The 85th percentile usually will include peak (read: rush hour) times where speeds are significantly lower than the times where traffic is free flowing. So, the 85th percentile may be a lot lower than you think it would be.
The numbers, of course, are a bit biased because everyone is going to drive at a speed approaching the speed limit, hence the 85th percentile will always likely be really close to the speed limit. They would only really change the speed limit if it's a significant increase or decrease over a very long time, given no other external factors.
I've heard of some municipalities (city level) even going to a 95th percentile in urban areas.
Well, just because it's the rule does not mean that anyone pays attention to it. It's the government enforcing itself. We all know how well that goes.:D
The general rule that most DOT's (especially at a state level, federal doesn't really take interest at this sort of micromanagement) is to take the 85% speed percentile of a week long or month long speed study. There are several exception to this rule (school zone, residental areas, hospitals, fire/police entrances) that have fixed speed limits regardless of the 85% percentile.
Slashdot Mirror
The 85th percentile usually will include peak (read: rush hour) times where speeds are significantly lower than the times where traffic is free flowing. So, the 85th percentile may be a lot lower than you think it would be. The numbers, of course, are a bit biased because everyone is going to drive at a speed approaching the speed limit, hence the 85th percentile will always likely be really close to the speed limit. They would only really change the speed limit if it's a significant increase or decrease over a very long time, given no other external factors. I've heard of some municipalities (city level) even going to a 95th percentile in urban areas.
Well, just because it's the rule does not mean that anyone pays attention to it. It's the government enforcing itself. We all know how well that goes. :D
The general rule that most DOT's (especially at a state level, federal doesn't really take interest at this sort of micromanagement) is to take the 85% speed percentile of a week long or month long speed study. There are several exception to this rule (school zone, residental areas, hospitals, fire/police entrances) that have fixed speed limits regardless of the 85% percentile.