Driving While Stoned

What should the legal limit be for marijuana? Jacob Sullum examines the state of affairs:

While a blood alcohol concentration of 0.08 percent or more is treated as equivalent to DUI, there is no such per se standard for marijuana. Previous efforts to set a limit at five nanograms of THC per milliliter of blood—the same as the standard established by Washington's marijuana legalization initiative, effective last Thursday—have failed due to objections from critics who said it would unfairly penalize regular consumers, especially people using marijuana for medical purposes, who might exceed the ceiling even when they are not impaired. Now the Post reports that supporters of a five-nanogram rule are offering a compromise that has won over some critics: Instead of being automatically guilty of DUI, drivers who test above the limit could present evidence that they were in fact OK to drive.