Despite increasingly intense rhetoric from both politicians and members of law enforcement in regard to an alleged epidemic in "stoned driving," Congressional testimony last week by a representative of the US government's foremost traffic safety agency, the US National Highway Traffic Safety Administration (NHTSA), revealed that data in regard to cannabis effect on behavior and traffic safety is largely lacking.

Speaking before the US House of Representatives, Committee on Oversight and Government Reform, Dr. Jeffrey Michael, Associate Administrator for Research and Program Development at NHTSA, acknowledged that the agency is unaware of any figures specific to cannabis's role in US traffic fatalities. By contrast, NHTSA's representative was able to provide precise annual figures pertaining to the annual number of traffic fatalities caused by alcohol intoxication, distracted driving and aggressive driving. However, when asked by Virginia Democrat Gerry Connolly as to the estimated number of annual traffic deaths attributable to motorists under the influence of THC, Michael replied, “Currently, that is difficult to say, sir.”

NHTSA's representative further testified to the fact that, at this time, the imposition of per se thresholds (legal limits intended to delineate between impaired and unimpaired subjects) for cannabis are inappropriate, a position also acknowledged on the agency's website which states: “It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects. ... 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.”

Speaking to the Congressional Committee on Thursday, Dr. Michael explained, “The available evidence does not support the development of an impairment threshold for THC (in blood) which would be analogous to that (of) alcohol.” He added: “The available evidence indicates that the response of individuals to increasing amounts of THC is much more variable than it is for alcohol. So with alcohol we have a considerable body of evidence that can place risk odds at increasing levels of blood alcohol content. For example, a .08 blood alcohol content is associated with about four times the crash risk of a sober person. The average arrest is at .15 BAC; that is associated with about 15 times the crash risk. Beyond some broad confirmation that higher levels of THC are generally associated with higher levels of impairment, a more precise association of various THC levels and degrees of impairment are not yet available.”  

Dr. Michael also noted that NHTSA was conducting an ongoing study that, upon completion, will provide odds ratios -- assessments of whether an activity is associated with a risk greater than or less than 1 (1 equating to zero increased risk) -- for cannabis-positive drivers compared to other motorists. But data from similar case-control studies conducted by other researchers already exists.

Recently, a 2012 meta-analysis of 66 studies assessing drug positive drivers and crash risk concluded that marijuana-positive drivers possessed an odds-adjusted risk of traffic injury of 1.10 and an odds-adjusted risk of fatal accident of 1.26. This risk level was among the lowest of any drugs assessed by the study's author and it was comparable to the odds ratio associated with penicillin (OR=1.12), anti-histamines (OR=1.12), and antidepressants (OR=1.35). To put cannabis' odds ratios in context, a separate study published earlier this year in the journal Injury Prevention reported that drivers with a BAC of 0.01 percent are "46 percent more likely (OR = 1.46) to be officially blamed for a crash than are the sober drivers they collide with."

Not surprisingly, NHTSA's inability to provide hard data to the Committee raised concerns among Congressional members. In response to Dr. Michael's testimony, Rep. Connolly stated the obvious: “I just think it is amazing with some of the hyperventilated rhetoric about marijuana use and THC that 50 years after we've declared it a Class 1 substance, we still don't enough data to know just how dangerous it is in (regards to) operating a vehicle. That really raises questions about either the classification (of marijuana) itself, whether that makes any sense, or raises serious questions about how our government is operating in terms of the data it does not have and the science it does not know and yet the assertions that we (the federal government) make. That is not a good recipe for rational public policy.”

Indeed it is not.