When flashing

The sign reads
TRUCKS OVER 5T
GROSS WEIGHT
USE EXIT 27
WHEN FLASHING
I find the pragmatics of this sign fascinating.

We know it doesn't mean when the trucks are flashing (which is what the grammar tells us), nor does it mean when the exit is flashing (which is a somewhat less possible reading, requiring an elided pronoun - but possible on a sign).

We know it means when this sign is flashing.

And that's cool.