Often times clients ask under what circumstances a police officer can stop their vehicle. In order for an officer to make a stop he must have "reasonable articulable suspicion" that criminal activity is occurring or is about to occur. The rule comes from a United States Supreme Court case Terry v. Ohio which is why they are known "Terry Stops."
Often times people will shorten the phrase to simply "reasonable suspicion" to suggest the officer must have a logical gut feeling before stopping your vehicle. But the 'articulable' part is equally important. A gut feeling isn't enough for an officer to stop your car. He must be able to articulate his suspicion.
For instance, simply deviating slightly in your lane, or "swerving" isn't illegal. The officer must be able to tell a court for what legal reasons he suspected your vehicle was being operated incorrectly. Officers are not allowed to rely solely on their suspicion for stopping your vehicle. That's why in most cases an officer will stop you for speeding, changes lanes without signaling, or expired plates before inquiring about your potential intoxication.
If an officer stops your car without reasonable and articulable suspicion the evidence gathered against you after that point can be suppressed, or thrown out of the case. That's why its important to have an experienced attorney look through your case for potential illegal or unwarranted stops.