Now that marijuana is basically legal in CO and WA, there are more "public" and "objective" evaluations of the impact of marijuana on public safety. This video shows the impact of smoking on driving (from my experience, 1 gram is enough to put me out). Nobody wants accidents caused by marijuana, but at least we know that marijuana is not as dangerous as alcohol.*
Bottom Line: Moderation in all things.
* Over 10,000 people were killed in alcohol-related crashes in 2010 in the US. Russians have a big problem with drink, and the Aussies are down to only 300 dead.