other contextual constraints.
This paper presents a retrospective review of time-use studies and a prospective agenda for time-use research. The retrospective review is organized under two categories: a) Activity time allocation studies, and b) Activity episode analysis.
Activity time allocation studies classify activities into one of several target types and then examine the allocation of time to these activity types based on household/individual characteristics. These studies generally ignore the context in which activities are performed; that is, for the most part, they do not consider the time of day (or day of the week) of activity performance, the sequence in which activities occur in the continuous temporal domain, the duration of each activity participation, the location of activity participation, and the company (alone, with spouse, with children, etc.) in which activity participation occurs. To highlight the activity context (or activity setting, as Harvey, 1982 refers to it), we will follow Chapin and Hightower (1966) and use the term "activity episode" to refer to a discrete activity participation.
The term "activity" refers to a collection of episodes of the same type or purpose over some time unit (say a day or a week).
Studies reviewed under the category of activity episode analysis emphasize activity episodes and their associated spatial, temporal, sequencing, and company contexts of participation.
. Review of activity time allocation studies
The study of allocating time to different activities has received attention in the fields of psychology, anthropology, sociology, urban planning, geography, time-use analysis, and economics. We review theoretical developments in the field of activity time allocation in section 2.1 and empirical developments in section 2.2.
. Theoretical developments
Theoretical studies of activity time allocation may be sub-divided into three classes. The first category of theories, which we label as motivational theories, have their origins in anthropology and psychology. Theories in this first category emphasize the fundamental motivational process underlying activity time allocation. The second category of theories, which we label as sociological and planning theories, have their origins in sociology and urban planning. This category of theories provides more immediate insights into the process governing time allocation behavior. The final category of studies, labeled economic theories, have their roots in micro-economics. Generally, economic theories have an explicit mathematical formulation, though they may not incorporate some of the more subtle determinants of time allocation behavior identified by motivational and non-economic theories.
. Motivational theories
There are numerous theories of the process that motivates human activity time allocation behavior. The theories differ considerably in their views of the motivational basis of behavior but agree on the notion that behavior is dictated by felt needs whether innate or nurtured.
The earliest theory of human nature originates from the Greek philosophers Aristotle and Plato. They believed that people have a "free will" and that intellect and reason governed human activity behavior. Rene Descartes (late seventeenth century) put forth a dualistic approach to behavior in which he theorized that animals and humans were intrinsically different in the process governing their activity behavior. This dualistic theory was challenged by Darwin's (1856) theory of survival of the fittest. He argued that the basic force motivating all animals (humans included) is survival and any difference in behavior between lower animals and humans can be attributed more to the ability of adjusting to the contextual environment rather than a qualitative difference in the process determining their activity behavior. The next important theory was the psychoanalytic approach of Freud (1915). His theory demonstrated how the need to survive could motivate activity behavior, though humans may perceive other motives as dictating their behavior. Freud's view that humans are motivated by blind, pleasure seeking, animal instincts was modified by new theories. Amongst them are Jung's theory (1938), Hull's drive theory (1943), and more recently, the humanistic approach.
The humanistic approach extends earlier theories to include motivation to grow and mature. Rogers's (1959) theory was founded on "self concept". He believed that we are motivated to develop a positive image (perceiving our personality positively) and that we act to fulfill needs of growth to realize our inherent potential (he calls this self actualization). Maslow (1970) expanded the humanistic approach to clarify the conditions necessary to express human need for self actualization. He distinguished deficiency motivation from growth motivation and stressed that any deficiencies (physical or psychological) need to be taken care of before an attempt can be made toward the direction of growth.
Closer to the field of activity analysis, Fried et al. (1977) proposed a micro-theory of adaptational change affecting individual activity time allocation behavior. Their micro-theory presents a set of propositions about the adaptation process that reduces the imbalance between current or expected needs and resource opportunities and constraints. in this structure, the main influence on behavior is environmental opportunities which are modified by psychological, social, and economic influences to determine the spatial distribution of human activities and travel. The theory emphasizes the dynamic process of behavioral adaptation arising from an effort to bring environmental opportunities into finer balance with current and anticipated needs.
This second class of theories has more directly focused on time allocation relative to the anthropological and psychological theories discussed earlier. Sociological studies have centered on "resource theory" and the effect of attitudes regarding sex role behavior on time allocation of individuals living together. Most resource theorists relate "contributions toward resources" (which is the source of power) in terms of individual socio economic attributes (such as income, education, job status etc.). However, the issue of resources has been interpreted in a much broader sense by Heer (1963). He suggests that it is not just the value to one partner of the other's resources that determines power, but how much those resources are worth relative to those available outside the current co-existing living arrangement. Geerken and Gove (1983) laid the foundation for an integrated socio economic theory of "imperfect" utility maximization (implying that time allocation may not be the best of allocations and that under certain circumstances, a household may face only an array of bad choices). Geerken and Gove directed their attention on time spent on household work by individuals living together as a function of time spent at work. While recognizing and contributing toward the need for an integrated approach, they did not explicitly formulate or structure the time allocation process.
In the urban planning literature, Chapin (1974) postulated a framework in which constraints imposed by society (he classifies these constraints as preconditioning and pre disposing) interact with inherent motivations, to result in the propensity of activity participation. Reichmann (1976) classified activities into a) Subsistence activities or work related business services which are essential to provide the financial requirements for pursuing maintenance and leisure activities, b) Maintenance activities or purchase and consumption of goods to satisfy household and personal physiological & biological needs (hunger, thirst etc.) and cultural/consumption needs (needs for an individual or a household to establish its place in society), and c) Leisure activities or social, recreational and other discretionary pursuits motivated by cultural and psychological needs (needs for social interaction, achievement and self actualization). This classification or similar ones have been adopted in almost all empirical time allocation research. For example, Aas (1982) uses a four-way classification of activities rather than a three-way classification. Aas's classification includes a) contracted (paid) work time, b) committed (unpaid) work time, c) necessary (personal care) time, and d) free time.
. Economic theories
The economic approach to time allocation is based on the assumption that individuals (and the households of which they are a part) will always try to do their best to function as well as possible. Each person in the household allocates time as well as money income to various activities receiving income from time expended in the market place and receiving utility from spending this income on the consumption of goods and services (Gramm 1975, Gronau 1973, Becker 1981;1965, Mincer 1962;1963). Individuals "produce" non-market activities using "inputs" - their time and market goods and services. An individual's choice of work time and time in other non-market activities depends on market wages and prices of the "inputs" used to produce non-market activities. In particular, non-market time and consumer goods used in "production" of each non-market activity is chosen so as to maximize utility subject to constraints imposed by wages, prices of consumption goods, and time (Juster, 1990). Building on Becker's economic theory, Kitamura (1984), Kitamura et al. (1996), Kraan (1997), and Bhat and Misra (1998) have used a resource allocation formulation to determine individual participation in an activity and duration of participation. Townsend (1987), on the other hand, presented a formal integrated theory of time allocation among individuals at the household level. Many recent reviews (for example, see Kraan, 1996 and Pas and Harvey, 1997) have discussed the economic formulations of time allocation and we refer the interested reader to these studies.