It depends. It is a frequently used and sometimes disappointing answer to that question. But there is good reason for the inherent vagueness. Because of the Fourth Amendment to the U.S. Constitution, the police cannot simply stop anyone at anytime, just to check and see if they have committed a crime.
Law enforcement must have a reasonable suspicion that a criminal act has occurred or is about to occur. If they observe you driving down the highway and they suspect you may be intoxicated or impaired, they have to be able to articulate to a judge specifically what supports that suspicion.
The recent case involving the arrest of a state representative is notable, because the man was stopped and arrested after the wheel of his vehicle crossed the “fog line” on the edge of the road once.
The judge hearing the case dismissed the charges after viewing the video, as he thought it offered insufficient evidence of intoxication. The judge apparently believed the incident was too insubstantial to suggest impairment.
Of course if you ran off the shoulder, and then crossed back to the centerline, then slowed below the speed limit and made a sloppy stop at an intersection, a ruling could turn out different. With facts like that, the officer could have better grounds for making an arrest.
These standards have to be flexible, to both allow law enforcement to stop impaired drivers and to prevent them from being so inclusive as to allow anyone to be stopped. And this case is a reminder why you want to discuss your DUI arrest with an attorney, and the Chattanooga police or the Tennessee Highway Patrol are not infallible and, as with this case, may have “jumped the gun” when making the arrest.
Source: Tennessean.com, “Lawyers: Beck case falls in gray area of DUI law,” Stacey Barchenger, July 29, 2015