Probable Cause for DUI Stop by Law Enforcement

In the United States, police are required to establish "probable cause" in order to stop a vehicle for a potential drinking and driving offence. The following list of symptoms, from a publication issued by the National Highway Traffic Safety Administration (DOT HS-805-711), is widely used in training officers to detect drunk drivers.

After each symptom is a percentage figure which, according to NHTSA, indicates the chances that a driver, who displays the symptom, is over the legal limit.

Establishing Alcohol as "probable cause" of impairment
Symptom Percentage   Symptom Percentage
Turning with a wide radius
65%  
Following too closely
45%
Straddling center or lane marker
65%  
Tires on center or land marker
45%
Appearing to be drunk
60%  
Braking erratically
45%
Almost striking an object or vehicle
60%  
Driving into opposing or crossing traffic
45%
Weaving
60%  
Signaling inconsistent with driving actions
40%
Driving on other than a designated roadway
55%  
Stopping inappropriately (other than in lane)
35%
Swerving
55%  
Turning abruptly or illegally
35%
Slow speed (more than 10mph below limit
50%  
Accelerating or decelerating rapidly
30%
Stopping (without cause) in traffic lane
50%  
Headlights off
30%
Drifting
50%      

Probable Cause to Detain

The officer will typically approach the driver’s window and ask some preliminary questions. The purpose is to detect the possible presence of preliminary evidence such as:

If the officer's observations lead him or her to have a reasonable suspicion to legally justify a further detention and investigation, the driver will be asked to step out of the vehicle.

Field Sobriety Tests

Field Sobriety Test

The officer will administer one or more field sobriety tests if alcohol impairment is suspected. The most commonly administered tests include:

BAC does not have to be at or over .08 to establish probable cause for DUI. At the .08 level you are automatically assumed to be DUI. You can be arrested based on other "probable causes" above.

Probable Cause to Arrest

While every state and locality may have different procedures, generally, if the officer has sufficient facts justifying a reasonable suspicion that the suspect has been driving under the influence of alcohol, he will make the arrest, handcuff the suspect and transport them to the police station. En route, the officer may advise them of their rights and his legal implied consent to submit to an evidentiary chemical test of blood, breath or possibly urine.