There is limited research on marijuana impairment related to driving, but it differs significantly than the impairments caused by alcohol. The uncertainty surrounding this issue has resulted in laws throughout the United States that vary in determining what constitutes driving under the influence of drugs (DUID) and what the penalty is for doing so.

Studies have shown that even small amounts of cannabis detected in the blood have been shown to impair the cognitive perceptual functions necessary for safe driving. These findings are fueling arguments for zero-tolerance policies in which any trace of marijuana in the blood or urine results in an arrest.

The National Institute on Drug Abuse prepared a white paper on drugged-driving research in 2011 that identified DUID as "a serious public health and safety problem in the U.S." However, the paper advocated examining the effectiveness of zero-tolerance per se laws, which are more in line with current drunk-driving laws.

"For illegal drugs, zero-tolerance per se laws are those which set that limit at the drug detection cut-off level," the authors wrote. "In concept, it is not necessary to prove driver impairment to convict an offender under a per se law."

They further discussed per se drugged driving laws: "A per se drugged driving law is one in which a specified level of a drug in the body of a driver is defined as an offense. This may be a level at which there is evidence that the drug has been shown to affect driver performance such as the 0.08g/mL limit for alcohol."

When it comes to drugged driving, 15 states have a form of zero-tolerance regulations. While some exclude medical marijuana users, nine have more absolute laws with zero tolerance applied to all cannabis users.

Originally, Arizona had a zero-tolerance stance that applied to all users of marijuana, but a court challenge resulted in changes excluding medical cannabis users. A state Supreme Court decision clarified that the presence of nonpsychoactive cannabis compound did not constitute an impairment under the law.

Research is needed to address the impact of marijuana on the basic cognitive functions required for safe driving. As complex as the marijuana-induced cognitive dysfunctions are to interpret, many of the comorbid cognitive deficits as a result of the diseases for which cannabis is approved to treat can also seriously impact cognitive function as it relates to driving.

Even minimal detected levels of marijuana in those already diagnosed with Alzheimer's disease, glaucoma, severe MS, severe Parkinson's disease and severe HIV can create enough additional cognitive dysfunction for drivers to be seriously impaired for safe driving.

This was enough of concern to legislators in Tennessee that they proposed medical marijuana legislation with language that required those awarded a medical marijuana certification to surrender their driving licenses. This legislation was defeated in the spring of 2015, and there still is no medical marijuana allowed in Tennessee.

Zero tolerance for marijuana while driving should certainly not be the final answer. However, at this point it seems to be the only answer to assure many members of the public that the citizens of their states will have greater public safety.

It's clear that more research must be done in this area.