Two minutes per person per day each year: Exploration of growth in travel time expenditures

During the past several decades significant changes in travel behavior in the United States have occurred. Evidence from the Nationwide Personal Transportation Survey series and the 2001 National Household Travel Survey (NHTS) indicates that the average daily travel time per person has increased by 1.9 min per year between 1983 and 2001. The objective of this paper is to explore the growth of daily travel time expenditures. Changes in society, technology, incomes, attitudes, and sociodemographic and household structure have been hypothesized as having contributed to the travel time growth. This analysis explores those variables and their relationship to increases in travel time. Aggregate values are used to investigate the relationships between daily travel time expenditures and sociodemographic characteristics. This paper comments on the share of travel time growth that can be explained by the available variables and speculates on the implications of other factors on travel time expenditure growth. A review of the NHTS data set and an analysis of the relationships between the data set and travel time expenditures make clear that travel budget changes that appear to be related to the set of available variables do not explain the full reason for the significant increase in travel time spending. The increases are across all market segments: age, income, gender, ethnicity, household composition, and so on. Thus, shifts in demographic or other traveler conditions do not fully explain the increases in trip making. That has a significant implication for transportation planning - traditional sociodemographic predictors for trip making do not appear to be sufficiently causal to be useful for understanding current and future factors influencing trip making and travel expenditure changes.