Typical DUI investigation and arrest

Following are common procedures when a law enforcement officer has reason to suspect a driver is intoxicated.

Reasonable Suspicion to stop
There are several situations in which the officer will come into contact with a driver, some examples are:

* The driver has been involved in an automobile accident; the officer has responded to the scene and is conducting an investigation.
*The driver has been stopped at a sobriety checkpoint (also known as roadblocks).
*The police have received a report, possibly from an anonymous citizen, that a described car has been driving erratically. The officer should verify the erratic driving before pulling the driver over. In some cases, the driver will no longer be in the vehicle.
*The officer on patrol has observed erratic, suspicious driving, or a series of traffic infractions indicating the possibility that the driver may be impaired. This is by far the most common reason for stopping a suspect.
*A police officer has stopped a vehicle for a lesser traffic offense, notices the signs of intoxication, and begins the DUI investigation.

The following list of DUI symptoms, from a publication issued by the National Highway Traffic Safety Administration (DOT HS-805-711), is widely used in training officers to detect drunk drivers. After each symptom is a percentage figure which, according to NHTSA, indicates the statistical chances through research, that a driver is over the legal limit.

Turning with wide radius-------------------------------------65
Straddling center or lane marker----------------------------65
Appearing to be drunk----------------------------------------60
Almost striking object or vehicle----------------------------60
Weavin-----------------------------------------------------------60
Driving on other than designated roadway-----------------55
Swerving---------------------------------------------------------55
Slow speed (more than 10mph below limit)----------------50
Stopping (without cause) in traffic lane---------------------50
Drifting-----------------------------------------------------------50
Following too closely------------------------------------------45
Tires on center or land marker--------------------------------45
Braking erratically----------------------------------------------45
Driving into opposing or crossing traffic--------------------45
Signaling inconsistent with driving actions-----------------40
Stopping inappropriately (other than in lane)--------------35
Turning abruptly or illegally----------------------------------35
Accelerating or decelerating rapidly-------------------------30
Headlights off----------------------------------------------------30

If the officer observes enough to have a reasonable suspicion to legally justify a further detention and investigation, they will ask the driver to step out of the vehicle.
Reasonable suspicion requires less evidence than probable caus, but more than a mere hunc. A rule of thumb is that reasonable suspicion requires 25 % proof, and probable cause requires more than 50 % statistical chance. Therefore, if there is probable cause for arrest for DWI, as suggested by the research and examples used above, then there is reasonable suspicion to stop a driver.