To lawfully pull someone over, police must have a reasonable suspicion that the person has violated the law. However, it doesn't take much—even a minor traffic violation is enough. (But read about the exception to the rule for DUI checkpoints.)
And for a DUI arrest to be legally justified, it must be supported by probable cause. In other words, there needs to be evidence that could support a reasonable belief that the motorist was driving under the influence.
The facts of every case are different. But certain scenarios are frequently part of an officer's reasonable suspicion or probable cause determination in DUI cases. Here are some of the more common reasons police pull cars over and make DUI arrests.
Traffic violations. Any traffic violation can give police reasonable suspicion to conduct a traffic stop. And just because the initial stop is for a traffic violation doesn't mean the detention can't evolve into a DUI investigation. An officer who notices signs of intoxication (like slurred speech or the odor of alcohol) during a traffic detention will likely investigate further.
Traffic accidents. If you're involved in a collision, there's a good chance police will show up—especially when the accident involves injuries or significant property damage. Because so many accidents are caused by impaired drivers, officers are typically on the lookout for indications of intoxication when conducting accident investigations.
Bad driving. With traffic violations and accident investigations, the officer incidentally discovers evidence of driver impairment. However, it's also common for police to instigate a stop based on a suspicion of drunk driving. Such suspicions are typically related to unusually driving—things like swerving, driving the wrong direction on a roadway, or driving extremely slow.
In-person observations. In addition to driving patterns, an officer's observations during a traffic stop can supply the necessary probable cause for a DUI arrest. To justify an arrest, officers often cite factors like the odor of alcohol, bloodshot eyes, slurred speech, and odd behavior.
DUI testing. Police often use field sobriety tests (FSTs) and breathalyzers during DUI investigations. Generally, drivers don't have to participate in FSTs or prearrest breath tests, but many drivers take these tests voluntarily (or don't know they have a right to refuse). FST performance and breath-test results can be important components of an officer's probable cause determination.
The "exclusionary rule" may come into play when a stop wasn't supported by reasonable suspicion or an arrest lacked probable cause. Generally, the exclusionary rule requires the court to throw out any evidence obtained after the illegal police action. In a DUI case, excluded evidence can lead to a better plea bargain or dismissal of the charges altogether.
Reasonable suspicion and probable cause always involve fact-specific inquiries. And the law is constantly changing. To find out if you have a viable DUI defense, it's always best to talk with a knowledgeable attorney. A qualified DUI lawyer can tell you how the law applies to the facts of your case.