Understanding people's travel time expenditures and the factors contributing to such expenditures can provide valuable insights into activity travel demand under a range of policy scenarios. The role of time in activity travel behavior is therefore of great interest to researchers. While the travel time expenditure constitutes the actual time spent traveling in a day, the expenditure does not necessarily constitute the upper limit of the amount of time that people are able and willing to dedicate to travel. In this context, it may be feasible to define an unobserved travel time frontier that represents an upper bound (under usual conditions) on the amount of time that people are able to dedicate to travel. A stochastic frontier modeling approach is adopted to estimate the travel time frontier, as this is an unobserved behavioral characteristic. Models are estimated for a sample of households from 3 years of the National Household Travel Survey in the United States to explore the dynamics of the frontier over a 15-year period. The variations in frontier values—as well as trends in the ratio of expenditure to frontier values, which may be interpreted as a metric of time pressure—differ considerably across sociodemographic groups; this finding calls for the formulation of policies that consider the unique mobility needs of different market segments.