Speed Limits

There’s been a lot of debate in our newsgroup regarding if speed limits really help make safer drivers and if it really saves lives. I got the following quote from /. and I found it quite interesting. Then again, those little white signs with the numbers on it are just recommendations, right?

Look, traffic engineers know, and have known for a very long time, that the safest speed to set speed limits at is the 85% percentile speed: the speed which 85% of the free-flowing traffic on that particular road travels at or below. This is because the large majority of drivers are reasonable and prudent, and while they wish to reach their destination in a short amount of time, they also wish to remain alive and unwrecked.

If traffic engineers want this speed on a stretch of hypothetical road to drop, they do this by changing the road surface. Narrows, curves, crests, inclines, will all reduce the 85th percentile speed.

Setting a speed limit lower than the 85th percentile speed doesn’t reduce the speed at which traffic flows. I’m going to repeat that again, because it sounds vaguely important:

Changing the speed limit doesn’t change how fast people drive [state.tx.us]. The safe speed for a road is determined by the road design and the road conditions, and *not* by some arbitrary number on a sign.

The notion that traveling at the posted limit +5 is more dangerous than traveling at the posted limit, or than traveling at the posted limit -5, is reasonable only if the posted limit reflects the 85% percentile speed.

Sometimes it does. Some states even have it written into their laws that that’s how speed limits are determined.

But more often it does not. More often, speed limits are set artificially low, in order to provide a source of revenue for the state. If you set a speed limit below the 85% percentile speed, people will generally ignore it, drive at the speed dictated by road conditions and their ability, and then you can ticket them for speeding.

Here are the actual conclusions of that study I linked to just above:

Effects of Raising and Lowering Speed Limits

Based on the free-flow speed data collected for a 24-h period at the experimental and comparison sites in 22 States, posted speed limits were set, on the average, at the 45th percentile speed or below the average speed of traffic

At sites where speed limits were raised, there was an increase of less than 1.5 mi/h (2.4 km/h) for drivers traveling at and below the 75th percentile speed. When the posted limits were raised by 10 and 15 mi/h (16 and 24 km/h), there was a small decrease in the 99th percentile speed.

Raising speed limits in the region of the 85th percentile speed has an extremely beneficial effect on drivers complying with the posted speed limits.

Lowering speed limits in the 33rd percentile speed (the average percentile that speed were posted in this study) provides a noncompliance rate of approximately 67 percent.

Accidents at the 58 experimental sites where speed limits were lowered increased by 5.4 percent. The level of confidence of this estimate is 44 percent. The 95 percent confidence limits for this estimate ranges from a reduction in accidents of 11 percent to an increase of 26 percent.

Accidents at the 41 experimental sites where speed limits were raised decreased by 6.7 percent. The level of confidence of this estimate in 59 percent. The 95 percent confidence limits for this estimate ranges from a reduction in accidents of 21 percent to an increase of 10 percent.

Lowering speed limits more than 5 mi/h (8 km/h) below the 85th percentile speed of traffic did not reduce accidents.

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