This is actually what any other mode of transportation that uses a schedule has to do: if you have a schedule, you have to take some precautions to ensure you can keep to the schedule most of the time, otherwise you can forget about the schedule. Here in Germany, this is very obvious for bus lines on streets that are usually very busy - they have so much "padding" in their schedule that on less busy days the buses have to travel at 30 km/h instead of 50 km/h to avoid running ahead of schedule.