The only recognized scientific publications that have examined Field Sobriety Tests (FST) have determined that they are designed for failure.  The scientist that conducted the study examined police officers and their abilities to determine intoxication and impairment solely on the basis of FSTs and the results were shocking.  Trained police officers were unable to determine via FSTs whether anyone was impaired even when there was a high level of intoxication.

Horizontal Gaze Nystagmus

One of the tests given as part of the typical battery of FSTs is the Horizontal Gaze Nystagmus (HGN) test.  The officer holds a pen or his finger 12 to 15 inches in front of your face and looks for involuntary eye movement.  This test is completely unreliable because studies have shown that 93% of people have Nystagmus naturally.  Furthermore, you can have pronounced Nystagmus by many medications, energy drinks, aspirin and even some foods.

Walk and Turn and One Leg Stand

These tests involve balance and follow a long list of instructions.  In all cases, we have no idea how you would perform under laboratory conditions with no alcohol in your system.

Furthermore, these tests are more tests of clumsiness and balance which is different in everyone.  For example, most Olympic gymnastic teams have a team of ladies that are about 5 feet tall.  You never see 7 foot  Zulu tribe members as a member of a gymnastics team.  The answer is simple, the taller you are, the higher your center of gravity and it makes it too hard to do balance beams and floor exercises