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Elevator and Escalator Consulting Engineers

Dwell Time and Headway Time

Dwell time refers to the time that the doors stay open in response to a hall or car call. This is the same for all floors.

At the main floor there is an additional time inserted by a separate circuit or by other software that is designed to keep the doors open to allow for loading or to provide separation in time from the departure of a previous elevator. This is referred to as "loading time" or "headway time" or "dispatch time".
The headway time, in most systems, varies depending on the traffic pattern. On down peak, for example, there is no good reason to delay the cars at the main floor so the headway time is set to zero. On up peak, the time may be in the order of 20 to 30 seconds subject to the dispatching algorithm. Some systems attempt to regulate the spacing of the elevators by calculating the average interval between cars arriving at the main floor and then trying to maintain that interval using headway timing. The idea is to provide a more or less even level of service and to prevent "bunching" of cars.
In more recent systems the principal idea seems to be to place cars in the hoistway subject to the expected hall calls. The cars are "zoned". The first available car - meaning a car without car calls or other prior assigned task - will go to the first zone, which is almost invariably set as the main floor. The other zones will be assigned more or less in proportion to number of cars and number of floors so as to have a car handy to any hall call. This zoning can be dynamic or fixed. Typically, it is fixed since the nuances of dispatching systems are generally perceived in practice to be of only marginal benefit. The zoning is, of course, adjustable.
There are innumerable other permutations on this theme including number of cars at main for up peak, heavier up or balanced and zoning predicated upon a high profile tenant.