As an officer begins to follow a vehicle, there are a number of erratic or abnormal behaviors that will tip the officer off that the driver has been driving while intoxicated. The most common of these is to determine if the vehicle is swerving or crossing over the painted lines on the road. This does not necessarily mean that the vehicle is careening back and forth; many times it is just hitting the yellow lines a couple of times.
For example, a vehicle may cross the line on the right side of the road enough that the tire is on the other side of the line. This one crossover is generally not enough to constitute a traffic stop because even a sober driver does that from time to time. However, what the officer is looking for is how many times the driver repeats this action in a short amount of time driving a short distance. The exact amount of time is determined on a case-by-case basis.
Further, another action an officer will watch for is erratic speeds by the vehicle. If a driver starts out going 50 m.p.h, but then slows to 30 m.p.h., and then increases the speed again, this will peak the officer’s interest because it is not a behavior that is normal when operating a vehicle. Generally the officer will detect how many times this will happen and determine whether the driver has indicated enough to support reasonable suspicion to pull the car over.
The main thing officers are looking at is whether a vehicle is maneuvering in an abnormal or erratic fashion as compared to how the average sober driver handles his vehicle. The most common being vehicle swerving which does not necessarily mean an abrupt or obvious swerve. In some cases an officer may not feel like there is enough erratic behavior to justify stopping the vehicle on that alone and will look for another reason to stop the vehicle in order to investigate further.
