One of the most important factors in any DUI traffic stop is that the police must have a reason to make that stop in the first place. A stop can never be random. They must have either reasonable suspicion that they found an intoxicated driver or they must at least have seen that driver break a traffic law. For instance, they could see the driver roll through a red light and then pull them over for that infraction, only making a DUI arrest after they smell alcohol on them when talking and give them a breath test.
What they cannot do is pull people over at a certain time of day or target certain types of drivers, making random stops and looking for those who are intoxicated.