Reasonable Suspicion for a Traffic Stop
Most DUI cases have a traffic offense listed on the ticket as the reason for the stop. The constitution prevents officers from pulling you over for just a hunch. They also cannot pull you over for looking lost. They cannot stop you for going under the speed limit because they think you have something to hide unless there is a minimum speed posted or you are impeding traffic.
They will also need reasonable suspicion to ask you to perform field sobriety tests. This is because they are only allowed to detain you for a reasonable amount of time to issue a traffic citation. By extending the time allowed they will require an additional expectation that another crime is being committed. This can be an odor of marijuana coming from the car or an odor of alcohol from the driver or car.
They also look at where you are coming from. They want to know if you came from a place that serves alcohol. Unusual actions can be an indication of intoxication like not rolling the window down, not pulling over right away, rooting around inside the car, or nervousness/shaking/sweating.
Mental confusion and loss of coordination can also be indications of intoxication. The officer will often ask you for your license, registration, and insurance while then asking you where you are going. If you fumble your license, present the wrong document, or give an answer that is not consistent with your direction of travel the officer will consider that proof that you might be intoxicated.
You have to stop your car when the lights and siren come on. You have to give up your license when asked, identify yourself if you do not have your plastic, and exit the car upon request. You do not have to take any tests or answer any other questions.