Marijuana and Driving Impairment

When most people think about driving under the influence, alcohol plays the villain. But that’s only part of the story.

Increasingly, officials and drug experts are focusing on the risks of marijuana and driving. Evidence suggests that the active ingredient in marijuana — tetrahydrocannabinol or “THC” — can seriously degrade driving performance for a period of several hours.

Marijuana and THC Basics

THC and several other chemicals found in marijuana belong to a class of substances known as cannabinoids.

Inside the brain, these chemicals attach themselves to certain areas, called receptors, located on the surfaces of individual nerve cells (neurons). Receptors come in a variety of types, and each type only responds to the presence of certain chemicals; the receptors that respond to the presence of THC and the other cannabinoids are called cannabinoid receptors.

When activated, the cannabinoid receptors promote changes in the brain’s neurons that produce the classic effects of marijuana intoxication, including relaxation, euphoria, distorted input from one or more of the five senses, and reductions in the normal ability to recall memories, solve complex problems, or think coherently.

Depending on individual circumstances and preferences, marijuana can be smoked or eaten. Typically, smoking produces relatively rapid THC-related effects, and most people experience a maximum buildup of the chemical in their bloodstreams before they end a smoking session. Conversely, ingestion produces a relatively slow buildup of THC-related effects; compared to smoking, marijuana ingestion also typically produces lower levels of THC in the bloodstream at any given time.

However, it’s impossible to discuss these factors without speaking in generalities. The actual effects of marijuana in any given person depend upon things such as the amount of the drug smoked or eaten, the potency of the specific batch of the drug in use, the level of tolerance that the individual has for the effects of THC, and the setting in which marijuana use occurs.

Effects on Task Performance

For a minimum of one to two hours after smoking or eating enough marijuana to get “high,” all marijuana users experience significant decreases in their task performance skills. In some cases, residual task-related impairments remain in effect for as long as 24 additional hours.

In addition to the previously mentioned sensory distortions and declines in memory, thought processing and problem solving, specific forms of impairment associated with marijuana intoxication include decreased hand/eye coordination, reduced ability to rapidly shift attention from one object or subject to another, reduced ability to track the passage of time or accurately compute distance, and increased sleepiness.

Contributions to Driving Impairment

According to the National Highway Traffic Safety Administration, forms of driving impairment attributable to the use of marijuana include poor reaction time, poor ability to judge distance or the time needed to travel a given distance, poor ability to maintain proper distance from other vehicles, a tendency to move laterally while driving (i.e., swerve), reduced overall body coordination, an increased likelihood of falling asleep, and inability to maintain the ongoing vigilance generally associated with safe driving.

On average, these driving impairments remain in effect for roughly three hours following marijuana use. Critically, they affect the abilities of experienced marijuana users equally as much as the abilities of users with little or no experience coping with THC’s effects on task performance. Once intoxicated, all marijuana users make dangerously poor drivers.

Statistics and Considerations

Out of all illegal drugs in the US, marijuana is the substance most frequently present in people who get stopped for impaired driving, get involved in vehicular accidents or crashes, or die from involvement in a vehicular crash or accident. Depending on the specific city or region under consideration, rates for marijuana-involved accidents that produce injuries or death range from approximately 4-14% of all vehicle-related injuries and fatalities.

Significant numbers of teenagers drive while under the influence of marijuana, the National Institute on Drug Abuse reports; however, generally speaking, higher rates for marijuana-influenced driving occur among older individuals. Still, because of their relative driving inexperience, teenage marijuana users quite possibly have higher overall risks for harmful outcomes of substance-influenced driving than adults.

As with all other forms of substance-influenced driving, men who drive under the influence of marijuana significantly outnumber women who drive in the same condition.

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