Four lifts, all serving every floor, and everyone crowds into the same one — why does that happen?

Bunching is a known and slightly counter-intuitive phenomenon, and it is the same effect that makes buses arrive in threes.

The mechanism is a feedback loop. A lift that falls slightly behind picks up more waiting passengers, which makes it stop more often, which makes it fall further behind. The lift behind it now finds fewer people waiting, so it moves faster and catches up. Left alone, the system converges on clumps rather than an even spread.

So it is not that nobody solved it — it is that even spacing is an unstable equilibrium under simple rules, and any dispatch policy that just sends the nearest available car will drift into bunching under load.

There is a cheaper intervention worth knowing about, which is to stop all four cars serving all floors identically.

Splitting the bank — two cars serving lower floors, two serving upper, or one car reserved for the top floors and roof — reduces the average number of stops per trip without any new hardware or any new interface. It is why tall buildings have sky lobbies and express cars.

The trade is fairness: somebody's floor is now served by two lifts instead of four, and they will notice. In a residential building that is a political problem rather than an engineering one, which is often why it does not happen.

The design that avoids it is the one you have probably seen in newer office towers: destination dispatch.

Instead of pressing up or down and finding out which car arrives, you enter your floor at a panel in the lobby and the system tells you which car to take. That single change gives the controller information it otherwise never has — where everyone is going, before anyone boards.

With that, it can group passengers heading to nearby floors into the same car, which cuts the number of stops per trip dramatically. Fewer stops means shorter round trips, which means the cars stay spread out, which is precisely the loop you want.

The cost is a real interface change. People have to interact before boarding, there is no button inside the car, and getting in the wrong one is confusing in a way it never was before. It is a genuinely better system that feels worse for the first week, which is a hard thing to retrofit into a residential building where nobody signed up for a new interface.

On the piling-into-the-first-door part specifically: that is rational behaviour under uncertainty rather than a mistake.

A lift in front of you now is worth more than a lift that might arrive shortly, because you cannot see the other one's position and you have been burned before. Buildings that display where each car is, and which is arriving next, get noticeably better spreading — not because people become more cooperative, but because waiting stops being a gamble.

Information displays are usually cheaper than a new dispatch system and get part of the benefit.