Anyone who has ever been arrested for drunk driving has likely experienced being transported back to the police station to have a breath or other chemical test performed to determine intoxication. Most people assume police will look at the result and either charge or release the driver based on the result. Anything 0.08 percent or higher will lead to charges and anything 0.79 or less will result in a trip home.
Unfortunately, that is seldom how things actually work. Police can instead use something known as retrograde extrapolation science to try and estimate what someone’s BAC would have been hours earlier, meaning that even a reading below the legal limit can still result in charges under the right circumstances. Experts say this kind of retrograde extrapolation is used most commonly in cases where it took officers a long time to get the driver from his or her vehicle in to be tested. In these cases, the process of estimating backwards allow officers to charge those whose results may be just under the wire with impaired driving.
One recent appeals court case out of Illinois thankfully threw out charges against one woman, with the opinion finding the estimate to be inherently unreliable. The case concerned a 2011 arrest of Chrystal Floyd. Police say they received a call about a disturbance at a family aquatic center and arrived to find Floyd exhibiting signs of impairment. Floyd was arrested and transported to a local police station where she submitted to a breath test at 10:30 p.m.
The test results revealed a BAC of 0.69 percent, well below Illinois’ legal limit of 0.08 percent. However, that didn’t stop police and prosecutors from bringing a DUI case against Floyd. Instead, authorities simply hired an expert witness to perform a retrograde extrapolation calculation on the figure to estimate what her BAC would have been at 9:15 p.m. that night. The expert guessed that Floyd’s BAC would have been somewhere between 0.82 and 0.95 percent, just above the legal limit.
Though Floyd was convicted on the charge, her defense attorney appealed the case, claiming that the expert witness in charge of performing the extrapolation failed to consider an array of important factors that could have dramatically altered the results of the test. These included how much food Floyd had consumed that evening, the type of alcohol she drank and other personal metabolic factors that would have impacted the rate at which her body processed the alcohol.
The Illinois appeals court agreed with Floyd’s attorney and decided that the state’s expert failed to consider important information when performing the retrograde extrapolation. As a result, the calculation was unreliable and Floyd’s conviction was tossed.
Source: “Court overturns Carpentersville woman’s 4th DUI,” by Harry Hitzeman, published at DailyHerald.com.