Decided March 27, 1979
At about 7 pm, an officer saw a car driving down the road. Since the officer wasn't doing anything, he decided to randomly stop the car to check for license and registration. He didn't observe any moving or equipment violations, and didn't have reasonable suspicion of criminal activity. During the stop, he found weed in the car.
The US Supreme Court held that the stop violated the Fourth Amendment (go figure). The court recognized that the state has an interest in catching unlicensed drivers, but reasoned that this interest is adequately served by checking for license/insurance/registration on stops where there's some reason to stop the car. At the end of the decision, the court pointed out that it wasn't barring the use of checkpoints with this decision. The difference is that checkpoints "do not involve the unconstrained use of discretion."