	Introduction

A growing number of Americans report that they use marijuana. In a survey, the majority of state highway safety offices rated drugged driving an issue at least as important as driving while impaired by alcohol. Studies of marijuana's impact on a driver's performance have thus far found that, while marijuana usage can measurably affect a driver's performance in a laboratory setting, that effect may not translate into an increased likelihood of the driver being involved in a motor vehicle crash in a real-world setting, where many other variables affect the risk of a crash. As of May 2019 33 states and the District of Columbia have enacted laws legalizing marijuana use under certain conditions, generally for medicinal purposes. These jurisdictions are home to one-quarter of the U.S. population. Several other states are considering legalizing recreational use of marijuana. Thus, the impact of a state's treatment of marijuana on the extent of marijuana usage is not clear. Other state laws, known as per se laws, provide that a driver is automatically guilty of driving while impaired if specified levels of a potentially impairing substance are found in his or her body (e.g., blood alcohol content (BAC) of .08% or higher, or, in some states, THC in the blood; see Table 1 ). Detecting Impairment

Currently, detecting marijuana impairment through a standardized test is more complicated than detecting alcohol impairment. Evaluating impairment due to alcohol is relatively straightforward. Alcohol is a central nervous system depressant, the effects of which have been extensively observed and studied for a century. It is a liquid that enters the bloodstream quickly and is metabolized (converted into other substances) by the body fairly quickly. However, studies indicate there is some degree of impairment at far lower levels of BAC. This is due, in part, to the requirements that must be met to use marijuana in studies due to its status as a controlled substance under federal law and many state laws. Another factor complicating studies of marijuana's effects on drivers is that the potency of THC in marijuana (i.e., the concentration of THC) can vary from one plant to another. The lack of correlation between both marijuana consumption and the level of THC in a person's system and THC levels and driver impairment reduces the usefulness of rule-of-thumb guides of impairment. Studies of Crash Risk Associated With Marijuana Usage

To date, results from studies that have examined the association between marijuana use and crash risk have been inconsistent. In reports examining many aspects of marijuana use and its effects, the National Academy of Sciences (NAS) in a 2017 report and the National Institutes of Health in a 2018 report reference various studies on the impact of marijuana consumption on driver performance to state that cannabis use prior to driving increases the risk of being involved in a motor vehicle accident. These include
continued research into whether a quantitative standard can be established that correlates the level of THC in a person's body and the level of impairment, and better data on the prevalence of marijuana use by drivers, especially among drivers involved in crashes and drivers arrested for impaired driving. Considering the length of time that marijuana is detectable in the body after usage, and the uncertainty about the impairing effect of marijuana on driving performance, Congress and other federal policymakers may elect to reexamine the rationale for testing all safety-sensitive transportation workers for marijuana usage.
A growing number of Americans report that they use marijuana. Most states now allow the use of marijuana for treatment of medical conditions. Ten states and the District of Columbia, representing a quarter of the U.S. population, have decriminalized the recreational use of marijuana, and other states are considering following suit.
As the opportunity for legal use of marijuana grows, there is concern about the impact of marijuana usage on highway safety. In a 2018 survey, the majority of state highway safety officers considered drugged driving an issue at least as important as driving while impaired by alcohol (which is associated with over 10,000 highway deaths each year). As of May 2019, 18 states have enacted laws declaring that a specified concentration of THC in a driver's body constitutes evidence of impairment and is inherently illegal (referred to as per se laws), similar to the .08% blood alcohol content (BAC) standard of alcohol impairment.
Advocates of loosening restrictions on marijuana often compare marijuana usage to drinking alcohol, which may contribute to some stakeholders viewing marijuana use and driving as similar to alcohol's impairment of driving. Research studies indicate that marijuana's effects on drivers' performance may vary from the effects of alcohol, in ways that challenge dealing with marijuana impairment and driving similarly to alcohol-impaired driving.
Alcohol is a nervous system depressant that is absorbed into the blood and metabolized by the body fairly quickly, such that there is little trace of alcohol after 24 hours. Its impairing effects have been extensively studied over many decades, and the association between levels of alcohol consumption and degrees of impairment is well-established. By contrast, marijuana is a nervous system stimulant. It contains over 500 chemical compounds, only one of which, tetrahydrocannabinol (THC), is significantly psychoactive. Its effects are felt quickly after smoking, but more slowly when consumed in other forms (e.g., in food). It is metabolized quickly, but the body can store THC in fat cells, so that traces of THC can be found up to several weeks after consumption. Its impairing effects have been the subject of limited study, due in part to its status as a controlled substance under federal law.
Although laboratory studies have shown that marijuana consumption can affect a person's response times and motor performance, studies of the impact of marijuana consumption on a driver's risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage. Levels of impairment that can be identified in laboratory settings may not have a significant impact in real world settings, where many variables affect the likelihood of a crash occurring. Research studies have been unable to consistently correlate levels of marijuana consumption, or THC in a person's body, and levels of impairment. Thus some researchers, and the National Highway Traffic Safety Administration, have observed that using a measure of THC as evidence of a driver's impairment is not supported by scientific evidence to date.
Congress, state legislatures, and other decisionmakers may address the topic of marijuana use and driver impairment through various policy options, including whether or not to support additional research on the impact of marijuana on driver performance and on measurement techniques for marijuana impairment, as well as training for law enforcement on identifying marijuana impairment. Other deliberations may address federal regulations on marijuana use and testing for transportation safety-sensitive employees.