Blood THC levels after smoking pot are useless in defining “too high to drive”
Measuring ‘drunk’ is pretty easy; the more alcohol someone drinks, the more alcohol shows up in that person’s blood and the more impaired that person becomes, falling somewhere on a scale of tipsy to wasted. Measuring ‘high,’ on the other hand, is far hazier—much to the dismay of some states' law enforcement.
Blood tests that try to quantify marijuana use are in fact useless at assessing how impaired a driver is, according to a study by the AAA Foundation for Traffic Safety. In other words, the study found that people with low blood amounts of THC—or delta-9-tetrahydrocannabinol, the main psychoactive component of pot—may still act as if they’re really stoned. On the other hand, some people may have THC measurements off the charts yet still act normally.
The finding is critical because several states have already set legal limits for the amount of THC a person can have in their blood while driving. AAA concluded that such limits are “arbitrary and unsupported by science, which could result in unsafe motorists going free and others being wrongfully convicted for impaired driving.”
For the study, AAA researchers combed through arrest records for impaired driving as well as results from toxicology tests and Drug Recognition Expert (DRE) exams. This exam includes roadside sobriety tests such as walking and turning, standing on one foot, and nose touching.
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http://arstechnica.com/science/2016/05/legal-limits-for-marijuana-dui-are-arbitrary-and-unsupported-by-science/
= new reply since forum marked as read
What in the world are we going to do? 
