This paper summarizes and analyses findings from more than two dozen aggregate and disaggregate studies of travel time (and sometimes money) expenditures, exploring the question of the existence of a constant travel time budget. We conclude (with prior researchers) that travel time expenditures are not constant except, perhaps, at the most aggregate level. Nevertheless, individuals' travel time expenditures do show patterns that can be partly explained by measurable characteristics. Travel time expenditure is strongly related to individual and household characteristics (e.g., income level, gender, employment status, and car ownership), attributes of activities at the destination (e.g., activity group and activity duration), and characteristics of residential areas (e.g., density, spatial structure, and level of service). To the extent that travel time expenditures are constant at the aggregate level, the underlying mechanisms explaining that regularity are not well understood. Consequently, further research into explaining travel time and money expenditure patterns is justified. (C) 2004 Elsevier Ltd. All rights reserved.

This paper focuses on the tradeoff in time allocation between maintenance activities/travel and discretionary activities/travel. With the recognition that people are not completely free to allocate their time between activities and travel, we propose a linear constraint in time allocation between activities and travel, which indicates a minimum amount of travel one must do in order to allocate one unit of time to the activity. This minimum amount of travel is represented by the travel time price, a ratio obtained by dividing the total amount of time traveling to maintenance or discretionary activities by the total amount of time spent on activities of the same type. This travel time price is the time equivalent of the monetary price for performing an activity (whether in-home or out-of-home). We ask two questions in the paper: “If the travel time price of performing maintenance or discretionary activities increases, how will that affect one’s time allocation to maintenance and discretionary activities? And if one had one more unit of available time, how would this affect one’s time allocation to maintenance and discretionary activities?” We use the San Francisco Bay Area 1996 Household Travel Survey data and apply the Almost Ideal Demand System (AIDS) of demand functions. The empirical results provide the following answers to our research questions. With respect to the time equivalent of income elasticities of maintenance and discretionary activities, we found the former to be less than unity and the latter to be greater than unity. In other words, maintenance activities are a necessity and discretionary activities are a luxury. With respect to the own travel time price elasticities, if the travel time price of performing a certain type of activity increases (for reasons such as traffic congestion), one would reduce the time allocated to that type of activity. As expected, time spent on maintenance activities is less elastic than the time spent on discretionary activities. As for the cross travel time price elasticities (changes in time allocated to activity type i itself in responses to changes in the time price for activity type j), we found that εdm > 0 and εmd > 0, suggesting a substitution effect between maintenance and discretionary activities.