On Monday, 19 September 2016 15:11:33 UTC+1, wrote:

How do the drivers know if anyone is on the stairs?

Mirrors. They can see the top deck and so will know if someone is coming
down or hasn't yet got up there. Plus I imagine they can hear the footsteps
too since on a normal DD they're right behind the cab.

LOL! Is that the first thing they think about? They don't give a toss!