For a variety of reasons, medical marijuana leads to a 9 percent drop in traffic deaths in states where it's legal, according to a new study by economists at the University of Colorado and Montana State University. The study takes a look at statistics from the 16 states with MMJ and hints that traffic deaths decline because:

1. A lot of folks would rather smoke than drink, and marijuana use among adults (but not kids) increases with MMJ passage. This means there are more adults smoking and fewer drinking and driving.

2. Marijuana's effect of driving ability is not as catastrophic as alcohol's. For this, the authors compared driving course studies of both drugs. No contest. In fact, experienced pot smokers had significantly less functional impairment than noobs.

3. MMJ use usually happens in the home. People drive around a lot (drunk) to get alcohol.

The study warns that other things could contribute to the correlation between traffic death and MMJ, but it's clear that in many ways MMJ is good for Arizona. You can read the study here.