Marijuana may lead to increased rates of traffic accidents, or maybe not - as researchers are currently divided over the subject.

Washington State, Montana, and Colorado have set an intoxication threshold of five parts per billion in driver's blood. A handful of other states have developed thresholds for marijuana intoxication, although most do not yet have a specific level.

Marijuana can slow decision making, according to studies, and can make multitasking more difficult. Peripheral vision can also be hampered by the use of cannabis. These skills are essential to safe driving, potentially making marijuana-impaired drivers a hazard on the road.

Alcohol frequently causes drivers to behave recklessly, driving faster than is safe, often passing other vehicles. Drivers under the influence of marijuana tend to be more cautious than sober operators. They often drive more slowly than drivers not under the influence of cannabis, leaving additional room between their own automobile and others. They also pass other cars less often than other, completely sober, operators.

Drivers who operate their vehicles under the influence of marijuana are generally aware of their impairment, while drinkers are not. After a few drinks, many users of alcohol believe they can operate their vehicles better than if they were not imbibing.

The use of marijuana along with alcohol can cause greater impairment to drivers than either intoxicant alone, according to some research.

Cannabis is now legal for recreational use by adults in two states, and approved for medical use in more than 20. Lawmakers, law enforcement and private groups are currently trying to decide how severely marijuana can affect driving skills.

"We don't know enough about the scope of marijuana-impaired driving to call it a big or small problem. But anytime a driver has their ability impaired, it is a problem," Jonathan Adkins, executive director of Governors Highway Safety Association, said.

Young males, including teenagers, are the most likely group to drive recklessly, and suffer accidents. They are also the most likely to smoke marijuana before getting behind the wheel of a car. Testing for cannabis cannot usually show if a driver is under the influence of the intoxicant. While the effects last just a couple hours, the substance can be detected for days or weeks after intake.

Efforts to legalize marijuana are underway in several other states, including Vermont, Alaska, New York, Oregon and Massachusetts.

Washington State showed an increase of 25 percent in the number of drivers testing positive for marijuana in 2013. However, there was no increase in the number of car accidents, fatal or otherwise.