Tuesday, November 9, 2010

A Car Crash

Ever wondered what it's like to crash into a freeway overpass at high speed in a convertible? What would it be like? Would you survive and with what sort of injuries? What's in those yellow barrels, anyway?

I wondered. Yeah, it's weird, but I have my reasons. I'm sure you can guess what they are.

The yellow barrels you see in front of overpass supports and barriers, especially on the freeway, are impact attenuators. More specifically, they're called Fitch barriers, named for John Fitch, who invented them. There are other types of impact attenuator, but Fitch barriers are the simplest and probably the most effective.

Each barrel in the line is filled with a progressively greater amount of sand or water. When a car hits the line of barrels, the sand/water is scattered in all directions, taking with it some of the car's momentum. Each barrel slows the car down a little more, as the amount of energy absorbed (and speed reduction) increases with the mass of sand/water in the barrel.

A car that hits a Fitch barrier at highway speeds is decelerated over nine or ten meters (or more) instead of the one or two that the front of the car would smash in if it hit a concrete pillar directly. This is a lot faster than a car could stop on its own, but a whole lot better than a direct impact. Some of the tests I looked at had the occupants experiencing less than 10 g, and 15 g at most. Easily survivable.

Vehicles can be flipped around by the impact. (Video of test) In a convertible, you'd end up covered in sand. It's also conceivable that a car could roll after hitting a Fitch barrier, but much more likely in high center-of-gravity trucks and SUVs.

The impact at the front of the car is more than enough to set off the airbags and will smash the front in pretty good.

Now, if the car is moving well over 100 mph, is a small convertible, and the occupant isn't wearing a seatbelt, it's possible they'd survive—maybe even likely. But at that speed, rollover risk is greatly increased, as the car could be deflected by the barrier and slide sideways at a fairly high speed. It's also possible that the car could nose down enough that you'd slide over the top of the airbag, but probably not very likely. Sports cars tend to keep your legs way out in front of your torso.

So, I didn't find a definitive answer. It seems like a rollover would be the most likely cause of fatality. That's a matter of luck. At any rate, I wouldn't try it at home.

2 comments:

  1. Yes, don't drive this way at home. You may leave tire tracks on the living room carpet. :) Why are you even looking this up? I mean, I know why, but, not REALLY.

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  2. Fascinating stuff. Thanks for sharing your research!

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