These are also great for heavy vehicles that go long distances down deep grades. There was a time, not long ago, when the service brakes that came on big vehicles were the things that kept them from stopping. Those proved to be only so-so for slowing big vehicles down on steep grades because one could burn through them fairly quickly under heavy use.
You can still see a lot of those mountain highways with special ramps on hills for runaway vehicles. The scary thing is those used to be necessary. They aren't needed so much anymore and that is a good thing.