(WAND/NBC) - A new study suggests that some heavy marijuana uses are dangerous drivers regardless of whether they are under the influence.
Researchers reported the bad driving appears to be isolated to those who started using marijuana before the age of 15.
This report was shared in Drug and Alcohol Dependence Tuesday.
The theory is early marijuana use changes the brain and causes people to be more impulsive and more likely to make rash decisions.
The study tested participants in a driving simulator.
Researchers from McLean Hospital in Boston found sober cannabis users who started using the drug in their teens had more accidents, drove at higher speeds and ran more red lights compared to people who had never used marijuana.
"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.
In the experiment, researchers looked at 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.
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.
When the researchers took 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.
However, this data cannot show whether the early starters were impulsive to begin with or cannabis use in the teen years made them that way.
However, officials said there are studies that suggest that cannabis use while the brain is developing can lead to changes, including an increase in impulsive behavior.
The National Highway Traffic Safety Administration reports 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.