If you’re Googling this from a bar or a party, chances are good you’re not ready to drive yet! If you’re not sure, do yourself, your loved ones and everyone else on the road a favor: Consider calling a cab, a ride sharing service, or getting a ride home from a sober friend.
If you’re not absolutely sure that you're unimpaired by the effects of alcohol (or anything else), it’s not worth the risk of driving.
That said, let's take a closer look at the facts.
The “Legal Limit.”
The maximum blood alcohol concentration (BAC) you can have in your system, and still legally operate a vehicle, is 0.079 percent. Blow a 0.08 percent in a field sobriety test, and if you were driving, you are almost certainly going to jail. That 0.08 percent cutoff is consistent in all 50 states, and in the District of Columbia.
Once you get above that BAC level, you will experience substantially slower reaction times and increasingly poor hand-eye coordination. This will definitely affect your ability to safely operate a motor vehicle.
That said, it’s not like a BAC of 0.07 is all that great either. In fact, in many states, you can be arrested if you blow a 0.05 percent or higher, depending on the circumstances. Furthermore, impairment begins with the very first sip of alcohol you take. Even if your BAC is below 0.08, the driving mistakes you make because you are already impaired can still get you a reckless driving charge, and they can still cause a wreck – even a fatal one. In some circumstances and in some jurisdictions, a BAC as low as .05 can earn you criminal charges when combined with other driving problems and violations.
Your target BAC level should be zero.
Note: If you are under 21, you can be arrested if you are caught operating a vehicle and you blow any BAC over 0.0.
Getting to Zero
It can take up to six hours for the average person’s blood alcohol level to fall from 0.08 to 0.0; that’s how long it takes your bloodstream and kidneys to expel the alcohol from your system.
Coffee and food do not shorten the amount of time it takes to get your BAC down to zero. Food can help slow down absorption and keep your BAC from spiking too fast, but it alone won't push your BAC lower once the alcohol enters your bloodstream.
Calculate your BAC
There’s an easy online BAC calculator here. You can get a rough estimate of your BAC based on your sex, weight, the drinks you’ve consumed and the period of time over which which you’ve been drinking.
Rate of Absorption
If you’re trying to estimate your blood alcohol level after you’ve been drinking, you need to know the ‘rate of absorption;’ that is, the rate at which your body processes alcohol and removes it from your bloodstream. Generally, humans eliminate alcohol from their bloodstream at a rate of about 0.015 of BAC per hour. This is true for both sexes, regardless of age and weight. A rough way to calculate how long you need to wait before driving is to estimate your BAC, then calculate how many hours it will take your BAC to drop to a safe level. That’s going to take some math, which most of us shouldn’t be doing when we’re drinking! But if you want to target a BAC of less than 0.05, then you need to subtract 0.05 from your current BAC estimate, and then divide that number by 0.015.
[(BAC) – 0.05]/0.015 = number of hours to wait.
A Much Easier Rule of Thumb is the "45-Minute Rule”
If the rate of absorption method doesn’t work for you, or you’re intimidated by the math, you can do something much simpler - plan to wait an average of 45 minutes per standard drink before you get in a vehicle. If you have a drink, and wait 30 minutes, and have another drink, you have an hour and fifteen minutes, at least, before you are reasonably capable of driving again: The 15 minutes left over from the first drink, which is still in your system, and the full 45 minutes for the second drink.
What’s a standard drink? According to the National Institute of Alcohol Abuse and Alcoholism, a standard drink is any alcoholic beverage that contains about 14 grams of alcohol; that’s a 12-ounce beer (assuming 5 percent alcohol by volume) 8 ounces of malt liquor (about 7 percent alcohol by volume), a 5-ounce glass of wine (12 percent alcohol by volume), or a 1.5 ounce glass of spirits (scotch, whiskey, tequila, vodka, gin, etc.), with about 40 percent alcohol by volume.
You’ll have to adjust based on your specific drink. For example, some beers have a much higher alcohol content than others.
The smaller you are, the longer you should plan to wait. If you’ve had three drinks or more in one sitting, plan on waiting another hour. Order dessert with coffee.
Note for women: Women, on average, are smaller than men, and each drink will tend to cause their BAC to elevate faster than men, all else being equal. If you weigh 120 pounds or less, you can easily become legally drunk (BAC of 0.08 or higher) after only two drinks, according to information from Drinkinganddriving.org, but impairment begins after only one drink. For women around the 100 pound mark, even a single drink can push you into the 0.05 BAC range.
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