A high percentage of DUI traffic stops are justified by police officers on the grounds that the car was "weaving" — one of many possible signs of drunk driving, if the weaving is pronounced (the National Highway Traffic Safety Administration lists 19 common driving symptoms in DUI cases). More importantly, the officer knows it is difficult to cross-examine him as there are no objective criteria for "weaving" and nothing but the defendant's word to contradict him. Unless, of course, a camera is rolling…
Take a look at this videotape (click on "Video: unedited traffic stop"), taken from a car following a deputy sheriff who follows and finally stops another car. The deputy appears to be the only one weaving — and constantly tailgating the car he's following (tailgating can cause the car being followed to weave and is, incidentally, another of the 19 symptoms of drunk driving).
For a continuing education, click also on the previous link, "Video: CBS4 Miami story".
(Thanks to Richard.)