Conservatives have used scare stories of high drivers as reasons to oppose both legalization of marijuana and even use of medical marijuana. As with a recent study on prostitution reducing the risk of rape and sexually transmitted diseases, eliminating laws against victimless crimes may be beneficial, and at least typically does not do the harm claimed by conservatives. Since marijuana was legalized in Colorado, highway fatalities are at near-historic lows.
It might be the case that other factors are reducing the fatalities, but at very least it does not appear that legalization of marijuana increases the risk. It is even possible that the legalization of marijuana might directly be related to the reduction in highway fatalities:
…some researchers have gone so far as to suggest that better access to pot is making the roads safer, at least marginally. The theory is that people are substituting pot for alcohol, and pot causes less driver impairment than booze. I’d need to see more studies before I’d be ready to endorse that theory. For example, there’s also some research contradicting the theory that drinkers are ready to substitute pot for alcohol.
Regardless of whether the reduction in fatalities is a result of marijuana legalization, the data in Colorado does further undermine the conservative argument for the continuation of marijuana prohibition.