Blood-Alcohol Level When Tested vs When Driving

Posted by Lawrence Taylor on July 20th, 2013

It is illegal to have a blood-alcohol concentration (BAC) of .08% or greater while driving a vehicle. It is not illegal to have a BAC of .08% or greater while blowing into a breathalyzer in a police station. In other words, just because a breath test shows a level of, say, .09%, it does not mean that the BAC when the suspect was driving an hour earlier was .09%.

So what was the breath alcohol level when driving?

Well, we’ll never know: There is no evidence of the BAC at the time of actual driving. However, we can be fairly sure that it wasn’t .09%, since the body is constantly either absorbing or eliminating alcohol and the BAC is therefore constantly rising or falling. If it was falling, then we can expect the BAC when driving was higher — .10% or more. But if it was rising…..

Let’s take a typical example. The subject — let’s call her “Janet” — finishes dinner by throwing down “one for the road”, a 12-ounce can of beer containing .05% alcohol. She is stopped by an officer soon after leaving the restaurant, alcohol is smelled on her breath and she is given field sobriety tests. She does marginally well but, to be sure, the officer takes her into the police station for breath testing. About 45 minutes after drinking the alcohol, Janet breathes into the breathalyzer. The result: .09%. She is booked and his driver’s license confiscated.

It will take, on average, about one hour for the alcohol to be absorbed and reach peak levels of concentration in the blood, thereafter to be eliminated from the body. This is only an average; it can vary from 15 minutes to 2 hours; some invidividuals can reach peak concentration ten times faster than others. Dubowski, “”Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects”, Journal on Studies of Alcohol, Supp. 10 (July 1985). This makes trying to estimate earlier BAC levels no better than a rough guess, and scientists have unifromly condemned the practice. See, for example, “Breath Alcohol Analysis: Uses, Methods and Some Forensic Problems”, 21 Journal of Forensic Sciences 9.

Applying averages to Janet, though, we can expect the last drink to have had little if any effect on her blood-alcohol concentration while she was driving. By the time she is being tested at the station 45 minutes later, however, she is reaching peak concentration. In other words, Janet’s BAC has been rising. At about 120 pounds, we can estimate (read “guess”) that the can of beer has increased her BAC by about .031%.

Translation: the breathalyzer reading of .09% at the station indicates a BAC while driving of only .06%. She is not guilty. But the “evidence” will convict her.

Just to make things worse….As I indicated, attempts to guess BACs when driving earlier than when tested have been condemned by scientists. This makes things tough for prosecutors. Solution? As I discussed in an earlier post, “Whatever Happened to the Presumption of Innocence?”, most states today have passed laws — directly contrary to scientific truth — which presume that the BAC at the time of being tested is the same as at the time of driving!

In other words, unless the defendant can prove that his BAC was different than when tested, the jury will be instructed that they must find that it is the same. In effect, the defendant is presumed guilty. And since there is no evidence of the BAC when driving, there is no way for the defendant to rebut the presumption.

These laws do, however, make getting convictions much easier.

  • Art

    I’ll clarify the misconception of the .08 BAC needed for a DUI conviction, people: In more states than ever, the BAC really means nothing. You can be arrested for being ‘impaired to the slightest degree’. If you blow anything above a .00, you’re screwed, anyway. This was carefully crafted by lawmakers (yes- lawyers), to make convictions far more easier, and far more numerous, and far more lucrative (as in Ca-Ching for the states). The BAC limit is a MYTH.

    Driving after drinking a single drink means you are playing with fire and risking ruining your life, unless employment isn’t one of your basic needs. Impaired or not, or above .08 or not- if you’ve been drinking and get pulled over, it’s over with for the vast majority- only a few people will ever be lucky enough to come upon the likes of a Lawrence Taylor- who has motives that are altruistic in nature, more so than just making another buck. Even he can’t save us from the ruination of our Constitutional rights, all by himself, however.


  • Art

    Why Legal Marijuana WILL Get You Arrested For DUI:

    Even as more states ‘legalize’ medical or recreational marijuana, those same states are covertly crafting DUI laws that make ‘any trace level of THC or metabolites’ an automatic charge of DUI. That means if you smoked pot a month ago, and you are stopped at a DUI checkpoint, and have blood taken- you will be charged with DUI. I’ll name two:

    CA- legal medical marijuana. However, for a DUI arrest, any trace of

  • Art

    (cont’d) …THC or metabolites is grounds to charge you with DUI. With at least 100,000 medical pot cards in the state, you can bet the cops are laying in wait to arrest YOU.

    CO- legal recreational marijuana. What you DID not hear was that even before they legalized that, they drafted a marijuana DUI law that states a blood level of 5 nanograms of marijuana is cause for arrest for DUI. Basically, the level of 5 nanograms is what you’ll get walking by a person smoking marijuana on the sidewalk, even if you ‘don’t inhale’.


  • Nelson Donnell

    What you’re seeking is that old time religion that the state use to operate under, since the Civil War and the advent of the 14th Amendment, it is different then the new and revised religion that it operates under now:

  • Art