Quote:
Originally Posted by Bear Islander
We don't set speed limits on our roads by studying how fast cars are going on them. We set them by road conditions and accident data.
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If I may interject this. Been done this way almost forever.
http://www.cabobike.org/2010/01/30/a...ed-limits-set/
Quote:
Since speed limits do not have a measurable influence on actual speeds, it makes sense to set speed limits so as not to make lawbreakers out of a large fraction of motorists. Speed limits that are set too low makes law enforcement more difficult and leads to a disrespect for speed limits.
Shown here is a speed distribution on a rural road. Notice that the distribution is quite narrow, meaning that most motorists drive at nearly the same speed. You will see that changing the speed limit by just 5 mph will make a big change in the number of violators.
So the 85th percentile rule is simply a recognition that speed limits are set for the purposes of enforcement. And since law enforcement only has the resources to cite the most egregious violations, the speed limit is established at or near the 85th percentile, making only about 15% of free-flowing motorists violators.
You may also notice that motorists who go a few mph over the speed limit are not cited. That occurs for two reasons. The first is the lack of law enforcement resources. But where the system really falls down is at the judicial level. Traffic court judges routinely give a 10-12 mph leeway on speeding tickets out of a false belief that radar is inaccurate. So even if some motorists do base their speeds on the speed limit, they routinely go about 10 mph over and almost never get a ticket.
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