# What does it take to prove a car safe?January 29, 2008 10:44 AM   Subscribe

Why would a car perform significantly differently in two relatively similar crash tests? For instance, why would a rear passenger in the Honda Accord do better in side crash tests from the IIHS than the NHTSA? Is it simply because the NHTSA crash is 7mph faster?
posted by Dasein to Technology (4 answers total)

Yep, 7 mph can make a huge difference. The energy being dissipated is a function of square of the velocity. So (without looking up the speeds), if the NHTSA tests at 42 mph versus 35 for the IIHS, the NHTSA imparts 42^2/35^2 = 1.44 times more energy to dissipate.
posted by Doohickie at 10:55 AM on January 29, 2008

+1 Doohicke.

A small increase in speed is a major increase in energy of the crash - this needs to be absorbed by the car. Some cars may perform well up to a certain theshold, but then go completely to pieces afterwards ,some may have a less severe fall off past their 'nominal' crash energy threshold.

Incidentally, a 30 mph test isn't testing for a crash that occurs while driving at 30mph. It's for a 30mph impact. You can have less than a 30 mph impact in a crash that occurred while you were initially driving along at 55-60 or so. You get a lot of speed off by slamming the brakes on.

/slightly irrelevant tangential point.
posted by Brockles at 11:20 AM on January 29, 2008

I've just noticed you title: "What does it take to prove a car safe?"

You can't. Nothing is 'safe'. It is just better or worse than another vehicle according to a given set of (potentially arbitrary) tests to try and quantify it. Being as accidents vary enormously in impact and style, it really is a way of trying to demonstrate some sort of resistance to the sorts of impacts that are present in most accidents. It is only useful as a comparative, and even then only with interpretation of the results. It is by no means definitive.
posted by Brockles at 12:14 PM on January 29, 2008

There has been quite a bit of controversy over the years about the parameters of various tests such as impact safety, impact repair expense, and mileage being biased to give an apparent advantage to one manufacturer over another (usually domestic over foreign).

At first blush it seems to me that you can probably expect the Insurance Institute to tune their tests to minimize claims-- which correlates pretty well with your safety-- whereas the NHTSA is more vulnerable to political pressures, and those often correlate with safety, too-- negatively.
posted by jamjam at 1:00 PM on January 29, 2008

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