Toll roads in Colorado aren't nearly as common as those in other states, but of the few there are, two are some of the fastest growing in the nation.

According to the International Bridge, Tunnel and Turnpike Association (IBBTA) Colorado's Northwest Parkway, Broomfield and E-470 Public Highway Authority, Aurora both showed double-digit traffic growth from 2014 to 2015.

The IBBTA report notes that while the northeastern part of the US has a long history of toll roads, it was the southern and western parts of the US that showed the largest growth.

Nationwide, toll road usage increased 7% in 2015. In Colorado, Northwest Parkway traffic increased 13% while E-470 saw a 12.4% increase.

The two US toll roads showing the most increase (25%) were the Tampa-Hillsborough Expressway in Florida and the North Carolina Department of Transportation.