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



kevinatk@home.com (Kevin Atkinson) wrote:

>In article <46r6pd$1qt@fullfeed.msn.fullfeed.com>,
>NMA [Bob M] <nma@genie.geis.com> wrote:
>>gribble@primenet.com (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.



References: