Lawmakers in Texas passed a medical marijuana bill in 2016, but the recreational use of the drug remains illegal in the state. States including California, Colorado and Washington have approved recreational marijuana use, presenting law enforcement agencies across the country with a serious challenge. While field sobriety, breath and blood tests are scientifically proven ways to detect alcohol impairment, there is currently no reliable method police can use to determine whether an individual is under the influence of marijuana.
Drunk driving charges generally hinge on blood alcohol levels, but blood tests are of little use in marijuana impairment cases. This is because the human body processes marijuana very differently than it does alcohol, and THC levels can remain elevated months after the drug has been consumed and its intoxicating effects have diminished.
Marijuana users also develop a tolerance for the drug, and seasoned smokers may be relatively unaffected by THC levels that would leave novice drug users highly impaired. With chemical testing considered unreliable, police departments have relied on field sobriety tests to detect marijuana impairment, but they were designed with alcohol in mind. The tests are effective at revealing the balance and coordination problems associated with alcohol consumption, but marijuana affects response times and the ability to cope with distractions.
There are several marijuana-specific tests being developed for police, but experienced criminal defense attorneys may object to their use in DWI cases until they have been scientifically evaluated and determined to be reliable. Lawyers may also seek to have impaired driving charges dismissed when the results produced by unreliable roadside tests are not supported by more exhaustive laboratory testing. The portable testing kits used by many police agencies have been known to identify benign substances like baking powder and sugar as illegal drugs.Source: The San Antonio Current, Texas Could Finally Blunt Its Harsh Marijuana Laws Next Year, Michael Barajas, Nov. 16, 2016