Low-clearance warnings can help avert unintentional head-banging (as opposed to purposeful head-banging to, say, Metallica).
In the U.S., we shout these warnings (Danger! Watch Your Head!) like we shout everything. But the Brits take a more proper, refined approach, as shown by the sign on the right, taken in a London pub.
The only thing missing is an “Old Chap” at the end, as in “Please Do Mind Your Head, Old Chap.”
Thanks to Pat Crowell.