Before you go any further, you need to read the NHTSA validation studies which I previously posted about here. You also need to read the Cole and Nowaczyk study from Clemson University which was published in a journal called Perceptual and Motor Skills in 1994.
Having recently been certified in DWI detection and Standardized Field Sobriety Testing I have some thoughts on the subject. Mostly the tests seem to be very subjective and are poor indicators of impairment when someone has low and mid-range levels of alcohol in their system. I recently performed the tests on my mother-in-law, sister-in-law and my wife. Using the NHTSA scoring, the only one not going to jail is my wife. They were sober, indoors with no environmental difficulties commonly found on the side of the road and had no fear of being arrested.
So, how have these tests become so popular and widely accepted in court? Society wants to convict drunk drivers. In the 1970’s, NHTSA paid a research group to come up with some tests. Many tests were tried and almost none were any better than a coin flip at determining whether someone was impaired or not. The three that did come up over that 50% that NHTSA adopted are the three test battery that we are all familar with. HGN, Walk & Turn and One Leg Stand. 70-80% accuracy is good enough for government work, so there you have it.
There are some more recent validations studies too that make the numbers even better according to NHTSA. The same lady (Burns) that originally proposed and validated the tests was behind the new validation numbers as well. It’s like asking Ford Motor Company, “Which American company produces the best cars?” You knew what the answer was going to be before you even asked the question.