We’ve all been there. You’ve had a long day so you decided to meet some friends for happy hour after work. You had a beer – ok, maybe three[1]. But it’s all good because you also ate a bunch of french fries and you remember that they absorb the alcohol so you’re pretty sure you’re not over the limit. And besides, you’re a big guy so drinking three beers doesn’t have the same affect on you that it would on smaller guys[2]. Just to be safe, you wait half an hour after you finish your last beer before you decide to drive home. You’re almost home when suddenly, only a couple blocks from your house, you see those infamous blue lights in your mirror.

Now you’re worried. As the officer approaches your window you think to yourself: Why did I get pulled over? Should I tell him I drank a few beers earlier? Will he be able to smell the alcohol? What if he asks me to step out of the car? If I get arrested I know my wife won’t bail me out. I might lose my job. I can’t afford a DUI.

This would be a great time to review DUI laws.

Driving Under the Influence (DUI) is a very dangerous and serious criminal offense[3]. According to the National Highway Traffic Safety Administration, in 2014 there were 9,967 people killed because of alcohol-impaired driving[4]. A DUI can carry a punishment of up to twenty (20) years in prison[5]. If you’ve been drinking, you just shouldn’t drive.

In Arkansas, a person charged with a DUI can be found guilty through either of two ways[6]. The first way to prove a DUI is to show that the person was in actual physical control of a vehicle while intoxicated. A person is intoxicated when his reactions, motor skills, and judgment are impaired by alcohol or other drugs[7]. The most accurate way to measure the amount of alcohol in a person’s system is through a blood sample[8]. However, for obvious reasons, obtaining a blood sample from a driver is not always practical at the time of the traffic stop. Therefore, officers use a variety of other methods to assess whether a driver is impaired.

One method that an officer uses to assess a driver’s impairment is the so-called field sobriety test (FST). The FST is a series of tests, such as the “walk and turn,” the “one leg stand,” the “finger to nose,” and the horizontal gaze nystagmus, that can help gauge a driver’s concentration and coordination[9]. If the officer suspects that the driver is intoxicated, and the driver performs poorly during an FST, the officer is likely to make an arrest for DUI[10]. The FST is not used to prove that a driver is intoxicated to a specific level[11]. Instead, the FST merely indicates that a person is likely impaired due to the consumption of drugs or alcohol[12].

In addition to the FST, officers may also use a portable breathalyzer test (PBT) to help determine whether a driver is impaired. Like the FST, the PBT does not actually measure how much alcohol a person has consumed or the level of his BAC[13]. Instead, the PBT is used merely to indicate the presence of alcohol on the driver’s breath[14]. In fact, the results of a PBT are not admissible to prove a DUI, but only to support the officer’s decision to arrest[15].

The second way to prove a DUI is by showing that the driver had a blood alcohol concentration greater than the legal limit. The maximum legal limit of alcohol in a driver’s blood, or his BAC, is eight hundredths (0.08) of a percent[16]. But just because you had that extra beer before you hit the road doesn’t mean you’re over the limit when you get pulled over. What matters here is the driver’s absorption rate. Absorption rate is how long it takes for the alcohol that you’ve consumed to reach your brain and start affecting your reaction time and judgment[17]. This is when you’ve become intoxicated.

There are also a number of physiological factors that affect a person’s absorption rate, and in turn his BAC. For example, how much food you have eaten, your weight and metabolism rate, and how quickly you consumed those drinks can all affect how quickly the alcohol gets absorbed into your blood[18]. That means that one person who has had several drinks may have a totally different BAC than another person who consumed the same amount of alcohol over the same period of time.

So let’s talk about those french fries. On an empty stomach alcohol reaches peak absorption anywhere from forty-five to ninety minutes after consumption[19]. But alcohol consumed with or after a meal does not reach peak consumption until two to three hours after consumption[20]. Therefore, if you had three drinks and a basket of french fries, an hour later your BAC could fall within a very wide range. Different people might have BAC’s that range from 0.04 to 0.15. But there is also another important factor to consider in this situation, and it’s called Rising BAC.

The alcohol in your drinks is eliminated from your body by your liver at a rate of about half a serving of alcohol per hour[21]. When the rate of consumption exceeds the rate of elimination, your BAC level is said to be rising. Rising BAC means that even after you’ve stopped drinking, the amount of alcohol in your blood stream – affecting your judgment and your motor skills – could be continuing to rise. So you might be under the legal limit when you leave the bar, and yet be too intoxicated to drive before you get home.

When I was in college, my friends and I always used to try to find the most affordable ways to drink. One of my friends used to take several shots of alcohol at his house right before driving out to the bars, because it was cheaper than buying drinks at the bar[22]. He was hoping that he would get to the bar before the alcohol made it into his system. He was gambling on Rising BAC. Which leads us to a discussion about clearance rates and legal presumptions.

The clearance rate is how quickly alcohol is eliminated from the driver’s system[23]. Scientific studies indicate that clearance rates among different individuals vary greatly[24]. However, courts often erroneously assume that a person’s clearance rate is consistent and steady[25]. This assumption is frequently the only way to prove that a person did in fact have a BAC above the maximum legal limit while they were operating a vehicle. For example, if a person’s specific BAC is known at a given time, and we assume an average elimination rate, then it is possible to estimate that person’s BAC at an earlier time when he was driving[26]. This is called retrograde extrapolation.

But because of Rising BAC, there is a valid argument that a person could register a BAC above the maximum legal limit at the time he is tested, yet have been under the legal limit while he was driving. In Arkansas, a defendant can establish a legal presumption that he was not driving while intoxicated if his BAC is 0.04 or lower within four hours of driving[27]. The presumption does not attach if his BAC is between 0.05 and 0.07 within four hours[28]. However, even assuming that a defendant can establish the presumption that his BAC was below 0.08 at the time he was driving, he may still be convicted of a DUI through evidence of his intoxication from the officer’s testimony.

All of this talk about absorption rates, metabolism, Rising BAC, clearance rates, legal presumptions, etc., can be very confusing. So what does all of this mean? It means that if you’ve been drinking, you shouldn’t drive.

[1] An average serving of alcohol is either 12 ounces of beer, 5 ounces of wine, or 1.5 ounces; each contains approximately 0.6 ounces of pure alcohol according to the CDC (https://www.cdc.gov/alcohol/faqs.htm).

[2] We will assume our subject is a middle-aged man weighing approximately 200 pounds and in good health.

[3] Ark. Code Ann. § 5-65-103 (West)

[4] https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/812231

[5] Ark. Code Ann. § 5-65-111 (West)

[6] Porter v. State, 356 Ark. 17 (2004)

[7] Ark. Code Ann. § 5-65-102 (West)

[8] Jennifer L. Pariser, In Vino Veritas: The Truth About Blood Alcohol Presumptions in State Drunk Driving Law, 64 N.Y.U. L. Rev. 141 (1989)

[9] Steven J. Rubenzer, The Standardized Field Sobriety Tests: A Review of Scientific and Legal Issues, 32 Law & Hum. Behav. 293 (2008)

[10] Id.

[11] Whitson v. State, 314 Ark. 458 (1993).

[12] Id.

[13] Patrick v. State, 295 Ark. 473 (1988).