Schedule padding

In transportation, schedule padding, sometimes called padding, is some amount of 'extra' time added to a schedule that allows it to be resilient to unpredictable delays. Schedule padding is, naturally enough, required only for transportation that operates on a fixed, published schedule, including public transport and airlines, or if timed connections are important, like in many areas of passenger and freight transportation in which things that are being carried need to be transferred to another vehicle. It is not right, therefore, to think of something like a taxi as operating with schedule padding.

For simple trips, schedule padding may be relatively negligible. If a vehicle encounters little delay, it may simply arrive at its destination early and either transact its business or wait until the scheduled time for some rendezvous. In public transport, however, a number of timed stops may be strung together into a route, and some amount of schedule padding may be needed for each part of the trip. A transit vehicle must not pass a timed stop before it is scheduled to do so, and a conservatively-padded schedule, with much padding, can substantially reduce overall speeds and increase travel times beyond what might have been necessary. It is important, therefore, to think about the probability and distribution of certain types of delay events and the desired degree of on-time performance when padding in public transport timetables is established.

In air travel, the scheduled flight time is usually increased to about 110% of the expected flight time, because delays are both common and potentially expensive. Delayed flights can cause knock-on effects in terms of missing departure slots, which may be a problem in busy airspaces in which the departure time is not only determined by the schedule of the departure airport but also the sectors the aircraft has to fly through. In addition, aircraft usually fly multiple flights in short succession so a delay may cause later flights with the same aircraft to be delayed or additional aircraft to be chartered on short notice.

In addition financial costs can be incurred because passengers miss their connections and airlines may incur punctuality penalties. Departure delays are easily caused by passengers arriving late at the gate or unexpected business at the airport. By padding the schedule, the aircraft is less likely to miss slots in airspaces it has to fly through and may still arrive on time (or even ahead of schedule) at the destination airport even with a small departure and/or enroute delay.

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Wikipedia