# How do guard rails on the interstate actually work?

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I’m no engineer and obviously don’t understand physics very well, since I’m asking this question. But how can guard rails keep a car or semi truck driving 70+ MPH from leaving the road?

In: 4

For two reasons:

1) Most commonly it is a glancing blow. That is, the vehicle drifts into the guardrail and the guardrail just redirects it, It doesn’t stop the vehicle. A guardrail is unlikely to stop a large truck that hits it at nearly a right angle.

2) The sections of guardrail connected together. So, even when one, or a few posts break, and the guardrail bulges to the side, the guardrail can keep holding the vehicle from going further off the road because the part of the guardrail that moved is still connected to the part ahead (and behind) that is in good shape.

Simply put – they can’t. They’re design is more so to slow down a fast heavy object , a car , in breakaway stages and giveaways( of the guardrails) so that the momentum of the car is quickly but efficiently slowed without sudden jarring.

If you put that guard rail in a huge parking lot, and have a car or semi truck drive 70 mph straight at it, it will likely disintegrate from the impact.

But on the highway, cars and trucks are driving alongside the guard rail, and if (for example) someone falls asleep they’ll veer into the guard rail at an angle. So cars and trucks are heavy, but still they’re “easing into” the guard rail, so to speak.

So a car / truck’s own steering system, its wheels, can “ease” it back onto the road, if that driver wakes up in time. The sideways force doesn’t have to be that great, it’s just a matter of “nudging” the car or truck back onto the road. That’s what guard rails do.

70 mph is only the velocity in the forward direction. In case of lane departure, usually the vehicle is departing the lane at much less than 70 mph in that direction.