Safety effects of speed limits relative to the 85% speed

All the safety studies of speed limits cited in the Crash Modification Function Clearinghouse are for changing the speed limit from x to y, or for reducing or increasing it by Z. You know what? Taken as a whole, they are pretty inconclusive.

I found this one small study that looked at crash rates and the difference between the speed limit and the 85% speed rounded to the nearest 5 mph (called the engineering recommendation).

Their results showed that a speed limit 5 mph below the engineering recommendation is safest. Total crashes and injury crashes were lower than speed limits set at the 85th. When the speed limit was 10 mph lower, injury crashes increased. They didn;t have neough data for 15 mph to get statistically meaningful results.

So this report suggests that perhaps a lower cumulative percentile speed should be used, but excessively low speed limits degrade safety.

This somewhat supports the conventional wisdom, but it's a small study in a predominantly rural state. I think it's important enough to share here, but if you know of any similar studies, please share!

 

[BigInch]

No study to prove it, but my feeling is that speed limits have little to do with safety, as long as they are outside pedestrian zones where any reduction in speed has a greatly positive affect on reducing pedestrian injuries.

Low speed limits on a poorly configured highway won't help reduce accidents at all. That was exactly what was done a few years ago near here. Speed reduced from 100kph to 80kph. The accidents along a portion of S-turn highway continue, especially and primarily only at one particular entrance ramp of extremely poor design. The entrance onto the last curve in the S on the principal highway from a downward continuously curving exit ramp proceeding from the bridge above. The entrance ramp needs to be closed entirely.

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

 

[civilman72]

BigInch - While poor design plays a bigger part in accidents than speed limits do, speed limits do have an impact on safety. Particularly when unrealistic speed limits are set, which often creates large discrepancies in vehicular speeds, which leads to more accidents.

 

[BigInch]

I absolutely agree. I didn't say "nothing to do". I said "little to do".

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

 

[3DDave]

Speed limit reports are little different than proofs from the flat-earthers or perpetual motion machine sellers. The basis is fundamentally flawed - that if only all drivers acted in some way, especially the way the report writer wants them to, that things will be better.

Instead of whinging about the particular limits, it would be more valuable to call for the repeal of all traffic laws and the removal of all traffic controls, such as lane markings, traffic signals, lane usage signs, and the like. Since the speed reports all claim the majority of drivers will, out of self-interest, select the safest speeds, it is reasonable to assume they will use this same ability to make good judgements in all other areas of driving.