Marijuana, alcohol, and other substances that impair your ability to drive are illegal to use whike driving because they impair your ability to drive.

Even if MJ was legalized, using it while driving won't be permitted due to its effect on the body.

Coffee doesn't impair your ability to drive.

But the bigger issue with marijuana is this: since legalization in Colorado and elsewhere, more children have been hospitalized after MJ use. More students dropped out of high school. In addition, we are slowly learning that the super MJ that is currently bring produced does have adverse health effects. Contrary to popular belief, it appears that the stronger strains can be addictive as well.