I was told they’re for trucks with failed brakes, but wouldn’t the truck just roll back down the ramp? And the ones I’ve seen don’t look long enough for a big truck to fully lose its forward momentum…
In: Physics
Usually they’re set at an incline that will quickly stop their forward momentum. Often times they’ll also have loose gravel or sand incorporated that will stop and secure the truck. If a truck is able to come to a stop, even for a moment, the driver can downshift, lock in a low gear, and kill the engine. Like manual transmission cars, if you’re locked into a forward gear, you can’t roll backwards. This would stop it from heading backwards onto the highway.
>but wouldn’t the truck just roll back down the ramp?
This can be a problem, yes; this is why runaway ramps aren’t steeply graded.
>And the ones I’ve seen don’t look long enough for a big truck to fully lose its forward momentum…
If the ramp isn’t long enough for a vehicle to bleed its momentum normally, it’ll be filled with sand, gravel, or some other material that amplifies the dissipation of the truck’s forward momentum.
They have a sand or gravel pit at the end. Once the truck gets to this point it will start to sink in and usually takes a lot of work to get it unstuck afterwards. Those pits will slow a truck pretty fast. [Here is a dashcam video from one truck looking at another using a runaway ramp.](https://youtu.be/r7djN6bxHcU)
Also, most large airports have a similar system at the ends of the runways. Basically, in that case it is a soft tarmac product that crumbles under the weight of the plane and slows it safely, but rapidly. If you look at the aerial view of a major airport like LAX, you will see yellow chevron or arrows at the end of the runways, this is that area. A fully loaded 747 doing 200 mph will be stopped in that area. [Here is a pic of a smaller jet sunk into the safety area surface. ](https://images.app.goo.gl/uZ82p5BedppQvMo28)
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