Why are checkpoints legal if police need probable cause to make a traffic stop?
It depends on the type of motorist checkpoint. Roadblocks, as they are referred to by the courts, fall under what is known as a special needs exception to the 4th Amendment. The special needs exception requires that the primary purpose of the search/seizure be for regulatory, not law enforcement, purposes. Under this theory, things like drivers license/vehicle registration checkpoints pass muster because the purpose is to ensure motorists have complied with state licensing requirements. And as stated below, if there are observable signs of other wrongdoing, police may also gain probable cause for further searches at this type of checkpoint. DUI checkpoints require more though. When police officers extend the checkpoint from checking licensing with passive observation for signs of intoxication to actively testing every driver for intoxication (e.g. flashlight tests, simple cognitive tests, etc...), the purpose has shifted from regulatory to criminal enforcement. In Michigan v. Sitz, the court effectively ignored the regulatory requirement of special needs searches in coming to their holding that DUI checkpoints fall under the special needs exception (the court also likened DUI checkpoints to airport and boarder checks without addressing that there is no other way for the interests protected by those checks to be advanced). Rather, the court merely recognized that there was a compelling government interest, and that DUI checkpoints were a means of advancing that interest. In a later case, Indianapolis v. Edmond, the Supreme Court held that "narcotics checkpoints" were an unconstitutional violation of the 4th Amendment. While these checkpoints were identical to DUI checkpoints, except that police would also lead a drug dog around the motorists vehicle. The court refused to extend the special needs exception where the "primary purpose" is "to detect evidence of ordinary criminal wrongdoing." As such, motorist checkpoints are legal only so long as the primary purpose is regulatory rather than general criminal enforcement, except in the anomolous DUI checkpoint situation, and in boarder or airport security checks. A checkpoint is not a traffic stop. While it depends on the state, in Texas, a person is required to furnish their license on the request of a peace officer while operating a motor vehicle on a public roadway. Therefore, the checkpoint, primarily, is designed to ascertain whether the person is legally operating a motor vehicle. And, if the peace officer has reasonable suspicion to believe a person is intoxicated, he may perform a Field Sobriety Test. == == A checkpoint isn't a stop. It's designed with safety in mind, such as a seatbelt check. When they approach the vehicle to see that you're wearing a seatbelt, they also observe the driver to see if he/she is intoxicated. Since it's illegal to drive intoxicated and they observed you drive, they can stop you. I believe they get away with it because their checkpoints are announced publicly. They are in the newspaper, on the police departments website, and even sometimes the news will say it. This goes along too with "hiding places" you know when they sit in someone's driveway or so. Also, checkpoints are very well visible. If you think you might be stopped or questioned and you have something to worry about, stay put, or find another way out. All vehicles pass through the checkpoint and therefore any of the vehicles are subject to being stopped, the driver and passengers being asked for identification and so forth. Such action does not legally constitute profiling or harassment. In British Columbia Canada we often have check points and it is about seat belts (very strict here) as well as to check to see that the driver is not intoxicated. Police officers have that right. We see many cars pulled over and abandoned for towing.
Because the Supreme Court ruled in Michigan Department of State Police vs. Sitz that it was constitutional. The majority acknowledge that it violates the 4th amendment, but said that the state interest in reducing drunk driving outweighed the infringement. The dissenting justices pointed out that there is no "state interest" clause or anything else in the Constitution to provide an exception.