Determining whether a driver is impaired by the consumption of marijuana is not as simple as is the case with alcohol. The level of THC in a person’s blood has no scientific connection to the level of impairment. “There is understandably a strong desire by both lawmakers and the public to create legal limits for marijuana impairment in the same manner we do alcohol. In the case of marijuana, this approach is flawed and not supported by scientific research.” Any “nanogram limit for THC in blood (is) picked out of thin air by politicians. Innocent people are convicted of DUI because of this.”
The study commissioned by AAA’s safety foundation said it’s not possible to set a blood-test threshold for THC that can reliably determine impairment.
Source: Scientific basis for laws on marijuana, driving questioned – WTOP