There's nothing quite as scary as driving down the road and discovering your brake system has malfunctioned. It happened to me once when I owned a 1966 Dodge Dart. Slush and ice had packed on the firewall and limited how far I could turn the steering wheel. Then I discovered the accelerator pedal was stuck open. The final straw was that I couldn't press my brake pedal because of the ice.
That certainly contributed a few of my grey hairs.
Now imagine that scenario except you're driving in a mountain range at the wheel of an 80,000lb tractor trailer. If you're going uphill, you can just ease it over onto the shoulder. If you're going down a 6% grade, you're in trouble. Gravity will continue to accelerate you until you go off the road in the largest Dukes of Hazzard jump you'll ever experience.
Thankfully, someone thought about this and came up with an idea to help save lives and equipment. The highway maintenance folks have offramps that go up at an angle and they cover the ramp with a deep layer of small rocks. As the runaway truck enters the emergency ramp and rolls up the incline, it loses momentum and the rocks slow the vehicle down until it stops.
There are several signs on the highway to alert truck drivers of how far away the runaway truck ramp is. Please note it's not for taking your four wheel drive pickup for a spin.
Iron Noder 2017
Tem42 notes: In North Carolina, these ramps have big mounds of sand every few meters -- possibly filled with gravel, for all I know.