Michael,

Those round "speed" stickers on the back of the trucks and certain cars are 
the maximum speed those vehicles can travel....you exceed the speedlimit on 
your truck....and get caught you pay a hefty fine.

I seem to recall France having a similar system....in fact if you are a 
"novice" driver you need a sticker on the back of your car warning the rest 
of the world of your "noviceness"...

Ric