The Michigan State Police were administrating a DUI checkpoint program, to which Sitz & Co objected on constitutional grounds (as far as I can tell from the decision, Sitz wasn't stopped in or otherwise involved in the checkpoint. The suit was filed on the theoretical grounds that he and the other plaintiffs, who were licensed drivers in Michigan, might be unlawfully detained in future checkpoints). Due to the pending litigation, at the time of this decision there had only been one such checkpoint pursuant to this program.
During that particular operation, all vehicles passing through the checkpoint were to be stopped, and an officer would contact the driver to see if he noticed signs of intoxication. If he did, the driver would be directed to a secondary area of the checkpoint where further investigation would occur, possibly culminating in arrest. If not, the driver would be allowed on their way immediately. 126 vehicles were stopped over the course of 75 minutes, for an average of 25 seconds, only two drivers were detained for FSTs, only one of those was arrested, and a third driver was arrested for DUI after blowing through the checkpoint.
The US Supreme Court held that the States have an important interest in apprehending drunk drivers, with references to both anecdotal and statistical evidence of "alcohol-related death and mutilation on the Nation's roads." On the other hand, the Court held that the motorists' interest in not being briefly delayed at a roadblock was slight. Balancing the two against each other, the Court held that such checkpoints are reasonable. The Court relied on its reasoning from US v. Martinez-Fuerte, which held similar checkpoints reasonable for the purpose of detecting illegal aliens.
The Court did point out that the DUI checkpoint program minimized the discretion of officers on the scene (as compared to making random stops with roving patrols, which is what happened in Delaware v. Prouse), and also pointed out that this decision was about the initial brief detention of all the vehicles passing through the checkpoint. Further detention (at the secondary area) would require individualized reasonable suspicion.