Up until recently, the worst thing that may have happened to you while driving involved a speeding ticket. Then, after you had a couple of drinks with your friends or your family, you got behind the wheel. You hear sirens, see flashing lights and begin to wonder if your breath smells like alcohol.
For alcohol, the 0.08 standard is nationwide for ordinary drivers. If you have 0.08 blood alcohol or higher, there's pretty good scientific proof that you're under the influence. When it comes to marijuana's psychoactive component THC, however, it's not so clear. The majority of studies have indeed found that THC can impair your ability to drive, but there just isn't much evidence that the impairment is very substantial.