The law creating a limit for just how stoned a driver can be in Montana went into effect Oct. 1. Debate over the veracity of the new limit, however, continues.
The new statute says that if a driver is suspected of being under the influence of marijuana while operating a motor vehicle, a blood test may be requested to determine intoxication. If the blood test reveals a THC content of 5 ng/ml or higher in the driver's blood stream, the driver may be charged with a DUI, resulting in a possible license suspension, up to six months imprisonment and a fine of up to $1,000.
The 5 ng/ml limit has been criticized as an arbitrary threshold to determine driving impairment. The National Highway Traffic Safety Administration has said that the connection between the amount of THC in a person's bloodstream and their level of impairment is difficult to determine.
"It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH concentrations," the NHTSA said in a 2004 report. "It is possible for a person to be affected by marijuana use with concentrations of THC in their blood below the limit of detection of the method."
Missoula Deputy County Attorney Andrew Paul says in an email that he does not feel comfortable commenting on what blood level of THC indicates impairment, as he is not a doctor of medicine.
"You have to establish a standard," Paul writes. "Whether that (THC) standard is too high or too low should be a question addressed to the scientific community."
Ten states now have zero tolerance laws pertaining to drugged driving, and Montana is one of five others that have designated a nonzero limit related to marijuana.
Paul says that as far as he knows, there are no numerical or presumptive standards regarding DUI prosecution for other drugs such as narcotic painkillers.
"However, if we get a blood sample that contains evidence of such drugs in the blood system, we will get a doctor to testify regarding the interactions between the drugs and the alcohol and testify regarding the effect of the drugs over a person's ability to safely operate a motor vehicle," Paul writes.
According to the Missoula Police Department, no drivers have been cited under the revised law.