I think most of us chronic's agree, if you smoke a lot and use daily, impairment is rarely a problem. That why the level in a persons system, imo is wrong. I'd probably blow the meter off the scale but I can still drive rings around most people. I'm a better driver than the average driver and weed simply doesn't change that for me.
Now to be honest, for me, other variable's could affect my driving. I've noticed as I've gotten older I'm not quite as good as I used to be. Other idiot morons makes me aggressive. My mood. Selfish drivers. Those are just a few things that impair my driving more than weed ever has.