A 5 ng legal limit is a license to drive stoned.
The majority of marijuana-impaired drivers test below 5 ng/ml of THC. A ten-year study in Norway6 showed that over 90% of drivers arrested for driving under the influence of marijuana were below 5 ng/ml when tested. According to Fiona Couper, Washington’s State Toxicologist, 55% of drugged driving suspects in Washington tested below 5 ng in 2011.
Colorado’s data are between these two studies, with 72% of drugged driving suspects tested below 5 ng/ml THC (2012 data from CDPHE):