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How do field sobriety tests and chemical tests differ?

Erratic driving, falling asleep behind the wheel or causing an accident may be clear indicators that a Boston resident is driving under the influence. Yet minus these incidents, how is a law enforcement official to know if you may be intoxicated? There are subtle clues that one might rely on (e.g., bloodshot eyes, alcohol beverage containers in your vehicle, the perceived smell of alcohol on your breath), yet none of these are sufficient to prove that you are legally drunk. Only sobriety testing can do that. 

You may hear the terms "field sobriety test" and "chemical test" and think that they are synonymous. Yet are they really? Chemical testing is an actual chemical measurement of your blood alcohol concentration obtained from a sample of your blood, breath or urine. These tests are performed in a controlled laboratory environment and offer a high degree of accuracy. Per implied consent laws, you cannot refuse such testing without facing some sort of criminal consequence. 

Field sobriety testing, on the other hand, is simply a method law enforcement officials can employ to get an idea of whether or not you might be impaired. These are tests that measure elements like your standing equilibrium, response time or motor control. While a wide range of field sobriety tests exist, the three endorsed by the National Highway Traffic Safety Administration include: 

  • The one-leg stand test
  • Your horizontal gaze nystagmus (involuntary blinking of your eyes)
  • The walk-and-turn test

These tests, of course, still offer no direct measurement of your supposed level of impairment. While a breathalyzer does, it is considered to be a field sobriety test due to its measurement not meeting evidentiary standards. You are within your legal right refuse any of these tests. 

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