While I have never driven an ambulance, I drove a small truck about 20 years ago. The Mexican
migrants who worked in our warehouse would tap repeatedly and then pound once or twice on
the side of the truck as I backed up to the warehouse door. The tapping meant “keep going”
or “keep backing up”; the pounding meant “stop now.” I always closed up the back of the
truck myself so I don’t know if they had a signal for “The rear door is closed and you can go now.”
I got an ambulance ride last March, and after loading me in someone did in fact bang on the door to let the driver know I was in there and secured.
/the whole trip was a fiasco, medically and financially, but I’ve posted about that earlier.
??? Slots? Transaction?
On the old two-man crew trams here, passengers would ring the bell once to request a stop. The conductor would ring twice when the all exiting passengers were out, and clear. And, rarely, would ring three times if an immediate stop was required.
For example, if he signaled ‘go’, and the lady who just got off turned around to grab her kid/babby buggy/pram as the tram took off with her child, that would be an immediate-stop situation.
This kind of signalling used to be standard on trains/cranes/telephones/mines/factories everywhere a phone or radio is used now, and from what I’ve seen, the codes were fairly standard too: 1 stop, 2 go. 1 stop, 2 hoist
He he my friend works with the ambulance crew and they all do this all the time… just to take the p*ss out of what they do in the movies! 
Sarah
It certainly was in the UK on busses with a conductor.
That’s like what they said about gangster films in the 1930s. Real-life gangsters would copy the speech they heard at the movies until it just became standard.