Local truck driver here. Could be wrong but the guys that miss all the other signs are probably zombied out from road fatigue, aka at the stage of just keeping it between the lines and probably missing everything else. A sign with flashing amber lights, or the ones with a dozen smaller inset flashing lights around the perimeter staged at the last viable turn around would be the best bet for those guys. Maybe even put a few in short succession. No excuse for driving at that stage of fatigue, but as far as realistic solutions go, flashing lights might just catch their attention when nothing else will. Fair game to up the penalties for ignoring them or driving fatigue impaired as well.