There is probably no sinking feeling that matches getting pulled over after having had a bit too much to drink. As often as people will have thought, only moments before, that they were okay to drive, there is a sudden concern, if not realization, that their own assessment may have been off a bit as they put the car in park and wait for the police officer to approach. No one can really smell alcohol on his or her own breath, but everyone knows when he or she is unable to walk a straight line.
There is, of course, a lot to a Michigan DUI traffic stop. As a Detroit DWI attorney, I've written rather extensively about many, if not most, aspects of the traffic stop. The fact is that you could fill several volumes about the various facets of being pulled over for suspected drunk driving. To keep our mission manageable here, and to keep your attention, as well, we'll limit our ambitions a bit and take a brief look at one small part of a DUI traffic stop and the field sobriety tests, the horizontal nystagmus gaze (HGN, or as often referenced by the police in the Metro-Detroit area of Macomb, Oakland and Wayne Counties, the "horizontal NSG") test. This is the test where the officer asks a person to keep his or her head still, and follow an object (often a finger, or a pen) using only their eyes. This test has a remarkable capacity to detect that a person is impaired by alcohol.
The HGN test measures how smoothly a person's eyes follow an object moved from one side of their field of vision to the other. As it turns out, absent any of a few particular medical conditions, a sober person's eyes will follow such an object rather smoothly, like a perfectly round marble rolling on a piece of glass. Alcohol affect the movement of the eyes, and as a person becomes more inebriated, his or her autonomic motor functioning likewise become impaired, meaning that they eyes will "jump" (kind of like that same marble being rolled over a sheet of grainy sand paper) and not track smoothly. This is completely outside of a person's conscious control, and no matter how sober or drunk someone may become, they never notice this from their own perspective.
It would be impossible to appropriately summarize the science behind the HGN test beyond pointing out that even the American Optometric Association has passed a resolution endorsing it as an effective test for alcohol impairment. The HGN test is generally believed to be the most reliable of all field sobriety tests. The flip side is that it is also almost generally impossible to independently verify, leaving proof of a person's performance on the HGN test as almost entirely a matter of believing what the police officer says (or writes in his or her report), or not. Not every police officer can administer an HGN test, however. In order to do so, the officer has to be specially trained to administer it. Given it's high degree of reliability and ease of administration at the side of the road, it's little wonder that more and more police officers are receiving this training.
The few exceptions to the scientific reliability of the HGN test as evidence of alcohol impairment don't often occur in real life DUI traffic stops. If you have a brain tumor (and then you'd have to prove that it's the kind that would affect your performance on an HGN test), a brain disease, or an inner ear disease, then this should be explored as a defense. Most conditions that would affect a person's performance on an HGN test would likewise prevent them from driving in the first place. Even then, the likelihood that such a person would be driving, and also drinking (remember, it's not illegal to drink and drive, it's only illegal to drink too much and then drive) is rather remote.
For a moment, let's skip all the lawyer-talk exceptions and exclusions and limitations and focus on the "real world." In the real world, if you're pulled over, and the police officer has you out of the car taking field sobriety tests, it's rather likely you've had something to drink. The National Highway Traffic Safety Administration has sanctioned, or validated only 3 field sobriety tests: The HGN, the heel to toe walk, and the standing leg raise. While many police officers do alphabet and counting tests, neither of them, nor any other test beyond the 3 approved by the NHTSA, really have any real legal weight.