It is difficult to pin an increase in traffic accidents or traffic fatalities on marijuana legalization. Traffic deaths nationwide increased by 14% from 2014 to 2016, with 2016 being the deadliest year since 2007. So Colorado is far from the only state to see an uptick in traffic incidents.
Colorado's traffic fatality rate per 100,000 citizens is 11.0, which is actually below the national average of 11.6. The highest fatality rates are in Mississippi (23.1) and Alabama (21.3), states that most certainly are not known for lax marijuana laws.
If anything, I'd surmise that the increase in traffic deaths nationwide has more to do with a strengthening economy plus relatively low gas prices. These two factors helped make 2016 the year with the highest number of miles driven in the history of the United States. More cars on the road and more miles driven would logically lead to an increase in traffic accidents.
This is the part where I point out that the bike/walk/mass transit-friendly streets of Amsterdam, where car traffic is limited and calmed by engineering, law, and culture, are far safer, crash-wise, than those of basically every other large city. (Their national average is around 3.4 deaths per 100k—about a third of ours.)
This despite having pot and hashish sold on every other corner, drunk tourists roaming around every night, and a self-reported 2/3 of young people regularly biking while drunk.
The most effective remedy for the dangers of intoxicated driving, it turns out, is not reducing intoxication, it's reducing ... well, you know.