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

From: [email-masked] (Mr. Fun)
Date: Fri, 27 Oct 1995 04:06:33 GMT

[email-masked] (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.

---

From: [email-masked] (Mr. Fun)
Date: Sun, 29 Oct 1995 07:24:24 GMT

[email-masked] (Kevin Atkinson) wrote:

>In article <[email-masked]>,
>NMA [Bob M] <[email-masked]> wrote:
>>[email-masked] (Mr. Fun) wrote:
>>>I'm not sure "slow" drivers would be all that big a problem.  
>>
>>Slow drivers are a problem on two-laners.  Ever try to pass them?  Plus, 
>>they're just as likely to get into an accident as those who exceed the 
>>speed of traffic.
>
>Correction, *more*.
>
>>>For  example, 16% of the drivers are going over 65mph and 84% are 
>going >40mph.  The 85% speed will be 65mph. 
>
>
>> >The speed limit would be set at 40 or 45, not 65. 
>
>Unfortuntly true.  Speed limits tend to accomdate slow drivers making it 
>legal for then to annoy everyone else on the road.
>
>-- 

You folks aren't following how this works.  Here's a table

Table 1

Speed =>  30   50   65    70
Number
of cars =>70    10   10   10

Table 2

Speed =>  30   50   65    70
Number
of cars =>40    40   10   10

In BOTH cases, the 85th percentile speed that will be used to set the speed
limit, if it's set according to engineering instead of political criteria,
will be 65 mph.  That is in spite of the fact that in the second table the
slow cars were going a lot slower.  In BOTH cases, you don't include enough
cars to have included 85% of them till you get to the 65 mph point.

As a point of info, in the first example, the weighted average speed is 39.5
mph.  In the second example, it's 45 mph.

---

From: [email-masked] (Mr. Fun)
Date: Sun, 29 Oct 1995 07:24:24 GMT

Table 1

Speed =>  30   50   65    70
Number
of cars =>70    10   10   10

Table 2

Speed =>  30   50   65    70
Number
of cars =>40    40   10   10