Officers may suspect impaired driving for a number of reasons. Perhaps someone merges lanes or goes around the corner without using a blinker. Maybe they suddenly weave or jerk their vehicle across lanes of traffic or seem to keep moving from side to side in a single lane. They could also have pulled out in an overly aggressive manner from a business that currently serves alcohol. Sometimes, just forgetting to turn on your headlights could give officers a reason to suspect impairment.
However, just because you do something questionable at the wheel doesn’t mean that you are drunk or necessarily dangerous. Officers will have to determine when interacting with you whether there is reasonable suspicion of impairment. Otherwise, they have no grounds on which to detain or arrest you.
You may find yourself wondering how, exactly, officers establish reasonable suspicion of impairment. The answer is that they use a combination of field sobriety tests and chemical testing to determine if someone is likely under the influence of alcohol while driving.
Field sobriety tests focus on the science of how alcohol affects your body
When an officer asks you to step out of the car during a traffic stop, they do so to check you for signs of physical impairment. Some of the field sobriety tests they administer will look at your balance and equilibrium. For example, your ability to balance on one foot while moving your arms or torso could be indicative of the loss of equilibrium that comes from drinking.
Similarly, officers will often have a suspected impaired driver walk a straight line. Those under the influence of alcohol may struggle to do so. Another common test involves looking at your eyes in what is called the Horizontal Gaze Nystagmus test. Effectively, because alcohol affects the function of your eyes, the officers are monitoring you for jerky eye movements under specific conditions.
Field sobriety tests can lead to chemical testing
As it is certainly true that there are medical conditions that could produce behaviors or actions that an officer would consider indicative of impairment, officers typically won’t arrest someone based solely on a field sobriety test for exactly that reason. Anything from diabetes to mild equilibrium issues could result in someone totally sober failing a field sobriety test.
Officers typically use the failed field sobriety test as probable cause for additional sobriety testing, which usually involves a chemical test such as a Breathalyzer. The multiple-stage testing process helps minimize the number of mistakes that officers make when conducting impaired driving enforcement, but no field testing system is infallible.
Officers can mistake medical issues as impairment symptoms. They may also jump to conclusions that the evidence does not currently justify. The good news is that anyone who fails a field sobriety test or even a Breathalyzer still has the right to fight back.