1: No, you don't know what you are talking about. Again, THC stays in your system for weeks and even months after smoking weed. If you smoke a lot of weed, you will have a lot of THC in your system even when you are sober. If you usually never smoke weed but one day you get high, you will probably have less THC in your system (while you're high) than someone who usually smokes a lot of weed every day but doesn't smoke any one day (while he's sober). It's not like alcohol where they can just see how much is in your system and know if you are drunk or not.
2: Yes, smoking way too much weed will impair your driving. So will wearing pants that are way too tight, eating way too much pizza, drinking way too much coffee, or doing way too much of anything else. I don't think it's a big deal because most weed smokers don't usually hit that level. Just like most pizza eaters don't, most coffee drinkers don't, etc. The usual amount of weed smoked, even if someone smokes what is considered a lot, is not enough to make driving as dangerous as someone who is at the legal alcohol limit. And the studies I posted confirm this.