I totally agree. It goes back to the first motorway when some idiot 'explained' that the slow lane was for trucks, the middle lane for cars and the outer land for fast cars. Unfortunately John Q Motorist took it to heart. Why, why, why don't they put "Keep left unless overtaking" on the variable message signs instead of platitudes like "Take a rest"
going back to the original query, it's a chicken and egg situation. Motorists tend to ignore signs so they have to repeat them again and again for miles beforehand. Because the signs appear meaningless, motorists tend to ignore signs.
The other one that infuriates me is signs saying "Beware fog" and you can see clear to the horizon. It may have been foggy 5 hours previously but the numpty on duty hasn't switched the sign off.