When are police checkpoints valid?

Usually, a police officer must have reasonable suspicion of a crime before they stop a driver. Thus, it would be illegal for police to drive around and randomly pull drivers over in order to see whether or not they have been drinking. However, the law does allow police to set up checkpoints to stop drivers without individual suspicion of their wrongdoing as long as they follow some sort of predetermined rule as to which cars will be stopped. For example, they may agree to stop "every 3rd car," which means the unlucky driver of every 3rd car must stop for the police. In contrast, a standard that would not be "neutral" would be, "stop every car driven by a minority." Similarly, a standard that would not be considered "articulable" would be, "stop every car that I have a hunch is involved in a crime."

Note that this doesn't mean police are helpless to spot specific instances of reasonable suspicion. For example, if the rule is "stop every 3rd car," but as the 2nd car drives up to the checkpoint and a large cloud of marijuana smoke flows out of the window with a strong odor of whiskey. In that instance, however, it wouldn't be the checkpoint standard that allows an officer to stop the car, it would be the fact that the specific circumstances provide enough probable cause to stop the car (i.e., any car that smells strongly of alcohol and drugs that drives past a police officer is fair game to be stopped).