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September 20, 2017
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Elevator interval is the figure you are unconsciously measuring when you stand in a lobby thinking “a lift should have come by now.” It is the average time between car departures from the main floor, and for decades it has been the headline number traffic engineers use to describe how good a lift service feels. Here is what elevator interval is, what counts as good, and how it is worked out.
Elevator interval is the average time between successive car departures from the main terminal floor during the busiest period. If five cars leave the lobby over a hundred seconds, the elevator interval is roughly twenty seconds. It is, in effect, how long you expect to wait for the next lift if you arrive at a random moment, and that intuitive meaning is why it has endured as the classic quality-of-service metric.
The interval is closely related to average waiting time but not identical to it. As a rough rule, average waiting time runs at a little over half the interval for a well-behaved up-peak, because a passenger arriving at a random point in the cycle waits, on average, part of an interval. The two metrics move together, and a good figure generally delivers a good waiting time.
Interval targets follow building grade. As a general guide used in UK practice:
These bands are guidelines, not rules. The right target is whatever the building’s grade and the client’s brief justify, but they give a sense of where a design sits.
Interval comes directly from round trip time and the number of cars. The relationship is simple: interval equals round trip time divided by the number of cars in the group. If one car takes 120 seconds to complete a full cycle and there are five cars sharing the work evenly, a car departs the lobby every 24 seconds on average.
That simplicity is why the interval is such a useful design lever. To shorten it, you either reduce round trip time, with faster cars, fewer floors served per car through zoning, or quicker door operation, or you add a car to share the cycle. Each option can be tested before it is built.
The round trip time formula assumes cars share the load evenly and arrivals are smooth. Real lift groups do not behave so tidily. Cars bunch, the phenomenon where two cars end up travelling close together while a gap opens behind them, which stretches the effective figure well beyond the calculated average even when the arithmetic looks fine. Conventional control systems are prone to bunching during the up-peak, and the calculated interval hides it.
Simulation exposes it. By modelling each car’s real movement and the dispatch decisions that cause bunching, simulation reports that this metric passengers actually experience, gaps and all. For any building where control behaviour matters, and that is most modern buildings, the simulated interval is the one to trust.
The most direct way to understand a lift group’s interval is to watch it. AdSimulo renders the up-peak as a real-time 3D visualisation, so bunching shows up as cars clustering on the screen while a gap grows elsewhere, exactly the behaviour that inflates the real interval. The engineer sees the problem rather than inferring it from a number, and can then test a fix, a different dispatch strategy or an added car, and watch the gap close.
Because AdSimulo’s Expert System searches automatically for a configuration that meets the target level of service, the interval target is something you set rather than something you chase by hand. You state the service you want, and the software returns the group that delivers it, then reports the interval against the target in a form ready for the client.
The figure is the most intuitive metric but not a complete one. A group can hit its interval target while falling short on handling capacity if the cars are frequent but small, plenty of lifts arriving, but not enough room on each. A sound design balances interval against handling capacity and waiting time, which is why a full study reports the set and judges the design on all three together.
The metric bands and the round trip time methods behind them are set out in CIBSE Guide D, the UK reference for lift traffic design, published through the CIBSE knowledge portal. Working to those benchmarks is what lets a consultant state an interval figure with authority.
Use interval as your quick read on service quality, but verify it with simulation so bunching does not flatter the design, and balance it against capacity and waiting time. To set an interval target and have the configuration found for you, see the lift traffic analysis workflow.
Interval is the average gap between car departures from the lobby; waiting time is how long an individual passenger waits for a car. Waiting time is typically a little over half the interval for a well-behaved up-peak, so the two move together.
Under 25 seconds is excellent and typical of prestige offices; 25 to 30 seconds is good for quality commercial buildings; 30 to 40 seconds is acceptable for many standard buildings. Above 40 seconds, service feels poor.
Either shorten round trip time, through faster cars, zoning so each car serves fewer floors, or quicker doors, or add a car so the cycle is shared among more lifts. Both can be tested in software before construction.
Because cars bunch. Conventional control lets cars cluster during the up-peak, stretching the effective interval beyond the even-sharing assumption in the calculation. Simulation reveals the real interval that passengers experience.
