Losing control of a vehicle on the highway is a nightmare scenario for any motorist. The risk is even greater for truck drivers, whose big rigs are both harder to stop and capable of inflicting greater damage at high speeds. That’s why many highways include runaway ramps. When a truck’s safety features aren’t working, the last-ditch escape route may be the only way to prevent disaster.

In this news clip, you can watch an out-of-control vehicle use the ramp for its intended purpose. Semi-trucks can weigh up to 40 tons, and once they gain enough momentum, that weight can be incredibly difficult to slow down even with specialized brakes. Failing brakes are more common on high-elevation routes like Colorado’s portion of I-70, as seen in the video below. Trucks need to maintain momentum while climbing steep roads then immediately slow down before rolling into the descent. If drivers are unable to pull this off, their breaks may become useless, transforming their truck into a multi-car pile-up waiting to happen.

When a truck’s breaks aren’t working properly, a runaway ramp provides a backup solution. They’re usually built up a mountain or hillside, allowing gravity to help slow down the vehicle. If the grade of the road is10 percent, that’s enough to bring an out-of-control truck to a stop in 85 feet. Instead of smooth pavement, these off-shoots are made with gravel or sand, adding another layer of friction to the speeding wheels. The farther up the path the truck travels, the deeper the sand becomes, transitioning from a rough road to a sand trap. The sand or gravel may be piled up to 4 feet deep near the end of the ramp, which is deep enough to bury the truck’s wheels and keep them secure.

Runaway truck ramps are reserved for emergency scenarios. If a driver is forced to use one, they may find themselves stuck on the side of the highway for a while waiting for assistance. But in situations where there’s no other option, there’s no doubt they can save lives. Here are more secrets of truck drivers you should know.

This article was originally published on www.mentalfloss.com as How Trucks Use Runaway Ramps in Emergencies.