Reasonable Suspicion is what a police officer needs to briefly stop you and perform a very cursory search of your person (purportedly for his own protection. This is also known as a “stop and frisk”). Reasonable Suspicion connotes something less than Probable Cause, but something more than an inchoate and unparticularized suspicion or hunch.
That definition isn’t very precise, and the lack of precision in that definition has led to untold abuse of Fourth Amendment rights by police, prosecutors, and judges. But some lack of precision is unavoidable. The courts cannot define everything with exactness as each factual situation can be a little bit different.
Why did we do away with the Fourth Amendment’s Probable Cause or Warrant requirement in favor of this new and lower “Reasonable Suspicion” standard? As a society, we value the ability of police officers to use their intuition in the detection of crimes, even when they are observing otherwise lawful activity. If the police officer feels something isn’t right, then it is of value to our society that the officer should be able to minimally intrude upon our lives to investigate. In balancing the minimal nature of this intrusion against the police officer’s need for safety, it can be argued that it is reasonable to carve out an exception here.
But police intuition is a very nebulous thing. If the police are allowed to act without constraint on a mere feeling that someone is guilty, the opportunity for an abuse of our Fourth Amendment rights is manifest.
Thus in a drunk driving scenario, if a police officer observes someone stagger out of a bar and get into a car, start the engine and pull out onto the street, under the Fourth Amendment, if the police officer stops that driver without a warrant or probable cause, there would still likely be reasonable suspicion for the traffic stop.