Of course I've seen that, I've even been there.
It's very simple: From 1939-1975 the Border from Gibraltar to Spain was closed, but the british Royal Air Force needed an airport in WWII.
The only place they could build it was on the eastern side of Gibraltar, the land side.
Nowadays, the only road leading into Gibraltar crosses the runway.
This isn't much of a problem, as there is not much traffic neither by plane nor by car.
And, yes they do use stoplights for the cars