The things outside London Underground station entrances?

Question

What are those oblong black boxes outside the Underground station entrances (above the entrances) with 2 halogen lights at each side? they look like some kind of signs but they are never lit.

Answer 1

Safety lights that activate only in emergencies

Answer 2

Do Not Enter signs. They come on when an fire alarm is triggered, or if there is any other situation where it is important that people don't enter the station (ie the station is *severely* overcrowded in which case the lights and sign will be turned on to prevent a crush)

Answer 3

emergency lighting units... and youd better hope you dont see them lit

Answer 4

They are automatic "DO NOT ENTER" signs which are connected to and operated by the fire alarm system installed at that station. The lights are NOT floodlights - they are flashing warning lights rather like one would see at level crossing. Apart from the sign coming on and the flashing lights there is also a loud siren which sounds as well - loud enough to waken the dead, so that anybody who fails to take heed of the sign when it is operating is either deaf and blind or a complete idiot, for they may be walking into a fire situation ! ! !