Friday, December 8, 2017

Guardrail Lessens the Force of Impact During Car Crashes


            My Dad works in the guardrail business, so I’ve spent the majority of my childhood being lectured about the importance of guardrail and how it “saves lives on a daily basis.” From him, I’ve learned about the various purposes guardrail serves, but the one purpose that always grabbed my attention was the fact that metal guardrail can lessen the force of impact of car crashes, allowing drivers to sustain fewer injuries and reduce the amount of casualties.

            A guardrail’s end terminal functions to absorb the force of impact of cars that crash into it by collapsing. The head of the end terminal will push back onto the guardrail, forcing the metal sheet perpendicular to it to feed through a small opening, turn, and ribbon out the side to absorb the kinetic energy.



According to the Federal Highway Administration, a standard end terminal should be able to withstand four different collisions to pass inspection, but the one I will focus on will be a 0 degrees head-on impact with a 2000 kg pick-up truck going at a speed of 100 km/hr (27.8 m/s), or 62 mph.


            To put things into perspective, the force of impact of crashing this pick-up truck into a solid brick wall would be as follows:

W = KE

Fd = ½ mv2

F = (mv^2) / (2d)

F = [(2000 kg)(27.8 m/s)^2] / [(2)(0.75 m)]

F = 1.03 x 106 N

(52.6 g’s)

The only “give” in this scenario is the small amount of distance the front of the pick-up truck collapses (0.75 m). Chances of the driver surviving this type of crash are slim at best from the sheer amount of force felt in such a short amount of time.

            When the truck crashes into the end terminal, the force on the driver decreases significantly because the transfer of kinetic energy (and subsequent force) occurs over a longer period of time. This calculation assumes that 11 meters of guardrail ribbons out with 0.5 meters of the front of the truck collapsing.

W = KE

Fd = ½ mv2

F = (mv^2) / (2d)
F = [(2000 kg)(27.8 m/s)^2] / [(2)(11.5 m)]

F = 6.72 x 104 N

(3.43 g’s)

            Guardrail helps reduce serious injury and death during car crashes by absorbing the force of impact of the vehicle during a head-on collision. Guardrail’s ability to ribbon out allows the transfer of kinetic energy and the absorption of the force to occur over a longer period of time, significantly reducing the force of the collision felt on the driver. 


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