The first time you go through a DUI checkpoint, you may find yourself wondering how such a thing can be legal.
After all, each driver is pulled over and has to talk to the police. This is technically a seizure since drivers are not permitted to leave if they want to, and it can be done without reasonable suspicion. The police need probable cause for a squad car to pull over a driver, but they don't need it at a checkpoint.
This has been contested. The Fourth Amendment does say that an unreasonable search and seizure is illegal, essentially preventing police from detaining and searching anyone they desire for no reason. It has been argued that a DUI checkpoint does exactly this, making it illegal.
In 1990, a case called the Michigan Department of State Police v. Sitz made it all the way to the Supreme Court. That is when the court ruled in favor of the police, saying that the checkpoints did not violate the Fourth Amendment, and they've been legal ever since. This does not mean they aren't controversial, but they can technically be used.
The court's reasoning was that the good of the state was more important. They felt it was better to use checkpoints and prevent DUI accidents than to give everyone the freedom to drive without being pulled into a checkpoint against their will. Though this is inconvenient and annoying for drivers who are sober, the court essentially decided it was worth it for them to endure that if it meant there were fewer accidents.
Those who are facing charges in Maryland after being caught at a DUI checkpoint do still have the right to a fair trial.
Source: FIndLaw, "Are DUI Checkpoints Legal?," accessed Feb. 19, 2016