Abstract:This study is concerned with how routine an individual's routine really can be. This question is addressed by examining the day-to-day variability of the time co-ordinate of the vertex of a time–space prism; in other words, by examining how the timeframe which governs the individual's daily schedule varies from day to day. When the timeframe varies, it is likely that the individual's behavior also varies. When the timeframe is stable, on the other hand, a routine can be maintained. The analysis presented in this paper attempts to determine how much of the variation in travel is due to the variation in the timeframe. The origin vertices of workers' morning prisms, which determine how early they can leave home in the morning, are examined in this study, along with the departure times of the first trips in the prisms, which are mostly supposedly routine commute trips. The results indicate that the vertices are located with a much smaller variance, but vary more systematically than do the departure times of the first trips in the prisms. This implies that a large degree of variability is introduced when a trip is made within the timeframe as determined by a prism vertex. It is also shown that the departure time varies from worker to worker according to unobserved heterogeneity—i.e., unexplained differences across individuals—much more than does the prism vertex. The study results indicate that large degrees of flexibility are associated with trip making, and suggest the presence of room for behavioral modification with respect to workers' first trips in the morning.