(KRON) — Marijuana-impaired driving has been on the rise across the country. According to the National Highway Traffic Safety Administration, the percentage of weekend nighttime drivers who tested positive for marijuana went from 8.6% in 2007 to 12.6% in 2014.

However, the NHTSA study pointed out that “the presence of drugs does not necessarily imply impairment.”

Just how long does marijuana affect your motor skills or impair driving? Stanford University professor Dr. Keith Humphreys said the answer is a complicated one.

“The challenge you have is that marijuana doesn’t mean one thing anymore,” he told KRON4. “It used to be a typical joint was about a couple of hours, but I would never say that now.”

Humphreys explained how significantly the marijuana game has changed. Back in the 1970s for example, marijuana typically tested anywhere from 3-4% THC (the active ingredient of marijuana that has psychological effects). Now, many marijuana products test at a rate closer to 25%, he said.

Not only do marijuana products now contain higher concentrations of THC, there are so many different types of products that are absorbed in different ways. Humphreys said that products like shatter or dabs can have significantly stronger impacts than marijuana flower does.

How can you be sure that you’re safe to drive after you’ve smoked?

“The best advice is don’t drive and smoke pot. I know that may be unsatisfying, but you can’t say when is safe,” Humphreys said. Because so little research has been completed on the effects of the more potent marijuana products, there is no established guideline for how long marijuana impacts last.

“You can’t really say, if you got pulled over, when you would pass an impairment test,” Humphreys said. The most challenging part? Testing for marijuana impairment is also not as developed as testing for alcohol is.

After California legalized recreational use of cannabis in 2016, a new set of challenges arose when it came to marijuana use and getting behind the wheel. There is no hard and fast answer to how much marijuana is legal to have in your system.

“It’s lipophilic, so it stores in fat. Long after you’re high, if you’re a heavy smoker, you could test positive, because people metabolize marijuana at different rates,” Humphreys said. This means that heavy smokers who make an error behind the wheel could end up testing positive for marijuana, and earn a DUI, even if they hadn’t consumed marijuana that day.

However, if law enforcement has reason to believe you are driving under the influence of marijuana — if they can smell it in your car, for example — they may ask to you to complete a field sobriety test as well as a blood test. If you refuse, the Department of Motor Vehicles can suspend or revoke your driver’s license. Here in the state of California, a DUI, even for marijuana, can also cost over $10,000 in fees.

The DMV is very clear about marijuana use in vehicles. Not only is it illegal to driver under the influence of marijuana or other substances that can impact your ability to drive, it’s also illegal to consume marijuana products while riding as a passenger in a vehicle. If a driver chooses to carry cannabis in their vehicle, they are responsible to keep it in a sealed and unopened container and place it away from passengers.

Humphreys said one key thing to remember about marijuana use is that if you are impaired from marijuana, you may not even recognize it. Impaired people often cannot often tell that they are impaired, but it will show up in their driving.

KRON4 reached out to the Oakland Police Department for some clarity around DUI laws in California, and the office shared a University of California San Diego study on marijuana impaired driving. According to those results, most drivers experienced a noticeable impact on their driving until four and a half hours after consuming marijuana.