I always wonder how often a trucker has to use one of those. I never seen one after it has been used most of the time they seem pretty over grown.
There is a saying in the industry, "You can go down a hill as many times as you want too slow, you can only go down once too fast." a drive should go their whole career without ever using one. they get used by the driver who either have a mechanical failure, don't know how to properly down down a steep grade and smoke their service(normal/air) brakes or simply are too stupid to believe they need to slow down. You can bet that if you use one your career will be over.
It was more common to have to use them before jake brakes came into common use. Back then you would have to just use your service brakes to go down steep grades. You use to have to adjust the brakes properly before going down the grade. Trucks use drum brakes and when overheated the drums expand and you can't apply enough pressure to slow the vehicle anymore, they will usually start smoking before this happens, smells like burt clutch. To keep them from overheating you use to have to go down a 6% grade at around 10-15mph when fully loaded. In my current truck I can go down a 6% grade at 45mph when fully loaded.