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Re: 85th percentile speed and 2 lane roads...



kevinatk@home.com (Kevin Atkinson) wrote:

>I was wondering if the 85th percentile rule really works on 2 lane 
>roads.  Does it really reflect the true "safe" speed for that road
>(which many times is between 60-65 mph).  The type of roads I have
>in mind or two lane state highways (slightly curvy, a few hills etc).
>
>It would seam to me that on heavily traveled two lane roads slow
>drivers will severally effect the 85th percentile speed because unless
>the road is EXTREMELY passable a slow driver will cause everyone
>behind him to move at his pace (generally 45-50 mph).  In rush hour
>traffic that can often be anywhere from 10 to 20 cars.
>
>The roads I am thinking of (for those in my area) are MD 108 from Sandy 
>Springs to 216 and 216 to US 29.  I generally take both of these roads at 
>about 60 mph (I often slow down to around 50 on parts of 216 due to a lot 
>of turning traffic).  However I often get stuck behind people who take 
>them at around 50mph. (A speed that I find very borring)
>

I'm not sure "slow" drivers would be all that big a problem.  The 85% is the
speed 85% drive at or less then, it's not the average of all of them, it's
just the count.  As long as 15% of the drivers are able to go over the "right"
speed it doesn't matter what the average speed of the other 85% is.  For
example, 16% of the drivers are going over 65mph and 84% are going 40mph.  The
85% speed will be 65mph.  If it had been 16% of the drivers going over 65mph
and 84% are going 50 mph, the 85% would STILL be 65mph.