To the interested: The standard DUI enforcement traffic stop may go something like this: It is typically divided into three portions.
1: Vehicle in in motion, during which the officer observes the vehicle. This can be a simple thing, such as recognizing the vehicle has an equipment malfunction or it can be driving behavior, such as swerving across lane markers. If the officer observes a law violation, we move into phase 2:
2: Personal Contact: Here, the officer is simply speaking to the driver. Typically, we introduce ourselves, inform the driver of the reason for these stop and ask for their driver's license. While this occurs, the officer is making observations: What do you see, smell, hear? Typical clues can include the odor of an alcoholic beverage emanating from the vehicle, the appearance of bloodshot watery eyes, the sight of the driver fumbling through a wallet and handing you a credit card or a stack of receipts instead of a driver's license. These observations move us to phase 3:
3: Pre arrest Screening: I typically just ask the driver if they've had anything to drink that evening. What you, as the officer, have to be aware of is the simple smell of an alcoholic beverage doesn't mean the driver has been drinking. Maybe they are the DD, maybe they're an Uber driver, maybe a friend spilled a drink on them or in the car. You want to start eliminating those possibilities and working through any statements the driver makes to determine the truth as best you can. I typically ask the driver to perform pre-exit checks.
These checks include the finger dexterity test, reciting a portion of the alphabet and counting down from a higher number to a lower one. These tasks are kept simple and often times allow me, with a minimum of interference to the driver, to make an educated guess and allow the truly sober to go about their business quickly. If, however, the person shows indications of impairment, I then have them step from the vehicle. While they do this, I watch to ensure their safety from other traffic, and to observe their balance. I've seen a big bag of weed fall on the ground during this before. I usually write that down as a clue.
I then talk to them for a minute, to find out why the smell of al alcoholic beverage is coming from their car. I see if I can smell it coming from their person, again, to make sure it's not something simply spilled in their vehicle as they drove intoxicated friends home. While doing this, I'm again looking at the whole person: are their eyes bloodshot, watery, are they swaying as they stand, is their clothing disheveled, is there're vomit on them, did they pee on themselves, etc.
After making the determination further investigation is warranted, I ask them to perform the battery of Standardized Field Sobriety Tests. The tests consist of three tests, the Horizontal Gaze Nystagmus, the Walk and Turn and the One Leg Stand. I have further training so I am also able to perform additional tests if I feel they are warranted, such as the Modified Rhomberg Balance and the Lack of Convergence Test. If they show sufficient indicators during these tests, I then arrest them and read the Implied Consent Advisement form to them. From there, it's simply a matter of going to the ER (or magistrates office for a warrant if need be) and then the jail.
For the very, VERY interested: during HGN, i perform a medical check to ensure their eyes show no resting nystagmus, track equally and have equal sized pupils. The test itself consisted of checking for a lack of smooth pursuit, distinct and sustained nystagmus at maximum deviation and the onset of nystagmus at maximum deviation. I also check for vertical gaze nystagmus. Although not a clue, it can be a sign of impairment, specifically a high does of the particular drug for that individual.
During WAT, there are two clues during the instruction stage, failure to maintain the start position and beginning the test prior to directions to do so. There are six clues during the walking stage: Arms used for balance, stepping off the line, failure to touch heel to toe, incorrect number of steps, incorrect turn and stopping once you have begun the test.
During OLS, there are four clues: Putting your foot down, using arms for balance, swaying and hopping.