"Overheight must turn" means to virtually all "IF you are Overheight you must turn". Obviously that driver doesn't think he is overheight.
Change it to "Warning: Overheight truck detected in this lane" (with proper lights/arrows pointing to lane). And blink red as well.
The truck driver doesn't realize the sign is only on when he is there, he never sees it when it's off, he has no idea it's illuminated specially for him.
This is government saying "We did something", while doing nothing.
And this sign is above even the traffic lights. Even someone glancing at the colors would not see this.
What they should do is lower the sign to the height of the bridge - so people hit the sign (or something dangling from it).
But that would require government admitting that people hitting the bridge is their fault, and not stupid drivers.