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Colorado Official Clears The Air On Cannabis Consumption And Driving Ahead Of 4/20 Festivities (Op-Ed)



“While cannabis has arguably gained mainstream acceptance, driving high has not, thankfully.”

By Glenn Davis, Colorado Department of Transportation Highway Safety Office

As the wave of legalization continues to roll across the U.S., more people are embracing cannabis. And while cannabis has arguably gained mainstream acceptance, driving high has not, thankfully. In a 2022 Colorado survey, 85 percent of cannabis consumers said they do not drive after partaking. However, the state did see 101 fatalities in 2022 involving a vehicle operator with Delta-9 THC above the legal limit at the time of the crash, which illustrates the seriousness of the issue.

The Colorado Department of Transportation has been at the forefront of this conversation since Colorado became the first state in the U.S. to legalize recreational cannabis a decade ago. So, as you prepare to partake in 4/20 celebrations, let’s clear the air on a few common questions and misconceptions surrounding driving and cannabis use to keep everyone safe and DUI-free.

How long should you wait to drive after getting high?

Everyone is different, but here are some general guidelines to help you decide:

  • Wait at least six hours after smoking cannabis containing less than 35 milligrams of delta-9 tetrahydrocannabinol (THC) before driving or performing other safety-sensitive activities. If you’ve smoked more than 35 mg, you need to wait longer.
    • In Colorado, the typical cannabis cigarette product or joint contains approximately 0.5 grams of cannabis, which means consuming about 60 mg to 115 mg of THC. The standard serving size for a cannabis edible is 5 mg to 10 mg. However, consuming similar amounts of THC in edibles will typically have stronger effects than if smoked.
  • Wait at least eight hours after eating or drinking cannabis products containing less than 18 mg of THC before driving. If you’ve eaten more than 18 mg, wait longer.
  • If you’re mixing cannabis with alcohol, you need to wait even longer than that.

Mixing cannabis and alcohol

Speaking of mixing cannabis and alcohol, combining them enhances the impairing effects of both. It’s a combination that has created some serious problems on Colorado roadways. Why? Research shows that users are more likely to drive or take other risks after using both substances than after consuming cannabis alone. Considering impairment-related fatalities are on the rise nationwide, this is a huge area of focus. Nationwide, over 30 percent of traffic deaths involve an impaired driver (NHTSA). And these deaths are all preventable.

“Our advice? Just stay off the road if you’ve had any amount of THC or other impairing cannabinoids.”

Consider this: The Colorado Division of Criminal Justice (DCJ) published a report in July 2023 that analyzed 2020 data from more than 21,000 impaired driving cases filed in Colorado. Researchers followed the cases from arrest to final court outcome. The report found that 75 percent of people with detected delta-9 THC in their system also had some other impairing substance present. Alcohol was the most common one. You can’t deny that’s pretty alarming.

What’s the legal driving limit for THC?

Technically, the limit in Colorado is 5 nanograms—a nanogram is one billionth of a gram—of delta-9 THC per millimeter of blood, but it varies by state. However, there’s a lack of clarity surrounding cannabis intoxication and THC in the blood. It’s something transportation and law enforcement leaders are actively working to address.

With that said, Colorado law empowers law enforcement officers to assess impairment through roadside evaluations. That means if you demonstrate signs of impairment—no matter what BAC or THC blood tests reveal—you can be charged with a DUI.

Unlike alcohol, cannabis isn’t metabolized in a uniform way or on a predictable timeline—whether you’re smoking or consuming edibles, your tolerance and other factors all play a role. The subjective experience of cannabis impairment also varies wildly. This makes it tricky for consumers to assess if they’re too impaired to drive. Our advice? Just stay off the road if you’ve had any amount of THC or other impairing cannabinoids.

What if I get pulled over and have THC in my system because I consume cannabis frequently, but I’m not driving impaired?

A common misconception we’ve heard is the belief that having lingering THC in your system from regular cannabis use can result in a DUI, the way an employer drug test would show cannabis in a person’s system. However, drug tests commonly administered by employers detect inactive THC, while impaired driving investigations focus on active THC in the bloodstream. So, if someone drives sober, even with residual, metabolized THC in their system, they won’t test positive for active THC in tests conducted by law enforcement. Plus, blood tests in Colorado aren’t even conducted until after a DUI arrest (based on visible signs of impairment).

Can I drive if I’m microdosing cannabis?

It can be hard to tell how much you can “safely” consume or if you’re too impaired to drive, especially given the nuances of measuring impairment. So, your best bet? Don’t risk it. If you feel different, you drive different.

Numerous studies prove that THC slows reaction time, reduces focus, affects a person’s perception of time and distance and temporarily weakens problem-solving skills. And from our experience, those aren’t things you want working against you when you’re performing an activity that, if it goes wrong, can be deadly to you, other drivers, pedestrians or others who share the roadway.

Call a rideshare. Plan a ride ahead of time. Walk home if you can or find a safe place to stay until you’re sober. There are so many resources these days, driving impaired really shouldn’t be on the table.

For information and resources on cannabis-impaired driving, visit CDOT’s website. Visit CDOT’s YouTube channel to learn about the common myths about driving high. Additionally, CDOT has partnered with Learn Brands, an online budtender education platform, to provide a free interactive cannabis-impaired driving safety course. View and take CDOT’s Budtender Training Course.

Glenn Davis is the highway safety manager in the Colorado Department of Transportation’s (CDOT) Highway Safety Office. His areas of focus include impaired driving, police traffic services, motorcycle safety, young driver safety, legislative analysis and speed enforcement programs. Glenn has worked closely with members of Colorado’s cannabis business and advocacy communities since cannabis was legalized in Colorado in 2012. Glenn also represented CDOT on Colorado Gov. Jared Polis’s Marijuana Working Group and is CDOT’s governor appointee to the Marijuana Education Oversight Committee and POST Marijuana Working Group.

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