Since Washington State legalized recreational marijuana in 2012, road safety does not seem to have been affected by the change in drug laws. The state's own data shows that traffic fatalities have not increased since marijuana legalization too effect, (you can read an analysis of that here).
One source of confusion on the issue of road safety and marijuana use is that fact that marijuana remains in the body for much longer than the intoxication lasts. In other words, while alcohol impairment can be effectively measured by the driver's blood alcohol content - traces of marijuana will remain in the blood for days or weeks after use. Someone who uses marijuana at home on Friday night will likely test positive when they drive to work on Monday even though the psychoactive effects have long worn off. For a more complete analysis of the relationship between marijuana and driving safety, click here.