Fatal car accidents involving drivers who had recently smoked marijuana doubled in Washington after the state legalized the drug, a study claims.

Cannabis was found in 17 per cent of drivers behind the wheel in a crash in the state in 2014, up from 8 per cent in 2013. Washington decriminalized recreational marijuana in December 2012.

The spike coincided with a 6 per cent rise in traffic fatalities in Washington, while national figures declined.

The data was released on Tuesday by AAA as the automobile group lobbies states to change the way they test drivers for THC, the substance that makes users high.

Cannabis was found in 17 per cent of drivers behind the wheel in a crash in the state in 2014, up from 8 per cent in 2013. Washington decriminalized recreational marijuana in December 2012

'The significant increase in fatal crashes involving marijuana is alarming,' said Peter Kissinger, CEO of the AAA's safety foundation.

'Washington serves as an eye-opening case study for what other states may experience with road safety after legalizing the drug.'

However, the group admitted in another study that cannabis is different to alcohol: a high level of THC could be nothing for some drivers, and significant for others.

The AAA stopped short of blaming Washington's crashes on the increase in cannabis-related cases.

However, AAA traffic safety director Jake Nelson noted that traffic fatalities in the state went up 6 per cent during that same period, while the fatalities nationally declined.

The findings were published as part of an ongoing debate surrounding how marijuana can be monitored and controlled as more and more states legalize medical and/or recreational use.

One of the key concerns for states is how to monitor its use on the road.

There is no scientific basis for tests to measure if a driver is under the influence of marijuana, the AAA concluded in its study.

Unlike the simple breathalyzer test for alcohol, time-consuming hospital blood tests are used as the gold standard in six states to test for marijuana.

But in fact the results of the complex, time-consuming, and expensive tests often mean nothing, according to a study released on Tuesday by the AAA's safety foundation.

Cannabis stays in the system for up to a month, meaning it is impossible to know if a driver smoked hours ago or weeks ago. And some users could be far more affected by a small amount than others.

Most of the time the officer has no choice but to let people continue on their way - possibly driving under the influence - instead of getting a warrant to take them to hospital.

It would be more safe, the AAA claims, to scrap those laws and let specially-trained police officers make their own judgement based on indicators such as pupil dilation, tongue color and behavior.

Drivers with a blood-alcohol level above 0.08 are deemed legally impaired.

In many states that have legalized cannabis drivers are deemed over the limit if they have 5 nanograms of THC per milliliter of blood.

But marijuana and alcohol are metabolized different, the AAA warns.

It means a driver with low blood-levels of THC - the chemical that makes users high - could still be a serious risk behind the wheel if they aren't a regular user.

Meanwhile a regular could be fine with copious levels of THC in their system.

Yet the laws in five of the six states automatically presume a driver guilty if that person tests higher than the limit, and not guilty if it's lower.

As a result, drivers who are unsafe may be going free while others may be wrongly convicted, the foundation said.

Releasing the study on Tuesday, the AAA, America's largest automobile club, called on the six state to scrap their laws to avoid inaccurate verdicts.

The foundation's recommendation to scrap the laws in Colorado, Montana, Nevada, Ohio, Pennsylvania and Washington comes as legislatures in several more states consider adopting similar laws.

The study also slams the tests as inefficient.

Testing for THC in a user's blood requires getting a warrant to take the driver to a hospital to draw blood, which can take around two hours.

The foundation recommends replacing the laws with ones that rely on specially trained police officers to determine if a driver is impaired, backed up by a test for the presence of THC rather than a specific threshold.

The officers are supposed to screen for dozens of indicators of drug use, from pupil dilation and tongue color to behavior.