Yeah. I think that sign would be more effective if both lines were swapped: "Must Turn, Overheight" rather than Overheight Must Turn". It sounds like a generic warning, I bet these truck drivers don't think it's specifically about them.

Pays to be imperative: TURN NOW. Needs more bright red blinkenlight, too.

The sign that warns driver does actually blink though, for this exact reason. I believe there's a separate electronic sign further up the road as well that flicks on when a vehicle triggers the sensor. Even without all that, an educated driver should just see the sign on the tunnel and determine their truck doesn't fit...

Funnily enough, blinking taillights are actually banned here because it's very difficult to guage distance and speed from a blinking tail light.

That's why you have a blinking taillight and a continuous light.