Safety effects of speed limits relative to the 85% speed
Safety effects of speed limits relative to the 85% speed
(OP)
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!
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!





RE: Safety effects of speed limits relative to the 85% speed
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.
RE: Safety effects of speed limits relative to the 85% speed
RE: Safety effects of speed limits relative to the 85% speed
Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.
RE: Safety effects of speed limits relative to the 85% speed
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.