A person who is facing a drunk driving charge often has various options to consider as components of their defense strategy. One of these might be calling the validity of the traffic stop into question. In order to stop a vehicle, the police officer needs to have reasonable suspicion that the law was violated.
Reasonable suspicion is a much lower standard than probable cause. The officer doesn’t have to actually see a person break the law. Instead, they only have to see signs that the person might be breaking the law. There are several signs of drunk driving that they might look for when patrolling.