No, it is not safe to drive shortly after consuming cannabis. When you can drive after consuming cannabis depends on several factors. But let’s set up the challenge:

Centers for Disease Prevention and Control (CDC): “Marijuana use is increasing and 13% of nighttime, weekend drivers have marijuana in their system. Marijuana users were about 25% more likely to be involved in a crash than drivers with no evidence of marijuana use, however other factors–such as age and gender–may account for the increased crash risk among marijuana users.”

Governors Highway Safety Association: “54 percent of fatally injured drivers that year were tested for drugs and alcohol. Of those who had drugs in their system, 38 percent tested positive for marijuana, 16 percent for opioids and 4 percent for both.”

National Highway Traffic Safety Administration: “12.6 percent of weekend nighttime drivers in 2013-2014 tested positive for tetrahydrocannabinol (THC).”

Many of the states which have legalized sale and use of cannabis products have not had the time to gather statistically significant data. We do have information on the following.

Alaska: “In 2017, 16% of high school students drove a vehicle when they had been using marijuana.” However, while Alaska reports data on alcohol-related accidents, injuries, and fatalities, it does not yet do the same for cannabis-related incidents.

California: The state has no collected enough data to alter its claim that legalized cannabis use has had no effect on traffic fatalities. But it has outlawed smoking and consuming cannabis products while driving.

Colorado: “There were 51 fatalities in 2016 that involved drivers with THC blood levels above the state’s legal limit. [and] An online survey in April [2018] found 69 percent of pot users said they had driven under the influence of marijuana and 2 percent said they drove high almost daily.”

Nevada: “Automobile crashes related to cannabis are up roughly six percent in Nevada since the state legalized the creational use of cannabis in 2017.”

Oregon: In 2016, a year after legalization, Oregon reported, “Between July 1 and Dec. 31 of last year, 50 drivers were accused of driving under the influence of marijuana, compared with 19 for the same time period the previous year…Another 93 drivers were accused of having marijuana along with other drugs in their systems at the time they were stopped, compared with 44 the previous year. Overall, the agency's data shows driving under the influence of any substance rose by 7 percent in 2015.” At the same time, OPB notes, “Back in 2004, when Oregon only had medical and black-market marijuana, there were 13 traffic fatalities involving the drug. In 2015, with recreational cannabis on store shelves across the state, 16 people died in marijuana-related traffic accidents.”

Washington: “A 2016 report by the AAA Foundation for Traffic Safety found that fatal crashes of drivers who recently used marijuana doubled after the state legalized it.”

So, what’s the problem?

As usual, we do not have the in-depth research needed to make definitive recommendations or warnings. However, the American Journal of Addiction (2019) has done much to identify the problem and how it interacts with the individual user’s variable factors:

The higher the THC concentration in the strain, the higher it will be in the blood quickly. That means increased impairment while driving. However, the more frequently users smoke or ingest that strain, the less impairment they will experience. Those using the same strain at the same dose infrequently will experience more impairment than the frequent user.

This level of tolerance may relate to the user’s physiological factors of height, weight, gender, and age. Or, it may be a biochemical adaptation or accommodation. In any case, “Maximal impairment is found 20 to 40 minutes after smoking, but the impairment has vanished 2.5 hours later, at least in those who smoke 18 mg THC or less (the dose often used experimentally to duplicate a single joint).”

The lack of universal testing standards and effective testing devices and protocol reduce the quality and efficacy of the data gathered by police. But some testing indicates smoking a third of a joint (6.25mg) will affect some automatic skills like tracking. That varies with driving experience and physiologic profiles.

It also suggests it takes higher doses to impair complex cognitive and behavioral responses. And, even where cannabis users try to drive more carefully, they may not respond well to unexpected circumstances like oncoming high beam glare, icy bridges, and reckless drives.

We all know marijuana smokers too often mix their cannabis intake with alcohol and/or other drugs, thus, increasing the level of impairment. The testing does indicate the alcohol will affect drinkers’ cognitive capabilities before it affects automatic skills while cannabis hits the automatic responses before it reaches cognitive impairment. Obviously, smoking or ingesting cannabis while consuming alcohol doubles the whammy.

Testing also differentiated affected skills under the influence:

Increased failure to maintain the driving lane,

Reduce their speedometer monitoring,

Increased time in making a passing decision,

Increased time to brake at stop light change,

Increased tendency to crash on the appearance of a sudden obstacle.

The 420 proof

Researchers at the University of British Columbia theorized the statistics on driving incidents and fatalities should increase if, indeed, cannabis impairs drivers. They accumulated and processed data from January 1992 to December 2016.

“The primary analysis compared the number of drivers involved in fatal traffic crashes between 4:20 pm and 11:59 pm on April 20 each year to the number in fatal traffic crashes during the same time intervals on control days 1 week earlier and 1 week later (i.e., April 13 and April 27).”

Adjusted for weekday, season, vehicle design, distance traveled, medical care, and more, they confirmed their hypothesis.

They identified 1.3 million drivers in 882,483 accidents with 978,328 fatalities. Of that total, some 1,368 drivers were involved in fatal crashes after 4:20 pm on April 20. Compared to the incidents on control days, it represents a significant increase in the relative risk of fatal crashes during the time studied. The number of drivers involved in such accidents before 4:20 pm differed little from the same hours on control dates or days before and after 4/20.

Further analysis showed a higher incidence among young drivers and drivers in New York, George, and Texas with only Minnesota showing a decrease.

Facts are: Driving impaired puts you and others at risk. Driving while impaired from cannabis and alcohol doubles your problems. Using high THC strains exacerbates the impairment. Frequent cannabis use of low and moderate THC strains can develop some tolerance in some users. And, young people lack the judgment skills to make the right choices in cannabis consumption, alcohol intake, and driving decisions. So, drink, toke, and drive responsibly!