Subject:
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Re: Can't Drive 65 (was 'Re: Tolerance of vice')
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Newsgroups:
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lugnet.off-topic.debate
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Date:
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Wed, 24 Oct 2001 02:03:14 GMT
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Viewed:
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345 times
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In lugnet.off-topic.debate, Christopher Tracey writes:
> Larry Pieniazek wrote:
>
> > Bad example... since you're referring to noncommon law. Speed limits tend to
> > be revenue generation devices nowadays more than anything else.
>
> Please elaborate... with statistics if you so prefer. :)
I'll just cite what I learned in Transportation Engineering. The proper way
to set speed limits , on a road that wasn't explicitly engineered with
limits in mind (1) is the 85th percentile rule.
This rule assumes that people basically are good, conscientious drivers. You
go out to this road and you observe traffic speeds on a good clear day with
good visibility and adhesion, and you set the limit to the 85th percentile
(such that 15% of the people are going faster than the limit)
THAT is the correct speed for that stretch of road.
ANY lower speed is too slow for conditions and is set for some other reason
than safety of the drivers or pedestrians in the area. Typically revenue
enhancement.
Note that this works even in residential areas, even with kids in the area,
as long as the "kids are in the area" signs are up... because most people
(2) are good drivers.
> Are you denying that speed limits also serve to regulate traffic
> patterns, allow drivers to avoid accidents by reducing stopping
> distance(dependent on ~following the limit of course), etc ?
Certainly not. These are *all* valid factors for determining speed. But the
85th percentile rule takes all this into account. Because people are
generally good (drivers). And because the people acting in aggregate (market
forces) are more accurate than some one guy deciding without data what the
speed ought to be.
Revenue enhancement, however, is not a valid factor for determining speed.
Neither are the desires of the residents of the area (assuming those "watch
out for kids" signs are up, that is).
As for the cite, I've run across this explanation in several texts on
transportation engineering, it seems to be a well accepted empirical
mechanism if you didn't design the highway with a specific speed in mind, or
if conditions (such as traffic, the number of cutins, signage, etc) have
changed since then. Sorry I don't typically take my old college texts with
me on the road, though.
1 - This means designing using lanewidth, curvature, banking, lines of
sight, information about adhesion, prevailing winds, horsepower profiles,
acceleration capabilities and stopping distances, and a host of other info,
to get a "design speed" for a particular stretch
2 - Contrary to the bumper sticker I saw today (on a car vigorously
tailgating someone, ironically) "why am I the only good driver on this
planet?"...
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Message has 2 Replies:
Message is in Reply To:
 | | Can't Drive 65 (was 'Re: Tolerance of vice')
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| (...) Please elaborate... with statistics if you so prefer. :) Are you denying that speed limits also serve to regulate traffic patterns, allow drivers to avoid accidents by reducing stopping distance(dependent on ~following the limit of course), (...) (23 years ago, 24-Oct-01, to lugnet.off-topic.debate)
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