An officer must have "reasonable suspicion" to believe you have violated a law in order to make a valid traffic stop in a DUI case. Random stops and roadblocks that lead to a DUI arrest are not legal in Washington.

According to the National Highway Traffic Safety Administration (NHTSA), the following is a list of DUI symptoms, and the percentage chance that a driver at night is legally drunk:

However, the DUI police officer does not have to observe any bad driving to pull you over. A simple vehicle malfunction like a burned out license plate light or headlight is sufficient legal justification to stop your car and proceed with the DUI investigation.

While you stop your car and pull over to the side of the road, the DUI officer will continue to observe your driving for anything unusual, such as responding slowly or failing to respond to the officer’s command to stop, swerving abruptly, stopping suddenly or striking the curb when pulling over.  All important in a DUI case. 

As soon as you have pulled your car to the side of the road, you should immediately retrieve your driver's license, registration and proof of insurance before the officer approaches your window and asks for these documents.  This will eliminate the officer’s ability to observe your movements and assume, without more, that you are a DUI driver.  After you have retrieved these documents, roll your window down. If the DUI officer witnesses you fumbling for your paperwork, or having difficulty with the window (due to nervousness or unfamiliarity with the car) he will likely assume that these actions are due to intoxication rather than stress, and therefore once again assume you are a DUI driver. Avoid this possibility by having everything ready. Also, do not take off your seat belt until after you first speak with the DUI officer.

The DUI officer's objective every DUI stop is to gather as much evidence as possible. He will be using his senses of sight, hearing and smell to collect evidence against you in your DUI case, and will do very little to gather or record evidence that will help you and your DUI case.

1) The DUI officer is looking for:

• Red, watery, glassy and/or bloodshot and eyes
• Flushed face
• Soiled clothing
• Fumbling fingers
• Alcohol containers
• Disheveled or unbuttoned clothes
• Drugs or drug paraphernalia
• Bruises, bumps or scratches

2) The DUI officer is listening for:

• Slurred or thick-tongued speech
• Inconsistent and/or slow responses
• Admissions of alcohol consumption or intoxication
• Abusive language
• Unusual statements

3) The DUI officer is smelling for:

• Alcoholic beverages
• Marijuana
• "Cover up" odors such a breath sprays, mints, chewing gum or smoke
• Unusual odors

You should always be courteous towards the DUI officer. Never argue or debate with him. You will inevitably lose, and it will be used against you in a DUI court later. Most importantly, never lie about anything. In other words, if have been drinking, don't deny it. Doing so can damage your credibility later in court. If the truth hurts, it far better to politely decline to answer questions and ask to speak to a DUI lawyer.

If the DUI officer suspects that you are impaired, he will ask that you get out of your car. While you do so, the officer will pay close attention to your coordination during the exit, which will, in the officer's mind provide evidence of your intoxication. Specifically, the DUI officer will be observing if you:

• cannot follow instructions
• cannot open the door easily
• leave the ignition on
• leave the car in gear
• "stumble" getting out of the car
• are swaying or unstable on your feet
• use the door for support while exiting
• lean against the vehicle
• keep your hands on the vehicle for balance

After you have exited your car, the DUi officer will instruct you to perform a series of so-called DUI "field sobriety tests" including all or a combination of the following:

• Walk and Turn
• One Leg Stand
• Horizontal Gaze Nystagmus (HGN) – examining your eyes while you follow a pen or finger to see if there is any jerky movements
• Reciting the alphabet
• Finger counting
• Closing your eyes, leaning your head back and touching the tip of your index finger to the tip of your nose

Only the first 3 tests, the walk and turn, one leg stand, and the HGN, are considered “standardized,” and hence it is only these that recognized as reliable  indicators of intoxication.  However, it is imperative that the DUI officer perform these tests in total compliance with the NHTSA guidelines.  More details on the DUI field sobriety tests in provided in the next section.

Also included in the DUI field sobriety tests is a device known as a preliminary or portable breath test (PBT). This type of DUI breath testing device cannot be used in your DUI trial, therefore it will not fulfill your obligation to take a breath test at the police station.  If you take the test and fail it, however, you will be arrested.