by Kay Axhausen
You might think of your schedule as what shows up on your calendar or daily planner. But there are lot of activities you probably don’t typically record: Driving to work, going out to lunch, sleeping, and so on. To engineers and mathematicians, the daily schedule is a complex optimization problem: there are periods of time (windows) when you have to be at certain places in order to be available to others, you have to be able to get to these points with the (mobility) tools you can bring along, you want to spend certain amounts of time at each point to be able to achieve your goals; all of this within the 24 hours of the day and within your commitments and monetary budgets.
So how do people solve this problem everyday? Often, we start with what happened yesterday as a model, We also have many previous occasions of when we wanted to combine certain combinations of activities and location. We have building blocks, which reduces the complexity of the problem enormously. In addition, there are constraints that make many combinations of places infeasible within the allotted time.
Social scientists have thought about this problem for a long time. These are illustrated in the Figure.
Torsten Hägarstrand, a famous Swedish geographer, identified and visualized one set of these constraints in the 1970s. His insight was to see, that some activities in time and space are much more firmly committed to than others. Think of the work schedule of a nurse or teacher, or the need to drop a child off at school. These firm commitments form anchors within the daily schedule, constraining the time available for remaining activities, and where they might take place. We are all caught in time and space.
 Lenntorp, B. and P. Hort (1976). Paths in space-time environments: A time-geographic study of movement possibilities of individuals, Volume 44. Royal University of Lund, Department of Geography Lund.