That might depend on how much and how frequently you smoke though. That is, someone who has never smoked probably won't need a very high does of THC to be "high" and/or impaired. I may have more THC in my blood at any given time than that particular person.
I honestly don't know very much about this particular question, but I am acutely aware of when I am "high". That awareness is a lot different, in my experience, than people's awareness of when they're drunk. I still think roadside sobriety tests- which may have to be tweaked for cannabis- should be a part of any legalized regime. Make sure the tests are caught on video AND, if it's deemed necessary, run some sort of blood/plasma test (if it's legally/economically viable).