Driving After Drug or Alcohol Use Report

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CONCENTRATION (BAC) LEVELS

The analysis of driving behaviors following alcohol use described in Chapter 2 of this report incorporates estimates of blood alcohol concentration (BAC) levels. Estimated BAC levels were calculated using the method described by the National Highway and Traffic Safety Administration (NHTSA) in "Computing a Blood Alcohol Estimate." BAC levels were estimated for each respondent’s most recent experience of driving within two hours of alcohol use. The method used to estimate BAC levels is as follows:

1.Calculate the water content of the individual. Each respondent’s weight was converted from pounds to kilograms. This weight in kilograms was then multiplied by a conversion factor (.58 for males, .49 for females) to estimate each individual’s water content in liters. This number was multiplied by 1,000 to get each individual’s estimated water content in milliliters.

2.Calculate the alcohol concentration in g/ml of body water. The number of alcoholic drinks each respondent reportedly consumed before driving was multiplied by 23.36 grams (i.e., weight of one ounce of alcohol). This amount was then multiplied by .6 oz., the average of the amount of ethanol found in a 12 oz. beer, a 5 oz. wine, and 1.5 oz. of spirits (.54 oz., .65 oz., .62 oz., respectively). This product, which is an estimate of the total number of grams of alcohol consumed, was divided by the milliliters of body water for each individual.

3.Calculate the alcohol concentration in the blood. The measure of grams of alcohol per milliliter of water obtained in the previous step was multiplied by a conversion factor of .806 (.806 ml of water/ml blood). This product was then multiplied by 100 to obtain a measure in grams per deciliter units.

4.Classify individuals by metabolism rate. If a respondent indicated that he/she had consumed 60 or more alcoholic drinks in the past month, he/she was classified as having a high alcohol metabolic rate and was assigned a rate of decline in BAC level of .02 per hour. Individuals who consumed two or fewer drinks in the past month were classified as having a low alcohol metabolic rate. About one-fifth of current drinkers age 16 and older (21 percent) fell into this category and were assigned a rate of decline in BAC level of .012 per hour. Individuals who did not fall within either of these two extremes were categorized as moderate drinkers with an average alcohol metabolic rate, and a corresponding .017 per hour decline in BAC level.

5.Adjust estimated BAC levels for rate of metabolism. The rate of decline in BAC level determined by each individual’s alcohol metabolic rate was multiplied by the amount of time from consumption of the first alcoholic drink to when driving began. This product was then subtracted from the BAC level estimated in step #3 to produce a BAC level adjusted for metabolic rate.

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This page was last updated on June 03, 2008.

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