This archive report was first published on 30 August 2019.
On steep roads, runaway vehicles pose a significant threat to drivers and other motorists. To mitigate this risk, truck escape ramps have been installed on highways around the world. But how do these ramps work?
According to experts, the design of an escape ramp is carefully considered to take into account factors specific to the road, such as the steepness of the grade and the road conditions at the bottom of a hill. While escape ramps may look different depending on their location, they share some common features.
Most escape ramps make use of arrester beds, which are pools of sand or gravel. The material in the bed is selected for its low coefficient of interparticle friction, meaning that when a wheel or axle touches the bed, the material moves away from each other, allowing the truck to sink into the gaps. This gradual deceleration reduces the risk of injury to the driver and minimizes the risk of the truck flipping over.
When a truck enters the arrester bed, it meets little resistance at first, but as it travels, the resistance increases, causing the truck to slow down. The arrester bed is typically shallow at entry, around 3 inches deep, and gets deeper, up to 48 inches, over a distance of 100 to 200 feet. This design allows the truck to decelerate gradually, reducing the risk of injury to the driver.
There are three main types of escape ramps: the sandpile bed, the gravity escape ramp, and the mechanical arrester ramp. The sandpile bed contains loose rows of sand perpendicular to the direction of traffic, which are sent flying away at high speeds when a truck collides with the pile. This creates an equal and opposite force that acts against the truck, reducing its velocity.
However, impacting sandpiles can be jarring, putting the driver at higher risk of injury. This is why the gravity escape ramp, with its gradually ascending slope, is more effective. The sloped gravity ramp is more effective than a flat arrester bed, and a 10% grade can allow a truck to halt up to 85 feet sooner. Gravity ramps are the most cost-effective where natural rises occur adjacent to the road.
However, not all terrains allow for a naturally occurring gravity escape ramp. In such cases, state agencies have turned to a more experimental form of escape ramp, the mechanical arrester ramp. This ramp does not have an arrester bed but contains a series of stainless-steel catch nets that absorb the energy from a truck collision. The force exerted on the truck increases exponentially the further it travels, allowing the mechanical arrester ramp to stop a truck more quickly than gravel or sand arrester beds.
