blood alcohol level

Why are the blood alcohol limits for drivers so low? I can function perfectly well after a few beers.

Q.  Why are the blood alcohol limits for drivers so low?  I can function perfectly well after a few beers.

A. Alcohol, aside from its addictive qualities, also has a psychological effect that modifies thinking and reasoning…. — The American Medical Association, in an official statement issued July 31st, 1964

We now know that having a drink of alcohol inhibits the executive functions of our brains.  The inhibiting mechanisms that control judgement, decision-making, and overall self-control are the first things affected by alcohol.  That’s why we experience that feeling of “freedom” when we’ve had that first drink: the feeling that we can relax, that we don’t have to hold the reins quite so tightly, that enables us to be a little more daring, take a few more risks, makes us more handsome, more beautiful and wittier (at least in our own mind), and that convinces us that we can drive just fine, thank you very much.

The abilities to drive skillfully, operate machinery, and carry out other dangerous activities that require judgement, decision-making and self-control are the very first things that we lose when we drink.  As you can see from the table below, other critical skills aren't far behind.  Combine that with the poor judgement that can make driving seem like a good idea, and we have a recipe for potential disaster.

Blood alcohol limits are set where they are because long experience and tens of thousands of blood tests on drivers involved in crashes and other driving escapades have shown that higher levels greatly increase the potential for trouble.  It’s that simple.  We may believe that we can drive better after a few drinks but, recall that good judgement is the first thing to go.  For a similar reason, the legal drinking age is held at 21, because younger drivers have not yet developed the judgement skills needed to drive with maximum safety, and certainly don’t need further impairment.  (The physical skills associated with driving have nothing at all to do with judgement and emotional stability.)

In the table below, “‘The second column lists behavioral areas by the first BAC at which 50% of the behavioral tests indicated impairment. That is, the point at which the majority of behavioral tests showed impairment. Note that, with the exceptions of simple reaction time and critical flicker fusion, all driving-related skills exhibited impairment by 0.070 g/dl in more than 50% of tests.” [The table was simplified for easier interpretation. The original can be found at the link shown.]

BAC AND IMPAIRMENT, BY BEHAVIORAL AREA

Source: http://www.nhtsa.gov/people/injury/research/pub/Hs809028/Discussion.htm#Major

BAC
(g/dl)
First BAC at Which 50% or More of Behavioral Tests 

Indicated Consistent Impairment

0.100 Simple Reaction Time, Critical Flicker Fusion
0.090-0.099
0.080-0.089
0.070-0.079
0.060-0.069 Cognitive Tasks, Psychomotor Skills, Choice Reaction Time
0.050-0.059 Tracking
0.040-0.049 Perception, Visual Functions
0.030-0.039 Vigilance
0.020-0.029
0.010-0.019 Drowsiness
0.001-0.009 Divided Attention

In the case of alcoholics and other addicts who use drugs that depress the central nervous system, the risks are multiplied exponentially.  We become impaired even beyond others who have had a few drinks.  The ability of most alcoholics to “maintain” and appear relatively sober to others and themselves is a specific, learned behavior that does not translate to driving and other skills.  We learn to function in spite of being impaired.

There are countries where you can be put in jail for having car keys in your pocket if you’ve been drinking.  Our more reasonable DUI laws can be clearly shown to be not only for you own good but for that of everyone on the road.