Today I Learned

[lumberjim]

and pot can remain in your blood stream for 30 days or more. I call bullshit on that twice as likely statistic.

Still, it is clear that marijuana use causes deficits that affect driving ability, Dr. Huestis said. She noted that several researchers, working independently of one another, have come up with the same estimate: a twofold increase in the risk of an accident if there is any measurable amount of THC in the bloodstream.
The estimate is based on review papers that considered the results of many individual studies. The results were often contradictory — some of the papers showed no increase in risk, or even a decrease — but the twofold estimate is widely accepted

apparently, you just need to practice:

seems to depend heavily on whether the driver is accustomed to being stoned.