There is no scientific basis for tests to measure if a driver is under the influence of marijuana, a landmark study claims.

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.

reference: http://www.dailymail.co.uk/news/article-3582408/Scientific-basis-laws-marijuana-driving-questioned.html