The Effects of Cannabis on Driving

While many of us understand the consequences of driving while intoxicated, many drivers remain unconvinced about the impact that marijuana has on the brain, body, and reflexes of those behind the wheel. Today, we are taking a closer look at the effects that cannabis, (aka marijuana,) can have on a driver and how that can negatively impact the safety of our roads. 


The National Conference of State Legislatures reports that “Driving under the influence of drugs (DUID) appears to be a growing factor in impaired-driving crashes.” They explore the growing trend showing that the most used drug by drivers after alcohol is cannabis or marijuana. 


Fast Facts About Marijuana and Driving

The Centers for Disease Control and Prevention (CDC) reports that studies have shown the number of self-reported marijuana users is increasing. The percentage of weekend nighttime drivers who tested positive for the presence of marijuana rose from 8.6% in 2007 to 12.6% in 2014. That number has only increased in recent years, due in part to the legalization of marijuana for recreational use in many states.


Despite the AAA’s 2019 Traffic Safety Culture Index that reveals most respondents consider driving shortly after using marijuana (70%) and driving after using potentially impairing prescription drugs (88.3%) to be very or extremely dangerous, more and more Americans seem to be taking part in that activity.

How Marijuana Can Impact Driving 

While driving impared under any circumstance, including alcohol, sleep deprivation, and marijuana, there are some specific negative impacts related to cannabis. These include: 


  • Slowed reaction times and decision making 
  • Impairment of coordination 
  • Distorted perceptions 
  • Memory Loss 
  • Distortions increase when combined use of both alcohol and marijuana


Marijuana can remain in a user’s system for days or weeks after last use (depending on how much a person uses and how often they use marijuana). Therefore, if CDL drivers are tested it could produce a positive test even 30 days after use. 

Roadside Testing 

While there is not currently an accurate roadside test for marijuana, there are bodily fluid tests that can be completed at a hospital after an accident or an incident on the road. These blood and fluid tests can determine that there is indeed THC in the system but not the impairment level. 

According to the National Institute of Drug Abuse, (a part of the National Institute of Health), two large European studies found that drivers with THC in their blood were roughly twice as likely to be culpable for a fatal crash than drivers who had not used drugs or alcohol. CDL drivers should be acutely aware of the drug and alcohol testing that must be completed regularly and negative impacts of marijuana use on driving and employment eligibility.