Marijuana Per Se Limit?

An appropriately set marijuana per se limit has been discussed in many professional circles for a long time. But why was marijuana singled out over other drugs known to cause psychomotor impairment? The simple answer is its prevalence of use. Marijuana remains the most commonly used illicit drug worldwide and can be found in the blood of a large proportion of apprehended drivers. Besides alcohol, marijuana (delta-9-tetrahydrocannabinol, and/or its metabolite, delta-9-tetrahydrocannabinol-COOH) is detected more than any other drug. But, does marijuana intoxication result in a disproportionately high number of traffic accidents? Maybe not, or at least not when low concentrations are present. After smoking, the blood concentration of the primary psychoactive constituent delta-9-tetrahydrocannabinol (THC) falls rapidly and effects tend to drop off significantly after a few hours. However, THC may still be detectable when the risk of impaired driving is relatively low.

Of the handful of states that have adopted per se limits, a few (Ohio, Nevada, Pennsylvania) have included urine as an acceptable sample. Urine THC remains low and only detectable within the first several hours after administration. However, urine concentrations cannot be linked to impairment due to a multitude of factors such as large variability of effects between individuals, variable excretion and timing of collection relative to dose. In this sense, it can only be linked to recency of use.

On the other hand, blood THC concentrations may be linked to some of THC's observable effects (e.g., heart rate); however, there is scant evidence that the effects related to impaired driving (motor coordination, balance, etc) are reliably linked to an increased risk of traffic accident. A large part of this is due to the time delay between driving and blood collection. It may take a few hours after a traffic stop before blood can be collected, and by then, blood THC has decreased considerably, or may be undetectable.

Since 1979 (Los Angeles PD), law enforcement have been identifying drug impaired drivers by testing multiple aspects of driving performance, such as motor coordination, divided attention and visual acuity. They also obtain physiological data (e.g., blood pressure, pulse, etc) that may indicate the drug class that is causing the impairment. Drug Recognition Experts (DRE) are police officers that are trained to identify drivers that may be impaired by drugs other than alcohol. The tests are accompanied by the collection of blood or urine that can be tested for potentially impairing drugs. At the conclusion of the officer's investigation, they render an opinion on what drug category(s) is(are) causing the impairment. In this scenario, there should be sufficient evidence to demonstrate that the driver was impaired and by what. A positive THC result would provide further objective evidence that the driver was exposed to marijuana and the concentration may suggest how recently.

There will likely never be a convincing, scientific, causative link between blood THC concentration and risk of traffic accident. The studies that have demonstrated this link with alcohol would be extremely difficult to conduct for THC. Again, the time delay between driving and blood collection is too long and there is poor correlation between observable effects (that would suggest poor driving) and blood THC concentration. Establishing such a link might be possible if the time delay was shortened to a few minutes; however, this would not be feasible in practice.

So, should we have marijuana per se limits? Good question and I expect this debate to heat up as states continue to legalize medicinal and/or recreational marijuana. Here are a few questions that may ultimately be answered.

1) Is it necessary that marijuana per se limits be established based on the same accident causation model that supports the current per se limits for ethanol?

2) Will the current DRE programs adopt new policies on how marijuana impaired drivers are investigated?

3) Will traffic accidents increase as a result of increased marijuana use in states where it is legal?