From: troyf@microware.com (Troy Frericks)
Subject: Re: Seat Belts Releaseing in Crashes
Organization: Microware Systems Corp., Des Moines, Iowa
Date: Mon, 14 Dec 1992 18:37:10 GMT

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From: uunet!balltown.cma.com!welty (richard welty)
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To: troyf@microware.com
Subject: Re: Seat Belts Releaseing in Crashes
Newsgroups: talk.politics.medicine,talk.environment,misc.kids,rec.autos
Organization: New York State Institute for Sebastian Cabot Studies
Cc: 
Status: R

In article <1992Dec9.141341.13389@microware.com> you write:
>In Iowa, USA when the speed limit changed from 55 to 65, the death rate went down.
>Does that mean that the speed limit was responsible for the decrease in the death
>rate?  I think so (because the speed DIFFERENCE that cars travel decreased, so
>now most cars are traveling at about the same speed).  But, you know what,
>the death rate in Iowa has been steadly decreasing, year after year.  In fact,
>the death rate probably would have decreased that year, even if the speed
>limit was not raised.

>Point being: statistics are not reliable.  If you want to have reliable info, the
>most accepted method is a double-blind study.

with all due respect, i think that your analysis is superficial and
badly flawed (i've spent a great deal of time studying this subject.)

normally i'd post this rebuttal, but posting is broken at this site
at present; if i succeed in convincing you, i'd appreciate it if you'd
posting this for me.

on to the analysis:

it is true that total fatalities did drop when the speed limit was
reduced.  however, there is more going on than the one simple statistic
would tell you.

first of all, fatalities have been on a long downward trend since
the late 40s; crediting the speed limit reduction for the the drop in
total deaths in that single year seems highly  questionable.

secondly, it turns out that highway passenger miles were drastically
down that year.   if you normalize the death count by the number of miles
drive, you find that deaths per passenger mile were up (!?) that year.
this is looking more and more suspicious.

one point that is often missed is that if we want to prove that a speed
limit change has altered the accident or fatality rate, we need to
prove that driver behaviour was significanlty altered.  here is where
trouble appears once again.  it turns out that when the speed limits were
dropped (by 10mph or more across the US), average driver speeds only dropped
by about 2mph (similiarly, when speed limits were raised from 55 to 65, 
average driver speeds went up by about 3mph.)  if driver behaviour didn't
change, it is very difficult to credit a speed limit change with a change
in fatality rates.

finally, as far as speed differential goes:  the only formal speed
differential studies ever done all apply to conventional, unlimited
access roadways.  no differential studies have been done for limited
access, divided highways.  there is some anecdotal evidence to suggest
that speed differentials may actually have been increased by the 55mph
limit; this seems somewhat counterintuitive at first but makes sense once
you think about it.

what happened is that a very small percentage of the drivers on the road
(probably less than 10%) actually do allow the speed limit to guide their
behaviour (perhaps via usage of cruise control or somesuch.)  when none
of the other drivers pay attention, and continue to drive at 65mph or
so, the result is that this group of drivers is now 10mph slower than the
prevailing rate, whereas before they might have been traveling at the
prevailing rate.  i have observed this first hand, back when states were
switching over to 65; when crossing from SC (then a 65 state) to GA (then
a 55 state), the traffic suddenly became vastly more congested when the
speed limit dropped, despite the lack of any major exits for large cities.
it was quite dramatic.

richard
-- 
richard welty        518-393-7228       welty@cabot.balltown.cma.com
``Nothing good has ever been reported about the full rotation of
  a race car about either its pitch or roll axis''  -- Carroll
Newsgroups: rec.autos