Traffic rolls along Interstate 25 in northern Colorado. 

(AP/David Zalubowski)

Colorado's traffic death numbers increased about 10 percent last year. That's got the state Department of Transportation worried -- it's the first spike after a significant downward trend. 

Many of the deaths in 2015 were related to behaviors that CDOT would like to change, said department spokesman Sam Cole. About half of fatalities involved people who weren't wearing their seat belts, even though approximately 15 percent of Coloradans don't wear their seatbelts, he said. And about one third of fatalities are related to drinking and driving.

Why there's been an upward trend in deaths:

"It's probably due to a number of reasons. We all know that gas prices are at historic lows and when you have gas prices that are low like that and you also have an economy that's humming along, more people are going to be driving. More people driving, more congestion -- certainly that has a lot to do with the fact that we're seeing traffic fatalities spiking.

"But there also may be some behavioral reasons -- are people on their phones more? Is there more distracted driving out there? Is there more drinking and driving out there? Are less people wearing seat belts? Those are all the issues that concern CDOT that we are working to rectify."

Other notable upward trends:

"Motorcycle deaths are up to an historic high. We saw 104 motorcycle fatalities in 2015. That's up by about 5 to 10 from last year. We have never seen motorcycle deaths that high. 

"As far as the reason, we don't know. Was it a long riding season? Were more people out riding their bikes? We know that you're much more likely to die if you're not wearing your helmet and Colorado does not have a helmet law."

What CDOT might do:

"Certainly, there's nothing that CDOT can do about increasing congestion due to a humming economy and low gas prices. But there's a lot that we can do when it comes to educating people about that they have that may be contributing to these fatalities. ... We're going to invest 3.5 million dollars in nonprofits and law enforcement across the state to try and deal with some of these issues."