How do the traffic lights on a garage door to an apartment building work?
Where the entrance to or exit from an underground car park of an apartment building makes it hard to spot any incoming or outgoing traffic, a priority system is put in place.
As soon as the door is operated both traffic lights (entrance and exit) will turn red. Once the door is fully open, the light on the side of the person who activated the door, so either on the entrance or the exit side only, will turn green.
If an incoming and an outgoing car were to activate the doors near enough simultaneously, the control mechanism can be programmed in such a way that it decides whom the light will turn green for first, depending on whether there is enough space for two cars to comfortably pass one another.
Even when there are no visibility issues, it may still be worth your while having a red light installed, which will stay on until the door is fully open, as this will prevent the garage door getting damaged by people driving in or out too quickly.
It goes without saying that, like in traffic, drivers must wait until the light turns green.