Speeding

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Speeding

Postby Giant Communist Robot » Mon Nov 13, 2006 12:59 pm

The headline in the local paper stated there would be a crackdown on speeders; the chief of police said more than 50% of the traffic fatalities on Oahu had speeding as a factor.

With these two assumptions

1. The chief thinks speeding is the culprit,

2. The standard method for determining speed limits is to sample the traffic and then set the limit at the 85th centile--I'll assume people will drive the same regardless.

An' plugging that into Bayes theorem I see only a .22 chance the chief is right.
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Postby Giant Communist Robot » Mon Nov 13, 2006 5:04 pm

What the heck? I make provocative use of a controversial technique, and support it with some flimsy and assailable premises and no one takes the bait.....


Where are all the lusty debaters?
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Postby Shapley » Mon Nov 13, 2006 5:17 pm

Beats me. I've been stirring the pot over in the 'environment' thread to no avail.

I read your post, looked at the link, and didn't want to try the math to verify your hypothesis. :dunce:

The argument given around here is that, contrary to the belief the 'speed kills', the reality is that 'speed differential kills'. It's not the gas-guzzling SUV roaring down the road at 85 mph killing people, it's the little old ladies doing 50 mph in their '82 Ramblers and refusing to get out of the way that results in death and mayhem on the highway.

I'm not sure how to express that mathematically, but I would expect that the macroscopic cross-section for collision for two bodies traveling in the same direction is greater if the speed of x is greater than or less than the speed of y. The probability of collision grows as the differential between x and y grows larger, and equals zero when x = y.

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Of course, it would depend on the starting point of x and y, among other factors :roll:
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Postby analog » Mon Nov 13, 2006 5:28 pm

I guess I didn't grasp what was the question.

That aside, I went through grade school before the "New Math", which is what that Wikipedia entry reads like. I never took statistics either.

I did however have a year of Latin in jr high school.
Upon reaching this phrase:
Bayes' theorem tells how to update or revise beliefs in light of new evidence: a posteriori.
I assumed it was a polite reference to a methodology of "Engineering Estimates" known as "USWAG" , the "Unscientific Wild-Ass Guess. "

a.
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Postby Shapley » Mon Nov 13, 2006 5:36 pm

I followed the link on a posteriori, and noticed that it had a reference to deCartes. I then followed the links further and found a reference to Horace.

I find it interesting that they seem to be putting deCartes before Horace. :wink:

V/R
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Postby Giant Communist Robot » Mon Nov 13, 2006 5:49 pm

Shapley wrote:

I read your post, looked at the link, and didn't want to try the math


The math is trivial--there is nothing wrong with it. Here it is:

.15X100=15
.85X100=85
15X.5=7.5
85X.5=42.5
42.5+7.5=50
7.5/50=.15

Yeow! Never trust me with arithmetic! Its .15, not .22!

analog wrote:



Upon reaching this phrase: Quote:
Bayes' theorem tells how to update or revise beliefs in light of new evidence: a posteriori.
I assumed it was a polite reference to a methodology of "Engineering Estimates" known as "USWAG" , the "Unscientific Wild-Ass Guess. "


Hey! Good one!
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Postby Haggis@wk » Mon Nov 13, 2006 6:26 pm

I find it interesting that they seem to be putting deCartes before Horace.
:roll:

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Postby BigJon » Tue Nov 14, 2006 3:27 am

Too much speed is the easy thing to write on the accident investigation form. Doesn't require any subjective evaluation or nuance. The reports are skewed, not the probabilities.
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Postby GreatCarouser » Tue Nov 14, 2006 10:18 am

Someone should arrest this 'speeding' person! Sounds like a serial killer. I had thought speed limits also had something to do with a vehicle's ability to maintain its control and direction on the road surface in question?
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Postby Shapley » Tue Nov 14, 2006 11:23 am

Prior to the nationwide 55 mile per hour limit, some states, particularly in the Northeast, as I recall, listed the speed limit as being a 'proper and prudent' speed for road conditons. I think Vermont had this listing. If you can find an old road atlas from the '60s and early '70s, you'll find a listing of speed limits by State, and there were a few that listed it this way.

Seems like it would have been a legal nightmare attempting to prove in court that someone was driving at an 'improper and imprudent' speed, but those were simpler times, when a cops' word held sway in court. You couldn't fight City Hall in those days, and you were polite to the cops or you got your head knocked in, and then got fined and jail time for making the cop knock your head in.

That was before rudeness became a constitutional right.

V/R
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Postby OperaTenor » Tue Nov 14, 2006 2:07 pm

This is old information, and it pertains specifically to motorcycles, but as far as I know, the Hurt Study is still considered the centerpiece for any discussion on motorcycle safety. The link is to a list of conclusions from the study, and the pertinent conclusion to this thread is:

42. Injury severity increases with speed, alcohol involvement and motorcycle size.


I've known a lot of cops through my work in the motorcycle biz, and they, to a person, would cite speed as an accident severity factor.
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Postby Catmando » Tue Nov 14, 2006 2:15 pm

Speed kills! I usually travel the posted speed limit or a bit higher, but that is only during ideal weather conditions (which are what the speed limits are for).

Excessive speeding or not slowing down below the speed limit to adjust to wintry weather conditions is a huge problem here in Manitoba (and I'm sure anywhere else that snows during the winter). Seems every year that people forget how to drive carefully upon that first blizzard or snow fall when the roads get really slick!

Excessive speed may or may not cause more or less than 50% of accidents, but it is a major problem of causing accidents (if not the greatest problem).

On the flip side, driving way under the speed limit is also very dangerous!
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Postby Selma in Sandy Eggo » Tue Nov 14, 2006 2:20 pm

If I recall an ancient physics class correctly, the force of an impact is easily calculated. It's the mass of the impacting body times the square of the speed of the impact, divided by two.

However, it's way too easy for accidents to be blamed on "driving too fast" or "loss of control" - it's a little like "pilot error" and really means that the investigating cops couldn't identify a cause for the accident so they pulled up the old catch-all.
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Postby Selma in Sandy Eggo » Tue Nov 14, 2006 2:22 pm

I'm trying to picture Sandy Eggo driving, with snow. :driver: :scooter: :thumbdown:

Bad. very bad. We don't even know how to drive in a little rain.
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Postby jamiebk » Tue Nov 14, 2006 3:34 pm

IMHO, speed(ing) is much less a cause of accidents as it is a factor in the severity of them. Speed exacerbates almost all the negative consequences of what could have been a minor event.
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Postby GreatCarouser » Tue Nov 14, 2006 4:29 pm

Selma in Sandy Eggo wrote:I'm trying to picture Sandy Eggo driving, with snow. :driver: :scooter: :thumbdown:

Bad. very bad. We don't even know how to drive in a little rain.


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Postby piqaboo » Tue Nov 14, 2006 6:00 pm

Someone actually studied this. Wish I remembered a reference. I believe I remember the conclusions correctly.

Death rate higher if speed higher.
The leading cause of all accidents is speed differential (vs average for that stretch of road at that time).
Its more dangerous (rate of accident occurence) to go 5-10 below the average speed than the same differential above it. (This one puzzles me... who is the second party involved in the accident? Must be someone traveling near the average. Or another slowpoke?)

Speed reduces your time available to react and correct, as well as increasing the force of impact.
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Postby piqaboo » Tue Nov 14, 2006 6:04 pm

The headline in the local paper stated there would be a crackdown on speeders; the chief of police said more than 50% of the traffic fatalities on Oahu had speeding as a factor.

With these two assumptions

1. The chief thinks speeding is the culprit,

2. The standard method for determining speed limits is to sample the traffic and then set the limit at the 85th centile--I'll assume people will drive the same regardless.


IF the speeds sampled follow a normal distribution, then roughly 85% of the people travel below the to-be-defined speedlimit. Thus, 15% of the drivers are involved in 50% of the fatal accidents (not that the speeders are necessarily the ones who die). Its possible. It just means the speeders are involved in fatal accidents ~ 5x as often as nonspeeders.
Since most of us fall into both populations at differnet times (speeders and non speeders), that 15% of the population is actually just a slice in time.

CRacking down WILL raise local revenues, if they actually catch anyone. But since they announced it, sounds like the motive really is to slow folks down.
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Postby BigJon » Tue Nov 14, 2006 7:53 pm

Except that a notation of speeding on an accident report does not indicate a violation of the posted speed. Only a speed that was too fast for conditions.

My poor wife crashed our new car this morning on her maiden voyage. :cry: :cry: :cry: :cry: She was well below the posted speed limit, but a combination of the rain-slicked roads, curves and an unfamiliar car caused her to spin into the ditch. Car is most likely totaled and dear wife is full-body stiff, but otherwise OK. She will be warned, but not cited, for speeding nonetheless.
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Postby Selma in Sandy Eggo » Tue Nov 14, 2006 8:04 pm

Oh, dear. :grouphug: to BigJane, plus the painkiller of her choice. Plus a long hot bath.
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