What's the real meaning of "No Trucks" signs?

I was running an errand over lunch and a couple of questions clicked in my mind. These have to do with “No Trucks” signs.

Sign 1: NO THRU TRUCKS This implies to me that trucks of any size and weight are allowed, but they can’t use that road as a through road—if they’re going to use the road, it’s because they are going to make a delivery somewhere along the way. I imagine that the County wants to discourage through trucks on that road and would prefer them to be on the state-numbered road a mile or two away.

How do they enforce it? Does a policeman follow the truck to see if he really is cutting through?

Sign 2: NO TRUCKS. This seems meaningless to me – you have to allow trucks along the road—garbage trucks, moving trucks, delivery trucks, fire trucks, etc. This simply sign doesn’t make sense. It can’t possibly be enforceable.

Sign 3: NO TRUCKS OVER 26,000 LBS. This is another one that confuses me. Are they saying that there is some bridge or culvert that is so weak that if a 27,000 lb. truck comes through that the road will cave in? If so, how do moving trucks and fire trucks survive?

Or is this a not-very-subtle way of saying “18-wheelers keep out”? And if that’s the case, why not say that?

Curious minds want to know.