After reading the report http://kdvr.com/2012/05/16/your-questions-answered-driving-u... and seeing this video, I am a bit confused on how they did this experiment and doubt their results mean anything.
In this kind of experiment you need a control group. So either we need to know the results of these people driving on the simulator before smoking weed or there needs to be a group of people who do not get the treatment/marijuana intake. If they tested these people on their driving skills before the treatment, then we need to know whether they passed or not. If they did not pass when sober then obviously they will not likely pass if they are intoxicated. Therefore, without the control group results, saying that the treatment group did not pass this simulated test does not mean a whole lot, especially if the simulator had problems.
Anyway, I am surprised about how long that stuff stays in your system.