Residents of Orange County are likely aware that a police officer cannot pull a driver over without a legitimate reason. While sometimes these traffic stops result in warnings or tickets, in other cases drivers can be arrested for driving while intoxicated.
Individuals who spend any time behind the wheel of vehicles around holidays such as Thanksgiving and Christmas may be aware of the existence of DUI checkpoints that are not otherwise present. The purpose of these checkpoints is fairly clear, to catch drivers who are over the legal limit of intoxication. By removing these individuals from the roads, other drivers will theoretically be safer on the roads. Since the purpose of these stops is to catch drunk drivers, many are likely surprised that the locations and times of such stops are so widely publicized.
The reality is because these checkpoints result in many drivers of the route being pulled over to determine if they have been drinking without any reason other than being in that location at that time, the Supreme Court of the United States determined that such stops are unconstitutional. Since any evidence recovered in an unconstitutional stop is inadmissible, the authorities had to come up with a way to make the stops constitutional. These public announcements fulfill that need.
Some could view this as good news all around. In addition to the constitutional rights of drivers not be violated, the practice also serves as a deterrent to drivers who have been drinking who would otherwise get behind the wheel to drive.
Source: Valley Independent Sentinel, "Why Do Police Announce DUI Checkpoints?" Ethan Fry, Nov. 20, 2012