With marijuana legal in Colorado, you may think that car accidents would be more common. That doesn't seem to be the case, but it doesn't mean they don't happen. Marijuana still has the ability to alter a person's thinking and to slow reaction times, putting other drivers, like yourself, at risk. If you're struck, you may still be able to file a lawsuit for the driver being impaired while behind the wheel.
A legalized medical marijuana was approved in Colorado in 2001. It's true that there was a surge in the number of drivers who were found to have smoked marijuana, and there were other studies that showed that the number of drivers testing positive for marijuana in fatal car accidents had also increased in other states. The anti-marijuana group, SAM, claims that the drug makes drivers more dangerous.
What's actually interesting about the science of drug use and how it shows up in tests is that metabolites for marijuana can stay in the body for weeks. A positive test only proves that the driver smoked marijuana at some point in the past few weeks. So in that regard, it does make sense that more drivers are testing positive for the drug.
Looking at the roads since the legalization of marijuana took place in Colorado, it's shown that fatalities in 2014 were down from the number of fatalities in 2013. By adding up the average number of miles Americans drive each year, which includes the years in the mid-2000s, you can see that the number of fatalities has dropped. Interestingly, the number of miles driven in Colorado has risen, making the drop in fatalities even higher.
Source: The Washington Post, "Since marijuana legalization, highway fatalities in Colorado are at near-historic lows," Radley Balko, accessed Aug. 11, 2015