Drivers under the influence of marijuana are significantly less likely to be involved in a car crash compared with drink drivers, according to new studies.

The US's National Highway Traffic Safety Administration also revealed that after taking into consideration age, gender, race and alcohol use, drivers who are stoned are no more likely to crash than someone not under the influence of any substance.

The study finds that while testing the exact impact THC (tetrahydrocannabinol) - marijuana's primary psychoactive ingredient – has on drivers is difficult, research shows people are vastly more likely to crash under influence of alcohol.

According to the report, drivers with blood alcohol concentration measured between 0.08 and 0.12 were estimated to be five to 30 times more likely to crash than sober drivers.

Those who tested positive for THC were estimated to be at elevated risk (around one to three times that of sober drivers), similar to drivers with BAC levels between 0.01 to 0.05.

However, the study says it is difficult to measure how the presence of marijuana effects a driver's impairment because of how the body breaks it down compared with alcohol.

"At the current time, specific drug [marijuana] concentration levels cannot be reliably equated with a specific degree of driver impairment," the report said.

"Most psychoactive drugs are chemically complex molecules, whose absorption, action and elimination from the body are difficult to predict.

"Considerable differences exist between individuals with regard to the rates with which these processes occur. Alcohol, in comparison, is more predictable."

THC can be detected in heavy users for days or even weeks after use, long after after any psychoactive effects remain.

Young men in increased risk group

A second survey released in comparison, the largest of its kind ever conducted, assessed whether marijuana use by drivers is associated with greater risk of crashes.

It found that while marijuana users are more likely to be involved in accidents, this increased risk may be due in part because users are more likely to be in groups at higher risk of crashes – mainly young men.

"This study of crash risk found a statistically significant increase in unadjusted crash risk for drivers who tested positive for use of illegal drugs (1.21 times), and THC specifically (1.25 times)," the report adds.

"However, analyses incorporating adjustments for age, gender, ethnicity, and alcohol concentration level did not show a significant increase in levels of crash risk associated with the presence of drugs.

"This finding indicates that these other variables (age, gender ethnicity and alcohol use) were highly correlated with drug use and account for much of the increased risk associated with the use of illegal drugs and with THC."