Interstate 70 - Twin Tunnels


The Interstate 70 Twin Tunnels, approximately one mile east of Idaho Springs, were constructed in 1961, as part of a widening of U.S. 6 & 40 through Clear Creek Canyon.

The original U.S. 6 & 40 paralleled I-70, on the south side of Clear Creek, through the Twin Tunnels area.  Portions of it still exist today as a frontage road.


High traffic volume on I-70, west of Denver, during the summer and winter, has lead to ever-increasing periods of slow traffic, which at times, is considered a gridlock situation.

The predominant period for gridlock is on Saturday and Sunday afternoons, on eastbound I-70, from Idaho Springs, through the Twin Tunnels, to the base of Floyd Hill.

The section of I-70 from the Twin Tunnels to Hidden Valley, the interchange just east of the Twin Tunnels (Central City Parkway exit) has identified geometric problems and is believed by many to be the starting point for congestion and backups on eastbound I-70.


In late February 2011, the Colorado Department of Transportation (CDOT) convened a week-long visioning workshop that included local, national and international design and construction experts to discuss a variety of short-term mobility options to aid in the alleviation of congestion in the Twin Tunnels area.

The Technically Recommended Concept that emerged from the team is a balance of the critical success factors that were developed by the I-70 Mountain Corridor Stakeholders.  The concept utilizes a third lane on eastbound I-70, beginning at the east Idaho Springs Interchange (Exit 241), and continues east to the base of Floyd Hill, where eastbound I-70 currently becomes three lanes.

This option includes flattening the curve on eastbound I-70, just east of the Twin Tunnels, enhancing safety and allowing for an increased speed limit through this section.


CDOT will engage in discussions with the Colorado Transportation Commission regarding the Technically Recommended Concept.   Other alternatives considered by the workshop team also will be discussed.

The cost of the “Third Lane/Twin Tunnel” improvements, as described, is estimated at $55 million.  However, no funding source has been identified at this time.

CDOT estimates it would take approximately two construction season (late spring to early fall) to complete all aspects of this alternative.  However, noticeable mobility improvements will be in place at the end of each construction season.

The proposals do not include improving the westbound Twin Tunnel bore or westbound I-70 between the base of Floyd Hill and Idaho Springs.  The recommended concept does not violate or preclude the long-term solution for the I-70 Mountain Corridor as outlined in the I-70 Mountain Corridor Final Programmatic Environmental Impact Statement.