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Lower legal blood alcohol concentration limit laws are passed under the assumption that a lower legal limit will reduce the number of accidents.

For example, the U.S government lowered the legal BAC for driving from 0.1% to 0.08%.

Is there any evidence to support that roads are made safer as a result of lowering the legal limit, or is it the case that beyond a certain level there are diminishing returns?

  • I've broadened your question slightly to make it less localized. – Sklivvz Apr 15 '11 at 23:30
  • Fair enough, same intent in the end. Although to be technical, the states all individually decided to lower the legal BAC. – GBa Apr 16 '11 at 1:51
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    Diminishing returns can be physiological or social. Setting the limit too low risks people not taking it seriously any more. I think the most important effect MADD and such groups had was making it socially unacceptable to drink and drive, not different enforcement practices. – David Thornley Apr 16 '11 at 14:08
  • Even 0.08% seems quite high. 0.03% means impairment, 0.06% being drunk. – vartec Oct 20 '11 at 11:46
  • "The U.S. government lowered the legal BAC"... AFAIK, there is no (and can be no) federal law to this effect in the U.S. I believe every state sets this themselves, with some lower than 0.08%. Do you have a source? – Flimzy May 17 '12 at 4:50
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There is clear evidence that driving performance is impaired even at those levels.

Examining the statistics of traffic accidents supports the conclusion that 0.08% is clearly related to accidents:

BACs of .08 g/100 mL or higher were found in 38% of killed and 30% of injured drivers, 37% and 27% of passengers, and 35% and 22% of male motorcycle riders.
source

More evidence, even for lower limits, comes from double blind experiments:

Statistically significant and meaningful decrements in driving-related performance were identified at 0.05% BAC or below in many studies. Younger and inexperienced drinkers and drivers appear to be a greatest risk for alcohol-related traffic crashes. It is concluded that on scientific grounds, there is support for setting the legal limit at 0.05%.
source

This paper also confirms the findings.

Now, are BAC lowering policies effective? Not very, unfortunately. This has been studied in Slovenia:

Further concern arises from the finding that heavily drinking drivers are not responsive to penalty increases. We came to the conclusion that, in addition to raising fines and expanding penalty points, the introduction and effective enforcement of complex legislative measures together with wide community action are necessary to resolve DUI problems in Slovenia.
source

  • I edited to emphasize the efficacy findings, since that appears to be the main point of the question. Feel free to rollback – user5341 May 3 '11 at 16:55
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    Of course, it is not effective because people don't measure their BAC before driving. They either drink and drive, or don't, so whether the limit is 0.1% or 0.08% doesn't matter. – psusi Jul 1 '11 at 14:58
  • What percentage had levels above .10 compared to between .08 and .10? Just because that's what they measured doesn't make it "clearly related". While it establishes a correlation between alcohol and crashes; it does not support .08 as the "magic number". – Random832 Sep 14 '11 at 21:01
  • @Ran I never said that 0.08% is a magic number of sorts. In fact the subsequent quote supports an even lower 0.05%. – Sklivvz Sep 14 '11 at 21:19
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    A full answer requires two parts: what is the effect of BAC on driving performance; and what is the effect of changing the rules on people's behaviour. Without effective enforcement, changing the rules makes no difference at all. – matt_black Nov 11 '11 at 10:50
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The article The effectiveness of reducing illegal blood alcohol concentration (BAC) limits for driving: Evidence for lowering the limit to .05 BAC supports this idea, saying

strong evidence in the literature that lowering the BAC limit from .10 to .08 is effective, that lowering the BAC limit from .08 to .05 is effective, and that lowering the BAC limit for youth to .02 or lower is effective.

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    This study was produced by an advocacy group, pire.org. As such, I don't think proper skeptics will find it compelling. – I. J. Kennedy Jun 19 '16 at 3:58

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