The police cannot stop you without legitimate suspicion that you committed a crime or a traffic infraction, such as speeding or failing to use a turn signal. Every citizen has the right to travel on the highways free from police intrusion. Random stops or roadblocks are illegal, as are "pretext" traffic stops. Careful scrutiny of the initial detention will always be part of an effective DUI defense.
Nowadays, people who are arrested for DUI were usually not stopped for "typical" drunk driving. A simple speeding infraction, for instance, will escalate to roadside tests, questioning, a formal arrest, handcuffs, a breath test at the police station, and finally - jail. The National Traffic Safety Administration publishes a list of traffic infractions with an associated percentage chance that a driver at night is "legally" drunk. Keep in mind that some of the items listed do not amount to a violation of law, justifying the stop of a car. Here is the list:
| Traffic Infraction and/or Suspicious Conduct | Chance that the driver is intoxicated or drunk |
| Turning with wide Radius | 65% |
| Straddling Center or Lane Marker | 65% |
| "Appearing" to be drunk (e.g. Slouching in the seat, Gesturing erratically or obscenely, Eye fixation, Tighly gripping the steering wheel, Face close to the windshield, Drinking in the vehicle, Driver's head protruding from the vehicle) | 60% |
| Weaving | 65% |
| Turning with wide Radius | 60% |
| Driving on other then designated Roadway | 55% |
| Swerving | 55% |
| Slow Speed (more than 10 MPH below speed limit) | 50% |
| Stopping (without cause) in traffic lane | 50% |
| Following too Closely | 50% |
| Drifting | 50% |
| Tires on Center or Lane Marker | 45% |
| Braking Erratically | 45% |
| Driving into opposing or crossing traffic | 45% |
| Signaling inconsistent with driving actions | 40% |
| Slow responses to traffic signals | 40% |
| Turning abruptly or illegally | 35% |
| Accelerating or decelerating | 30% |
If a car is illegally stopped, anything discovered by a police officer after the stop is "tainted" evidence, which generally cannot be used in court. Thus, the legality of the initial stop is very often a critical and a hard fought issue in a driving under the influence (DUI) case.
