Over the past decade, the tide on marijuana seemed to turn. Over half of U.S. states, including Arizona, have legalized the drug for at least medical purposes. Eight states and the District of Columbia have decriminalized it for recreational use.
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.