 Approaching the city of Glenwood Springs , the highway enters Glenwood Canyon . Both the federal and state departments of transportation have praised the engineering achievement required to build the freeway through the narrow gorge while preserving the natural beauty of the canyon . A 12 @-@ mile ( 19 km ) section of roadway features the No Name Tunnel , Hanging Lake Tunnel , Reverse Curve Tunnel , 40 bridges and viaducts , and miles of retaining walls . Through a significant portion of the canyon , the eastbound lanes extend cantilevered over the Colorado River and the westbound lanes are suspended on a viaduct several feet above the canyon floor . Along this run , the freeway hugs the north bank of the Colorado River , while the main line of the former Denver and Rio Grande Western Railroad ( now part of Union Pacific ) occupies the south bank . 
