I'd add here that statistics and studies have continued to show that being high generally only impairs driving while also drunk.
It has been shown that many drivers with marijuana in their systems have been involved in accidents, however these situations all generally share a common thread in that the drivers were also drunk on alcohol or using other substances.
Further, a new study confirms the implications of these statistics in showing that THC does NOT impair driving:
http://www.nhtsa.gov/staticfiles/nti/pdf/812117-Drug_and_Alcohol_Crash_Risk.pdf
I've always been disappointed with the narrative that legal marijuana will lead to more accidents, that we need some kind of highway breathalyzer for THC or any other such nonsense before legalization becomes acceptable, and now the governmenthas admitted as much.
We need to stop looking at marijuana and other drugs through the lens with which we perceive alcohol and tobacco.