For example, most trucks have a sticker with the number 60 on it. That means they can only go 60km/h (~35-40mph) no matter how high the speed limit for normal cars is. They are subject to the same limit as other cars if the road has a limit of less than 60km/h.
I hate it when anyone goes slower then the posted limit. It is just a dangerous, in my opinion, as going faster then the posted limit.