I have a rather different hypothesis in my mind: changing the speed limit does not change the speed of cars relative to the speed limit. People who go 10% over will continue to go 10% over, people who go 20% over will continue to go 20% over. Obviously this can't hold true for extreme values (changing from 50 to 5 or to 500), but in the range where speed limits actually do change from time to time, this is exactly what I see happening.

"Authorities consider this road safe for average drivers at x mph, therefore it must be safe for me at x * 1.y".

The y in the "1.y" pseudomath btw is proof enough to me that claims about enforcement not working are wrong: when you cross state borders it is easy to witness considerable changes in "typical y", the amount of relative speeding customary in the local population of drivers. Road design certainly has an influence on actual speed, but so do limits and enforcement.

> changing the speed limit does not change the speed of cars relative to the speed limit.

Studies [1] have shown that not to be the case.

[1] http://www.ibiblio.org/rdu/sl-irrel/

But why do you want to adjust all the roads in the country where maximum speed ever is 90mph, at least in the USA? Just pass the law to limit cars from driving faster than that, and problem solved!

It always amazes me how cars commercials show awesome dashboards with 250 mph speedos, perfectly knowing there is no single road that you can lawfully go that fast.

Heck, it probably won't cost much more than a thumb drive to stick into your car computer port to update speed limits built in.

Speed limits can act as a target, though. It'd be a little weird to have a 'realistic' speed limit of 35 mph, and then introduce traffic calming measures designed to slow people down to way lower than that, no?

The speed limit should reflect the reality of the road. No, you shouldn't go around tomorrow updating all your speed limits to 85% percentile graded and then try to redraw the roadways to traffic shape to desired speeds. You would do the latter first, then update your signs.

Except in most places, neither is happening. People are driving dramatically over the speed limit because that is the natural speed of the road, and no attempts to change the actual speeds of traffic are happening because they cost money.

So people get to die for the multitude of reasons the article outlines because of political deadlock.

The point of the article is that for 85% of the people driving on the road the speed limit most emphatically does not act as a target. If it did then modifying the limit would be a useful knob you could turn. Since it isn't all it does if give the non-automotive traffic a false sense of security regarding how fast traffic might be coming at them.

I think the idea is to use traffic calming methods to change the 'realistic' speed limit from 35 mph to 25 mph, and then post signs for 25 mph.