Jacob Sullum flags a study that suggests so. One of the reasons:
Alcohol is often consumed in restaurants and bars, while many states prohibit the use of medical marijuana in public. Even where it is not explicitly prohibited, anecdotal evidence suggests that the public use of medical marijuana can be controversial. If marijuana consumption typically takes place at home, then designating a driver for the trip back from a restaurant or bar becomes unnecessary, and legalization could reduce traffic fatalities even if driving under the influence of marijuana is every bit as dangerous as driving under the influence of alcohol.
Pete Guitner's two cents:
Now keep in mind that this study shows correlation, not causation, but it appears to be a very powerful correlation, in that it completely undermines the prohibitionist argument that legalization is certain to result in an Armageddon of fiery crashes on the highways.
The Dish 