I lean towards marijuana being a possible cause of the increase in bad driving but I can also see the huge influx of outsiders being another potential cause. People new to an area tend it make a lot more mistakes on the road.

This is the kind of poor correlative thinking that created the spurious war on drugs. Morally, you have to draw the line at what is "dangerous", when the danger emerges from a complex behavioral scenario.

Some drug is vilified as a lampshade, because a physical substance is considered easier to control than irresponsible behavior. There is a diminishing return for prohibition that is hard to cost-benefit justify at any scale. e.g. the substance can create revenue outside of prohibition. This is the core error in the prescriptive "war on drugs" and decades later it's still confounding audiences as to why it's an incorrect conclusion.

The punishment severity of drug offenses is something to look at (the inverse relationship between drug offenses recorded and severity of punishment). This has nothing to do with the danger of the drug itself, but does address the behavior more directly. "Treatment" for addiction is still in the dark ages as many debunkers study this topic routinely. If you chase that topic, you might see how consequence is the stick, not blaming the inanimate substance.

Just like alcohol is dangerous.

Precautions must be made, I agree with you. But this looks like all drugs must be included in the next studies, and articles.

To the best of my knowledge, there is no reliable way to test whether a driver is actively intoxicated from pot. Existing blood tests will tell you if a person has used it in the past week or two, but not whether they've used it in the past hour.

So average measured blood-levels could well be up because more Coloradans are smoking, but that's no proof at all that more Coloradans are driving under the influence.

I wouldn't necessarily say that weed is harmless or safer than alcohol, but articles like the Denver Post one are written with an unfair bias.