Before a police officer can make an arrest for "driving under the influence of alcohol or drugs" (DUI), he or she must have probable cause to believe that the driver was committing that offense. That "probable cause" is usually developed through roadside testing of the driver's physical coordination. However, before the officer can ask the driver to perform those tests, he or she must have good reason to detain and observe the driver.
The initial stop of an impaired driver is usually for a minor traffic violation: excessive speed, failure to stop at a stop sign, driving left of the center line. While impaired drivers often commit these violations, so do unimpaired drivers. Therefore, the officer must give other reasons for detaining the driver and asking him or her to perform coordination tests.
In Delaware v. Prouse (March 27, 1979), the United States Supreme Court held that an officer must have an "articulable and reasonable suspicion" that a suspect has committed an offense before the suspect can be detained. When a suspect is stopped solely for a minor traffic violation, the defense attorney will often file a motion to suppress evidence, claiming that the officer lacked reasonable suspicion to detain the driver beyond the simple issuance of a traffic citation. In order to establish a reasonable suspicion, the officer must be able to articulate reasons for suspecting a DUI violation. Therefore, at the suppression hearing, the officer should be prepared to express the reasons why the defendant was detained.
An observant officer may be able to articulate a few facts which arose before the defendant's vehicle came to a full stop: (a) the defendant's failure to respond to the officer's signal to stop and pull over, and (b) the defendant's poor performance in parking the vehicle. When describing the first contact with the driver, the officer should mention any traditional indicators of impairment, including:
In addition to the traditional indicators, the officer should be sure to describe any other reasons for suspecting that the driver was impaired, including:
A court's decision to allow or suppress evidence will depend on all the circumstances. Thus, it is important for the officer to describe as many indicators as possible. The more reasons an officer gives for suspecting an impaired driver, the more likely the court is to deny the motion to suppress evidence. And keep in mind that these same facts will continue to be useful; they will serve as persuasive evidence in front of a trial judge or jury.