It's a dangerous trend and it's becoming increasingly popular. A growing percentage of teens is smoking marijuana and many of them do not believe it is a distraction while driving.

A study commissioned by Liberty Mutual Insurance and Students Against Destructive Decisions finds that one in five teens, 19 percent, say they have gotten behind the wheel after smoking pot.

A different study conducted in December found that marijuana use among high school-aged children rose in 2011 for the fourth straight year. In fact, daily use was at a 30-year peak level among high school seniors.

"The news is very disturbing," said Gary Poedubicky, Acting Director of the New Jersey Division of Highway Traffic Safety. "The fact is marijuana does harm a driver's ability to drive safely and teen drivers already have the highest crash risk of any age group. Marijuana effects a person's power to focus, it effects coordination and their ability to react. The drug can make it harder for a driver to judge distances and react to signals and other sounds."

Among teens who have driven after using marijuana, 36 percent say it presents no distraction when operating a vehicle. Nearly one in five say alcohol is no distraction.

"The fact that more kids are smoking marijuana and fewer are concerned about the effect it has on their driving means that parents are going to have to step in even more," said Poedubicky. "They need to talk to their teens about the risk and responsibilities of having a license and also include in that discussion, the risk of substance abuse."