So how bad is smoking marijuana for you, anyway? Well, if you're a truck driver, it's very dangerous to your license and livelihood. 

In last November’s election, voters in Washington and Colorado made it legal to possess an ounce of pot and use it for recreation. And Massachusetts joined the 17 other states (and the District of Columbia) that allow medical marijuana.

But marijuana is illegal under federal law, and its use is expressly forbidden for transportation workers, including truck and bus drivers.

(And a number of states have zero-tolerance laws for driving with any evidence of pot in your blood.)

Toke up and Drive?

Shortly after we reported on Truckinginfo in late January that the DOT was warning truck drivers about not making the mistake of smoking marijuana just because their home state might allow it, we got a long email from a reader who took issue with the government's position on this issue. He included lots of examples of research. 

For instance, this lawyer's blog talked about research showing that those driving while high drove more slowly and were less likely to change lanes, he said. However, the same article noted that there's also evidence of "performance changes in braking latency," (which I understand to mean a slower reaction time when it's time to hit the brakes) and also some evidence of impairment of peripheral vision.

Plus, it says, Canadian researchers recently concluded that smoking pot within three hours of driving nearly doubles the risk of a vehicle collision.

My personal take? Even if driving under the influence of pot is less dangerous than driving while drunk, that still doesn't necessarily mean it's actually safe! 

In Your Blood

In addition, new research appears to show that you could be under the influence of marijuana for far longer than after drinking.

The research, published online last week in Clinical Chemistry, the journal of AACC, shows that cannabis (marijuana) can be detected in the blood of daily smokers for a month after last intake. The researchers hope the data could help promote "drugged driving" laws, saying marijuana is second only to alcohol for causing impaired driving and motor vehicle accidents.

In 2009, 12.8% of young adults reported driving under the influence of illicit drugs and in the 2007 National Roadside Survey, more drivers tested positive for drugs than for alcohol. These cannabis smokers had a 10-fold increase in car crash injury compared with infrequent or nonusers after adjustment for blood alcohol concentration, according to the paper.

In this paper, 30 male chronic daily cannabis smokers resided on a secure research unit for up to 33 days, with daily blood collection. Twenty-seven of 30 participants were THC-positive on admission. THC decreased gradually with only 1 of 11 participants negative at 26 days; two of the five remained THC-positive for 30 days.

These results demonstrate, for the first time, that cannabinoids can be detected in blood of chronic daily cannabis smokers during a month of sustained abstinence. 

However, there doesn't seem to be agreement as to whether THC in your blood actually means you're impaired. A 2007 study found that a THC concentration of 10 nanograms/mililiter — a level sufficient to prove DUI in several states — is not associated with elevated accident risk. 

Of course, this is all strictly academic, so to speak. No matter what various studies say, federal law says you can't smoke pot and drive a commercial vehicle.