Why can't impaired drivers sleep it off in their car? There's a rule (and a law) for that




 

Question: I've seen "plan ahead" messages to prevent impaired driving. They suggest having a designated driver, calling a cab, or letting someone sleep on your couch, which are all good, but might not be an option. Why don't they suggest sleeping it off in your car?

Answer: I have an additional question that I think can get us to an answer: How much do you trust drunk you?Or if this doesn't apply specifically to you, how much do you trust an impaired person to make a sound decision? If after drinking alcohol or consuming other drugs a person truly only had the option of either sleeping in their car or driving home, sleeping it off in their car is clearly the better choice. But that doesn't make it a good choice (or a legal one).

There's some science behind why we're bad at judging how intoxicated we are, and we're about to get into it. Back in 1919, Sir Edward Mellanby found that perceived behavioral impairment at a specific blood alcohol concentration was greater when the blood alcohol concentration (BAC) was rising than when it was falling. Or said another way, given the same BAC level, you feel drunker while you're consuming alcohol than you do when you're working it out of your system. Many studies have confirmed Mellanby's research of what we now call the Mellanby Effect.

The brain is kind of a difference engine. We're good at recognizing the difference between what we were just feeling a moment ago and what we're feeling right now. We're not as good at recognizing what we're experiencing on an absolute scale, and that's a problem. Consider a typical sleep-it-off-in-the-car scenario: A person realizes they're impaired and shouldn't drive, so they crawl in the back seat to sleep. A couple of hours later they wake up feeling a bit rested and more sober. But that's just compared to how they felt before the nap.

Let's say they started their nap at a .12 BAC level (roughly four to five drinks depending on a variety of factors). Two hours later, that person is likely still above a .08; they might think they're OK to drive, but their perception does not accurately represent reality. People actually perform worse at a .08 BAC level when their level is falling than when it is rising.

Meanwhile, people think they're more sober than they really are, because of the Mellanby Effect. I'll also mention that I've been referencing a .08 BAC level because that's Washington's per se level for impairment. However, in the '80s we already had scientific consensus that a .05 BAC causes deterioration of driving skills. Forty years later only one state (Utah) has actually followed science, and the recommendation of the American Medical Association, to set the per se limit at .05 BAC.

The Mellanby Effect is one reason why, in addition to "Driving under the influence" laws, we have "Physical control of a vehicle under the influence" laws. In an effort to grossly oversimplify, the big difference is that in the first one a vehicle is moving and in the second one it's not. I'm no lawyer, but if you are impaired, you're in a car, and you have keys to start the car, you're probably guilty of physical control.

Last year, impairment was a factor in 56% of fatal crashes in Washington. Less than 2% of drivers in Washington drive with a BAC above a .05 level. Combine alcohol-influenced poor judgment with the Mellanby Effect and it's clear that planning in advance for a safe ride home is the best choice.

Ask Road Rules a question using our form. Target Zero is Washington's vision to reduce traffic fatalities and serious injuries to zero by 2030. For more traffic safety information visit TheWiseDrive.com .

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