That's one thing that London Transport drivers never could claim.

I once discussed the 'safety' of the Routemaster braking system with Colin Curtis then Chief Engineer. I questioned what prevented drivers from continuing with inadequate system pressure. He responded that 'driving with the flag down' was a sackable offence. "No driver can claim they didn't notice the flag." He then took me to a Routemaster cab and showed me 'the flag' - a metal arm about a foot long that dropped-down from the cab header rail into the driver's field of view and perforated with the word STOP whenever the hydraulic pressure fell below the residual performance level. Indeed it would be almost impossible to see the road ahead whilst the 'flag' (which had to be reset by a mechanic in the workshops) was 'down'.
I had to concede that he had designed an unmissable warning device . . .

Likewise the height of the vehicle was written in large letters on the header rail of the (single width) cab where no driver could claim they couldn't see it. I admit that the first time I drove a Routemaster under a bridge I checked and double checked the figure against the sign on the bridge (and that was around the Chiswick Test Centre driver's test route).