What I haven't heard in Colorado's discussion of setting legal limits for "driving while stoned".

You may be aware that the Colorado Senate is once again trying to impose some kind of limit on "driving while stoned" but continues to struggle with the problem of measuring impairment.

from Westword:

A pair of studies suggest that marijuana users are more responsible drivers: German academics (again with those Germans) stated last year traffic fatalities in medical marijuana states like California have gone down as much as 9 percent. Why? They argued that pot users get stoned-in at home and don't go out drinking and driving as much. Really.

Likewise, researchers at Hartford Hospital in Connecticut and the University of Iowa Carver College of Medicine found that stoned drivers did just about as well as sober ones. (High five). Researchers there noted that high drivers slowed down and were more deliberate. (emphasis mine)

Additionally, no one seems to be talking about the cost of determining how many nanograms of THC you have in your blood. I've seen DUI arrests thrown out because the authorities could not prove that the alcohol measuring device was calibrated properly. Nanograms are really, really tiny - 1 billionth of a gram. Imagine the cost of maintaining and calibrating these machines. It makes my head hurt.