I did read the MUTCD and even quoted part of it in another post. It answered none of these questions. If you want to show me where these questions are answered in the MUTCD, please do so.
I believe I do understand the 85th percentile rule, which is why I feel it is so silly.
Here is what I believe
- The 85th percentile rule is arbitrary (why the 85th percentile and not the 90th percentile, or the 75th percentile) - There are no studies that I can find that "prove" that setting a speed limts at the 85th percentile speed results in the lowest accident and-or baneity rate (I am sure you have one that is not easily accessed) - The cheif rationale for using the 85th percentile rule to set speed limits seems to be - "People ignore speed limtis, so we might as well set them high enough so that only 15% of the people are vioating the law." - The 85th percentile speed is based solely on people's opinions of their capabilities. I know many people who have an unjustifiably high opinion of their driving ability. I know others who are competent to drive at well over the average speed of traffic. Why should they be limited to the speed that 85% of the people are comfortable with. Why should others be encouraged to drive faster (or be harbutted for driving at less than the speed limt) becasue the speed limit is set to high? - The 85th percentile speed is not the normal speed of traffic or the average speed of traffic. It is just the speed that 85 percent of the drvier are not exceeding. The average speed of traffic might be much lower. So setting the speed limit at the 85th percentile may not have any effect as far as evening out the traffic flow and preventing clumps of car where faster cars are trapped behind slower cars. - It appears to me that the 85th percentile rule is just a politically correct way to set speed limits so that you can identify an easy to persecute minority.
Ed
