What does this sign on a truck mean?

Don't follow it too close because it will make sudden stops and also you never know when some stone or other debris is going to fly off the truck and damage your car or cause an accident. I understand your question BTW

The sign appears only on highway construction vehicles, and people do just follow the traffic. The construction vehicle can go into a work area and stop, but the regular traffic is not supposed to be there and it can be complicated getting back into the traffic lanes.

You may not think you would follow a truck into a construction area, but if the truck is large and you are not paying attention - ipod or cell phone user, maybe - you can't see where the truck is going and all of a sudden, there you are - in a ditch!

It means what is says, don't follow it.

Many construction sites have specific access routes that are only for construction vehicles.

If you follow down one of those not only will you get stuck in construction traffic but have a high risk of damaging your vehicle.

It says that , because small rocks can fall from it and damage the following vehicle.

Admittedly, a badly worded sign. It really means keep back at least 50 ft. due to materials falling off. If you lived in Atlanta area you would already know what this means.

a lot of times they are painting or putting in reflecters and the glue or paint will get on the cars if you follow.

Perhaps it means that you shouldn't drive directly behind it, because it might be transporting loose material that might drop down on your car.