When a law enforcement officer suspects a driver of operating a vehicle while intoxicated, the officer will conduct a series of tests to determine whether the driver is actually intoxicated. These tests are referred to as field sobriety tests, and they are conducted on the side of the road. The tests are meant to determine whether a driver is under the influence of drugs or alcohol, and they are not a substitute for a formal chemical test.

The three most common field sobriety tests are the one-leg stand, the walk and turn, and the horizontal gaze nystagmus. The one-leg stand involves standing on one leg for 30 seconds, and the officer will watch to see if the driver is able to maintain balance and keep their foot raised. The walk-and-turn test requires the driver to take nine steps and turn around on each step. The officer will watch for the driver to take the steps in the correct order, and they will watch for the driver to take the steps without stumbling or falling. The horizontal gaze nystagmus test involves the officer watching the driver's eyes while the driver follows an object with their eyes. The officer will watch the driver's eyes for involuntary jerking movements.

If the officer believes that the driver is under the influence, they will bring the driver to a nearby facility for a chemical test. The most common chemical test is a blood test, but some officers will conduct a urine test instead. A blood test is more accurate, but a urine test is more convenient because it requires less time and less effort. A chemical test will check the driver's blood alcohol concentration (BAC), and the results will determine whether the driver is arrested and charged with DUI.