Previously, cyclists and pedestrians connecting between the existing terminus of the Boulder Canyon Trail at Four Mile Canyon Road and the Chapman Drive and Betasso Link trailheads had to ride or walk on the narrow shoulder of CO 119. The hazard was compounded because of speed differentials, poor sight distance in the canyon environment, and vehicles forced to enter the opposing lane of traffic to pass cyclists.

In response to these issues, Boulder County selected Muller to design a trail extension consisting of 0.75 mile of new multi-use path, including two underpasses crossing CO 119 and a trail bridge over Boulder Creek adjacent to Chapman Drive.

For pedestrian and cyclist safety, the alignment of the underpasses was selected to maximize sight distance and avoid blind corners. This resulted in crossings that were severely skewed with respect to CO 119. Combined with the narrow confines of the canyon, the underpasses posed a construction phasing challenge. Muller’s engineers developed an approach to phasing and temporary shoring that enabled the underpasses to be constructed while minimizing the disruption to traffic on CO 119. Extensive waterproofing keeps the inside of the underpass dry, improving the pedestrian experience and reducing icing on the trail.

The project was federally funded with oversight by CDOT and required environmental, right-of-way, and utility clearances. For construction efficiencies and to minimize impacts to the traveling public, the trail project was merged into a canyon-wide CDOT overlay project and was bid along with a separate permanent flood-recovery project on CO 119.