As marijuana use in the United States is on the rise, a growing number of drivers are getting behind the wheel high, while regulations and law enforcement agencies scramble to keep up with the danger.

Earlier in November, 19-year-old Luke Resecker was sentenced to 65 years in prison for a car crash that killed six people and paralyzed another; Resecker tested positive for THC (tetrahydrocannabinol, the substance responsible for the psychological effects of cannabis).

The case has shined a spotlight on the effects of marijuana use on driving, and the ways law enforcement are able to detect it. As more states move to legalize recreational use of marijuana, consumption has been on the rise – and so have serious car crashes with drivers under its influence, experts say.

Consuming cannabis can seriously impact a driver and cause a deadly crash, but unlike with alcohol, there still isn't a tried-and-true way to tell exactly how much will impair function.

"It's still a moving, developing story that has been going on for more than a decade, but there are still so many questions," said Dr. Guohua Li, an epidemiology professor at Columbia University who has studied the risk of cannabis use on car crashes. "The prevalence of cannabis among fatally injured drivers has increased so much in the past two decades."

What happens when you drive under the influence of weed

Consuming cannabis, whether by smoking it or ingesting it, can impact the part of the brain that controls your movements, balance, coordination, memory and judgment, according to the Centers for Disease Control and Prevention.

It can make your reaction time slower, impair your coordination and distort your perception, the CDC said.

The amount of time it takes to perceive a risk on the road and react to it by slowing down or stopping can increase while under the influence, Li said. Spatial perception is also a serious issue, he said; driving at night or under heightened weather conditions can be extra risky while using cannabis.

"Spatial perception is an essential function for driving safely, including how much distance you will keep from the other vehicles and what kinds of risk assessments you would need while driving," Li said.

Research shows a more than doubled risk of being involved in a fatal car crash while driving under the influence of cannabis compared to having none in your system, Li said.

While the effects of cannabis can clearly spell disaster on the road, the CDC notes that the research isn't yet clear on exactly how much is too much. The presence of cannabis is measured by detecting THC, but the CDC notes it's difficult to connect the presence of it to impairment for an individual person. A person's weight, body composition and tolerance will all impact it. But Li said there is emerging evidence in research that the higher the concentration of THC in your system while driving, the more likely you are to crash.

"It’s very individualized how much it will impact you. The effect of THC on cognitive functions and safety behaviors is much more unpredictable than alcohol," Li said. "That’s the challenging part of studying and regulating THC compared to alcohol."

Yet many Americans don't perceive driving after smoking weed to be an issue. In a survey of cannabis users by the AAA Foundation for Traffic Safety, 46.9% of respondents said they believed they drove the same as when they are unimpaired. Eighty-four percent of cannabis users smoke, eat, drink or vape marijuana within eight hours of getting behind the wheel, the survey found.

How do cops tell if you're high?

Driving while under the influence of alcohol or drugs is illegal in all U.S. states. Unlike with alcohol, for which almost every state says the driving threshold is 0.08 blood alcohol content, there is no universally recognized standard for the amount of THC in someone's system.

Some states, like Arizona, have a zero-tolerance law for the presence of any marijuana or its metabolites (the result of the body processing the drug). Colorado sets specific limits on how much THC in the body is considered impaired, and even advises drivers how long to wait after smoking or ingesting it before driving.

If a driver gets pulled over and is suspected of driving while under the influence of drugs, police officers will likely focus more on whether alcohol was consumed (and often, there may be a combined influence of alcohol and marijuana), Li said. Alcohol is easier to test for, and is still the most common culprit for DUIs.

They may conduct field sobriety testing (where a driver may perform exercises like walking in a straight line, balancing or reciting the alphabet backward) and use a breathalyzer. They might also collect a blood sample that can screen for a battery of drugs, including pot.

But it can be difficult to regulate and enforce limits on THC in the body for several reasons, Li said. Different amounts can affect people differently; for some people, cannabis is a depressant like alcohol, while for others, it's a hallucinogenic, he said.

There isn't widespread use of a portable device like a breathalyzer for weed, Li said. Over the last decade, there has been a "race" to develop a reliable, accurate and portable roadside detection device for THC, but even the molecule's large and heavy characteristics are hindering efforts, he said.

Plus, the presence of THC can stay detectable in the body for much, much longer than alcohol – up to weeks, even when the high feeling is long gone.

"If you have smoked a joint today, you may still test positive in urine or blood, two or three weeks," Li said, adding that an attorney could form a solid defense based on that fact.

Contributing: Greta Cross

This article originally appeared on USA TODAY: Here's what can happen if you drive under the influence of pot

Reporting by Jeanine Santucci, USA TODAY / USA TODAY

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