The Untied States Supreme Court said this year that a law enforcement officer can legally pull your car over based on an anonymous 911 call in some situations. The vote was split 5-4 in the decision of Naverette v. California.
In order for law enforcement to pull you car over, the has to be a standard of proof called "reasonable suspicion." If the officer has a particularized and objective basis for suspecting criminal activity, the officer has reasonable suspicion to stop the car. If the officer cannot provide enough information to support reasonable suspicion, a traffic stop will be considered unlawful and any evidence obtained will usually be thrown out.
Reasonable suspicion is not a very high burden for law enforcement to overcome. Usually reasonable suspicion is met by what the law enforcement officer personally observes, but that is not always the case. And this blog talks about one of those situations.
In this case, the Defendant was stopped by law enforcement after an anonymous 911 caller reported that Naverette has run her off the road. The Supreme Court has rationalized that in this particular case, that the caller bore "adequate indicia of reliability." One thing was the caller claimed to be an eyewitness to the event. This helped the Supreme Court determine her basis of knowledge. The Court also said that because the report was made so quickly in relation to when the event supposedly happened, that the caller had little time to fabricate a story. The part that makes me laugh is that the court reasoned that a person would be reluctant to make a false tip to 911 technology which can identify callers would result in people thinking twice before making false calls. In my practice I have found there are lots of people who would make false reports for any number of reasons. The most common reason is harassment of the party being pulled over.
So there you have it. If an anonymous caller to 911 accuses you of bad driving, and they give a little infomration to "verify" what they said, like the location and type and description of the car you are driving, and the officer then "verifies" that information, the officer will be allowed to stop you. Even if he never, ever, ever sees you do anything wrong. Enjoy your time on the side of the road America.