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Study Shows Marijuana DUI Laws Not Based in Science

13-May-2016

In every state, it is illegal to drive under the influence of alcohol or drugs. In states across the nation, the threshold for alcohol intoxication per se is a blood alcohol concentration (BAC) of 0.08 percent, although a lower BAC can still lead to criminal charges in some cases. This means that anyone driving with a BAC of 0.08 percent or greater is guilty of DUI, regardless of whether or not he or she shows any obvious signs of impairment.

Determining the threshold for legal impairment by drugs is somewhat more difficult. Some states, like Oklahoma, have zero-tolerance for driving after using marijuana. In these states, a person can be charged with DUI with any amount of THC, the active chemical in marijuana, in his or her system. And because THC can linger in the body for days or even weeks after use (and after any effects have subsided), a person can be arrested for DUI-Drug even without being under the influence of or impaired by marijuana.

Other states, particularly those which have legalized the medical or recreational use marijuana, have found it necessary to determine an "appropriate" level of THC in the driver's system. Much as alcohol impairment is determined by a BAC, these states have set a "legal limit" for marijuana use and driving.

However, a new study shows that these limits are arbitrary and have no scientific basis.  According to a study by the AAA Foundation for Traffic Safety, there is simply no way to accurately determine what level of THC is "safe" and what level would cause driver impairment.

In the study, researchers looked at several indicators of impairment among 602 people who were arrested for DUI with only THC as a contributing factor, and compared their performance on Drug Recognition Expert (DRE) exams to those of a control group of 349 sober volunteers.

Not surprisingly, the sober group performed better than the "high" drivers. However, when researchers attempted to correlate the stoned drivers' DRE performance with their THC levels, there was simply no connection. Some drivers with relatively low levels of THC were very obviously high and unable to perform well on the sobriety tests. Others with THC levels above the limits set by most states performed exceptionally well and gave no indication--other than THC level--of impairment.

The AAA Foundation study reached the following conclusions:

  • All of the candidate THC concentration thresholds examined would have misclassified a substantial number of driver as impaired who did not demonstrate impairment on the SFST [Standardized Field Sobriety Test], and would have misclassified a substantial number of drivers as unimpaired who did demonstrate impairment on the SFST
  • Based on this analysis, a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported.

The arbitrariness of these laws illustrates just how prevalent misconceptions about marijuana and driving can be. Obviously, driving while impaired by marijuana is dangerous: studies show that marijuana use doubles a driver's crash risk. But talking on a hands-free cell phone quadruples the risk. And driving with a BAC of 0.12--the average in DUI crashes--carries a 15 times greater crash risk than sober driving.

Should people take the wheel after using marijuana? It doesn't seem like a good idea--but states need to rethink how they classify marijuana impairment. THC levels alone are clearly not a reliable indicator.



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