Police officers in Illinois and around the country are able to use portable, and generally reliable, breath testing equipment to determine whether or not a motorist is driving while impaired by alcohol, but tests that reveal marijuana use are not currently available. Several companies are rushing to bring such tests to market, but many medical and scientific experts believe that they will be of little practical use.
Scientists and doctors are dubious about the merits of cannabis testing because the body does not deal with THC in the same way that it handles alcohol. BAC levels are viewed as a reliable way to determine alcohol impairment even among heavy drinkers, but a THC level that could leave a casual marijuana smoker incapacitated may have little effect on an individual who has developed a high tolerance for the drug.
Another challenge facing the makers of marijuana testing equipment is the way that the body stores the drug. THC is stored in fat cells, and the substance can remain in the system for a month or more after being consumed. Some states that have legalized marijuana for either medical or recreational use are thinking about setting maximum THC levels for drivers, but critics say that these measures are not supported by credible science.
Police generally rely on field sobriety exercises to determine impairment when a reliable chemical test is not available, but experienced criminal defense attorneys may seek to have drunk driving charges dismissed when they are not backed up by credible forensic evidence. Older or obese drivers may find field sobriety exercises difficult or even impossible to perform successfully, and certain medical conditions could also lead to unfounded accusations of drunk driving. Horizontal gaze nystagmus is an indicator of intoxication that field sobriety tests are designed to uncover, but it can also be caused by influenza, eye strain, vertigo or inner ear problems.