- The ramps use a combination of momentum-halting media like gravel or sand and an incline to bring runaway vehicles to a halt.
- For a deeper dive into truck escape ramps, watch the video above.
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Following is a transcription of the video.
Video: Oh! He took it! He took it, dude! Gravel is flying! Dirt is flying!
Narrator: If you've driven any one of the many highways crisscrossing the world, chances are you've seen one of these: a truck escape ramp.
Some escape ramps slope upwards. Others are flat. Some contain sand, others gravel. But regardless of design, they all serve one purpose: to bring vehicles with malfunctioning brakes to a safe stop. Just how do they work?
When designing a new ramp, state authorities consider factors specific to the road, like how steep the grade is and what road conditions look like at the bottom of a hill. Though they may look different depending on where you see them, escape ramps around the world do have some things in common.
Most escape ramps make use of arrester beds, pools of sand or gravel. The material in the bed is selected for their low coefficients of interparticle friction, meaning when a wheel or axle touches the bed, the material in it moves away from each other, allowing the truck to sink into the gaps.
Like a swimming pool, escape ramps are shallow at entry, anywhere from 3 inches deep, and get deeper, to around 48 inches at 100 to 200 feet in. When a truck enters the bed, it meets little resistance at first, then more as it travels. This means the truck decelerates gradually, reducing the risk of injury to the driver, and stands less risk of flipping over, or capsizing.
When observing escape ramps around the world, three designs stand out: the sandpile bed, the gravity escape ramp, and the mechanical arrester ramp.
This is a sandpile bed. This type of escape ramp contains loose rows of sand perpendicular to the direction of traffic. As a truck collides with the pile, the energy of the truck is transferred to the sand. As the sand is sent flying away at high speeds, an equal and opposite force acts against the truck, reducing its velocity. Impacting sandpiles, though, is... jarring, putting the driver at higher risk of injury, which brings us to the gravity escape ramp, distinguishable by its gradually ascending slope.
In addition to the friction of material in the bed, gravity works on the truck, pushing it down and back. The sloped gravity ramp is more effective than a flat arrester bed.
A 10% grade could allow a truck to halt anywhere up to 85 feet sooner. Gravity ramps are the most cost-effective where natural rises occur adjacent to the road. Some terrains, though, simply do not allow for a naturally occurring gravity escape ramp. So, state agencies have turned to a more experimental form of escape ramp.
The mechanical arrester ramp can be installed on flat ground or even downward slopes. Unlike other designs, this ramp does not have an arrester bed. Instead, it contains a series of stainless-steel catch nets. The nets absorb the energy from a truck collision. Like a rubber band being pulled taut, the force exerted on the truck increases exponentially the further the truck travels. The mechanical arrester ramp then can stop a truck more quickly than gravel or sand arrester beds.
Entering a truck escape ramp is a one-way trip. If the ramp works properly, a truck will either end up submerged or damaged, needing a tow to recover. Despite the expense, agencies urge truck drivers and other motorists to use the escape ramps.