How Drunk Were You At the Time? No One Knows, But the Answer is Crucial.

blood-alcohol-calculationDrunk driving collisions are a mess. Cars need to be disentangled and towed, drivers and passengers given medical attention (when they’re fortunate enough to have lived). Police need to interview witnesses, direct traffic, give orders, and make arrests. And somewhere along the line, a driver needs to be tested to find out how much he or she had to drink.

The legal blood alcohol concentration (BAC) number is well known : anything .08 or above is legal intoxication. Above about twice that (it varies by state) is sometimes considered “super drunk,” and punished more severely.

But with all the confusion, it’s sometimes hours between the collision and the test. If a driver crashes at 2 a.m. and shows a BAC of .06 at 6 a.m., what was the BAC at the time of the collision? Was it over the limit?

Calculating Blood Alcohol Backwards: Tricky Business

The devil is the rate of elimination of alcohol from the blood. Prosecutors use something called retrograde extrapolation to figure out what one’s alcohol content was hours before a given time, based on how many drinks were drunk over how many hours, plus a few other factors.

DUI-MeterAnd those other factors matter: the subject’s size, weight, sex, and general health. Estimates are iffy at best.  One method for calculating is the Widmark equation, which assumes a given absorption and elimination rate of alcohol, even though these vary. You can try out a Blood Alcohol Calculator here.

Which brings up another problem: you are not at your drunkest when you finish drinking. Even after you down your last whiskey and head for the parking lot, your body still has more alcohol to absorb before it starts eliminating. That adds to the confusion. And courts have to rely on the word of the drinker (or testimony of witnesses) as to the time of the last drink. So a driver might be getting drunker before starting to eliminate booze from the system. Where was he or she on that scale when the crash happened?

Does it Really Work?

A recent case in New York has brought the debate in the news. Defense attorneys tend to consider retrograde extrapolation pseudoscience, while prosecutors uphold it as an important tool to seek convictions. It’s doubtful that retrograde extrapolation will ever be unchallenged, since juries realize that every individual’s response to alcohol is different. To a drunk driver on trial, to victims and their families, everything hangs on that BAC number, even though it’s almost certainly unreliable.

But remember that the cause of many alcohol-related crashes is not ambiguous: the driver tests above the limit and is therefore intoxicated. And it’s possible to be convicted of DUI even if you are below the legal limit. If it can be proven that you’re impaired at .05, then you’re impaired.

The answer to “how much alcohol was in your system when you were driving?” should always be “none.” That’s the way to avoid the nightmare that is a drunk driving conviction. The science may never be accurate enough to prove what your blood was doing four hours ago, but you’re still responsible for staying sober behind the wheel.