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The Crown Jewel of America’s Interstate Highway System: Colorado’s I-70 Mountain Corridor

For many years, the I-70 Mountain Corridor, which runs from Denver, Colorado to Glenwood Springs, experienced severe congestion – particularly on weekends. In the winter, I-70 provides access to of the country’s premier ski resorts including Vail, Aspen, Winter Park, Keystone and Breckenridge. In the summer, I-70 also serves as a gateway into the Rocky Mountains for campers, bikers, hikers, climbers and kayakers alike.

When the Colorado Department of Transportation (CDOT) began exploring expanding the I-70 Mountain Corridor in the 1970s, however, they found the project presented unique challenges. Much of the interstate cuts through narrow valleys where there is little room to add additional lanes. Where it is feasible to add lanes, cost are high and there the risk of rockslides remains ever present.

In order to improve the corridor’s capacity and mobility, CDOT’s original proposal included blasting through cliff, building unattractive retaining walls, and channeling the Colorado River.

When CDOT began work on an Environmental Impact Statement (EIS) shortly after, a process that seeks to encourage public input in government decision-making with mandated comment periods, they discovered that a majority of stakeholders (including local residents) were firmly opposed to the plan. 

In response, used the National Environmental Policy Act’s (NEPA) review process to initiate a collaborative decision-making process to identify a new reconstruction plan. The Colorado Highway Commission’s lone environmental member helped to form a Citizens Advisory Committee (CAC) of design and ecological professionals, with members from the Colorado Open Space Coalition and western Colorado interests.

In 1978, after two years of design review, a new proposal was brought before the public that incorporated local input including the suggestion to place a section of the highway in tunnels to protect the scenic Hanging Lake area from noise and visual impacts.

The final design preserved the natural topography and maintains the integrity of the Colorado River and side rivers entering it. Eastbound and westbound lanes often diverge with one lane rising over a bridge or ducking through a tunnel, preserving the canyon floor, walls, vegetation, and river where possible. Forty bridges and viaducts (totaling 6.5 miles) and three tunnels minimized the highway’s impact on its surrounding environment while the speed limit was set at 50 miles per hour to improve safety.

A construction technique called balanced cantilever construction was also utilized. The technique allowed each section of the highway to be built on bridge columns, reducing damage to the canyon. Other features added to the final design included four rest stops, a bike and jogging path along the length of the canyon, a boat launch, and a raft drop allowed for canyon recreational use by tourists and regional residents.

The result of the NEPA process was a 12.5-mile stretch of highway with lower environmental impacts.

“NEPA helped engineers to understand ecology and environmental design. In this case, without it, the CAC would have been ignored or abolished and the unique Canyon would have been destroyed. NEPA ensured that citizens and design professionals were heard in preserving the Canyon,” said Bert Melcher, a citizen activist.

The project has since won more than thirty awards for innovative design and environmental sensitivity, with the American Society of Civil Engineers giving the project its Outstanding Civil Engineering Achievement Award in 1993.


[1] “I-70 Mountain Corridor History.” Colorado Department of Transportation. Accessed February 16, 2018. Available at:

[2] McNichol, Dan. “The Roads that Built America: The Incredible Story of the U.S. Interstate System.” New York: Sterling Publishing, 2006. pp. 230-258.

[3] “Glenwood Canyon I-70 Final Link.” Colorado Department of Transportation. Accessed February 26, 2018. Available at:

[4] “Final Supplemental Environmental Impact Statement: Glenwood Canyon I-70.” S. Army Corps of Engineers. June 14, 1982. Available at:

[5] “Outstanding Civil Engineering Achievement (OCEA) Award Past Winners.” American Society of Civil Engineers. Accessed February 26, 2018. Available at:


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