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Comment by rifung | Hacker News Reader

I am surprised that higher speed limits are so strongly correlated with higher mortality rates. I thought the trend was to increase speed limits, and I remember reading that at least for one of the states where they increased the speed limit, people were still pretty much driving at the same speed.

It seems that the general consensus is the speed limit should be at the 85th percentile to minimize accidents. However, this just considers minimizing accidents, and I suppose I can see how we can lower the number of accidents but also increase the mortality rate.

Also, I suppose it's not completely fair to compare different states, as what works in one might not work in another and vice versa.

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> I am surprised that higher speed limits are so strongly correlated with higher mortality rates.

This really shouldn't be surprising: higher speed limits correlate with higher average speeds, and while energy goes as the square of the velocity, mitigation efforts do not.

No matter how well you engineer crumple zones, improve traction, give automated braking responses... if you are going fast enough when you actually hit something, it's the kinetic energy that is going to kill you.

Bear in mind that mortality rates and incident rates don't really reflect the same things here.

It has a lot to do with reaction times as well. At higher speeds you've already run into the idiot swerving into your lane (or whatever) by the time your brain processes image.

I think the point of raising speed limits is to better match the natural safe speed for a road. The old 55 MPH limit was an over-correction because of high speed accidents. But over time roads were built better and cars became safer, so drivers felt more comfortable driving as fast as 75 MPH.

And, I'm not sure but wasn't there some accidents caused by people suddenly decelerating when they saw a police car? Or am I thinking of the rear-end collisions because of red lights.

The 55 MPH limit in the US wasn't even instituted for safety reasons; it was a reaction to the oil crisis. Any safety impact was probably due more to the improvements in car safety that you mention.

My guess is that higher average speeds are the confounding factor and that the speed people drive is mostly independent of the speed limit.

http://priceonomics.com/is-every-speed-limit-too-low/

Rural highways have much higher fatality rates. I'm not sure that actually changing the speed limit will change that, as drivers might just keep driving the same speed.

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