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Even when sober, some heavy marijuana users are dangerous drivers, a new study suggests.

The bad driving appears to be isolated to those who started using pot before age 16, researchers reported Tuesday in Drug and Alcohol Dependence. The theory is that early marijuana use changes the brain, leaving people more impulsive and more apt to make rash decisions.

In the new study, which tested participants in a driving simulator, researchers from McLean Hospital in Boston found that sober cannabis users who started using the drug in their teens had more accidents, drove at higher speeds and cruised through more red lights compared to people who had never used marijuana.

Marijuana is the most commonly used illicit drug in the United States, but there has been relatively little research on its health effects, particularly on teen brains.

"This research suggests that early exposure to cannabis may result in difficulties performing complex cognitive tasks," said co-author Staci Gruber, an associate professor of psychiatry and director of Marijuana Investigations for Neuroscientific Discovery at Harvard Medical School and McLean Hospital.

For the experiment, the researchers recruited 28 regular, heavy cannabis users (23 males and five females) and 17 non-users (six males and 10 females) whose driving abilities would be tested in a simulator. The participants' average age was 23.

Those in the cannabis group reported having used the drug at least five out of the previous seven days and at least 1,500 times during their lifetime. They were told to abstain for at least 12 hours before their study visit to ensure that they weren't high at the time of the test.

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Once they were at the lab, all of the study participants provided urine samples, which were tested for drugs of abuse, including cannabis. The participants also filled out a number of questionnaires, including a psychological assessment.

During the driving test, cannabis users were more likely than non-users to speed, hit a pedestrian, cross the center line, miss stop signs and cruise through red lights. The cannabis users were also more likely to score high in impulse behavior.

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When the researchers reanalyzed their data taking the age of first cannabis use into consideration, they found that the bad driving was almost exclusively limited to people who had started in their teens and that the young starters were also more impulsive.

One thing the data can't show is whether the early starters were impulsive to begin with or cannabis use in the teen years made them that way.

However, Gruber said, there are studies that suggest that cannabis use while the brain is developing can lead to a number of changes, including an increase in impulsive behavior.

Marijuana's role in crashes

The number of fatal vehicle crashes in which drivers tested positive for cannabis more than doubled from 2007 to 2016, rising from 8 percent to 18 percent, according to the National Highway Traffic Safety Administration.

As states legalize recreational use of marijuana, more research is needed on how consumption affects driving safety, said Ryan Vandrey, an associate professor in the behavioral pharmacology research unit at the Johns Hopkins School of Medicine.

"We don't fully understand the health impacts of heavy frequent cannabis use," Vandrey said.

The results of the new study wouldn't likely apply to people using medical marijuana, because they tend to gravitate to products that are low in THC, the psychoactive constituent in marijuana, Gruber said.

The results show educators and people involved in public policy how important it is to prevent early exposure to marijuana, Gruber said.

Researchers at the Insurance Institute for Highway Safety have noticed a trend of more accidents in states that have legalized marijuana.

"An analysis of insurance data in Colorado, Oregon, Washington and Nevada found a combined 6 percent increase in collision claims versus control states," said Joe Young, a spokesman for the institute.

A separate analysis of police-reported crashes just in Colorado, Washington and Oregon found a similar 5.2 percent increase, according to Young.

"Marijuana's role in crashes isn't as clear as the role that alcohol plays, but the early research suggests that states should proceed with caution when looking to legalize," he said.

It's legal for adults 21 and older to use marijuana for both medical and recreational purposes in 11 states, as well as the District of Columbia. Medical marijuana is legal in 21 states.