A police officer will use the same field sobriety tests for detection of a potential Marijuana DUI as they do for an alcohol DUI. These tests are; the Horizontal Gaze Nystagmus, the Walk and Turn test, and the One Leg Stand test. With marijuana use, the officers are looking for any indicators that show you are impaired during these tests. However, very few sober people can pass without practice. Since these tests are completely voluntary, and there is no penalty if you decline, you should politely decline the tests.
It Is Legal To Refuse A Field Sobriety Test
In response the officer may try to coerce you into doing the field sobriety tests by telling you that if you pass, you may leave. Do NOT believe this! If they suspect you have used marijuana, they are trained to gather as much evidence against you as possible. Decline in a manner that is apologetic but firm, such as, "Officer, I respect that you are doing your job, and I intend to cooperate with you so long as my rights are protected. I am now invoking my right to silence and to an attorney upon the advice of counsel, and I decline to perform any field sobriety testing."
Even if you believe that marijuana usage will not impact your performance in terms of balance, gait or eyes, it is unwise to perform any of these tests. Even a slight mistake can be used against you. In addition, officers rarely allow you to perform these tests in the view of a video camera (that is if one is even present at the time/location). Thus, they can say you failed even if you did the tests perfectly... and it is your word against theirs.
Measuring Alcohol Content With Portable Breath Tests
Whether or not you do the field sobriety tests, it is likely that the officer will ask you to do a preliminary alcohol screening(PAS) in order to check for alcohol (despite his/her suspicion of marijuana use). Again, this test is completely voluntary and there are no penalties for refusing the PAS. Do not confuse this for the Breathalyzer test which is administered at the police station; refusing this may result in a revocation of your driver's license for at least one year. If you have not consumed any alcohol and blow a 0.000, you may not be in the clear, because the officer has ruled out alcohol and may now have a greater suspicion of drug use. At this point they are likely to ask you to submit to a Drug Recognition Expert (DRE) Evaluation.