I believe there was a case in Colorado where a medical Marijuana user had enough THC in their blood to be considered impaired but they were able to pass a field

I believe there was a case in Colorado where a medical Marijuana user had enough THC in their blood to be considered impaired but they were able to pass a field sobriety test so they weren't convicted of a DUI. The problem is that the amount of THC you need in your blood to impair you is different for every person so it's hard to develop a test for it. A driving simulator makes sense to me, though once you have a bunch of adrenaline kicking in and are totally focused on not going to jail, you may perform better than when you are relaxed in the comfort of your own car.