Unless you cause a crash, police officers typically cannot summarily arrest you for suspicion of drunk driving. They first need to establish probable cause.
A few bad driving maneuvers on their own don’t necessarily indicate chemical impairment. You may have swerved to avoid a pothole or what looked like a piece of glass in the road. You may travel at a slower speed because of an issue with your vehicle, not because you want to avoid a police officer pulling you over for speeding.
To obtain probable cause during a traffic stop, a police officer will perform a standardized field sobriety test to gauge someone’s likelihood of being under the influence of alcohol. What are the standard field sobriety tests?
The walk-and-turn test
Just walking in a straight line can be a challenge when your blood alcohol concentration is too high. Police officers can check you for impairment by asking you to walk along the straight line, turn 180° and then walk back. People who lose their balance or who struggle to communicate while performing this test will make a police officer suspect chemical impairment.
The one-leg stand test
Another way to test your impairment also involves checking your balance. An officer may ask you to balance on one leg, possibly while counting backward or answering simple questions. If you cannot maintain your balance or handle questions during the test, you may have failed the test.
The horizontal gaze nystagmus test
Alcohol doesn’t just affect your balance and your cognition. It can also affect involuntary bodily functions. In the horizontal gaze nystagmus test, a police officer moves their finger from side to side and asks you to keep your head still while following their hand with your eyes.
Everyone will have some degree of muscle spasm while moving their eyes. However, very pronounced spasms in a horizontal gaze nystagmus before the eyes reach a 45-degree angle from the front are both warning signs of chemical impairment.