Happy Canyon Interchange

Project Overview

Construction is underway on the Happy Canyon Road and Interstate 25 (I-25) interchange reconstruction project in Castle Pines. The City of Castle Pines, in partnership with Douglas County and the Colorado Department of Transportation (CDOT), is working alongside contractor partner, Kraemer North America, to improve safety, address increasing traffic volumes, and enhance traffic operations, connectivity, and regional mobility.

The project, aimed at supporting continued growth in the surrounding community, includes replacing the existing Happy Canyon Road bridge over I-25, constructing an additional bridge north of the existing bridge, rebuilding the I-25 on- and off-ramps, and improving traffic signals and intersections throughout the interchange.

Construction began in the fall of 2026 and is anticipated to be completed in early 2029.

Questions? Contact the project team by emailing HCInterchange@castlepinesco.gov or calling the project hotline at ‪720-689-3590.

What to Expect During Construction

Crews are working to minimize impacts to the traveling public by completing as much of the reconstruction as possible outside of active roadways. The new bridge will be constructed to the north of the existing bridge, allowing Happy Canyon Road to remain open during the majority of bridge construction.

Motorists should expect the following traffic impacts throughout construction:

Temporarily narrowed and realigned travel lanes on I-25.
- To safely access areas in the I-25 median to construct foundations for the bridge over I-25, crews will narrow and realign I-25 travel lanes toward the outside of the roadway. This will allow crews to maintain four travel lanes in each direction on I-25 for most of the project. Nightly I-25 lane closures will be implemented from 7 p.m. until 6 a.m., starting Monday, Nov. 16 at 7 p.m. until Thursday, Nov. 19 at 6 a.m., to safely realign the travel lanes. Travelers will still be able to access and travel on I-25 during these temporary nightly lane closures.
I-25 speed limits will be reduced to 65 mph in the construction zone.
Intermittent I-25 lane closures and on- and off-ramp closures, primarily during nighttime hours.
Full nighttime closures of I-25 for a limited number of major construction operations.
Reduced speed limits throughout the work zone.
Changing traffic patterns as construction progresses.

Drivers are encouraged to use caution, follow posted signage, and allow extra travel time when traveling through the interchange.

Sign up for project updates to stay informed about upcoming construction activities, traffic changes, and anticipated closures.

Construction Update

Fall 2026 – Summer 2027: Crews will construct a new bridge north of the existing bridge. This first phase will allow traffic to continue using the existing bridge during construction.

Anticipated Impacts:

I-25 travel lanes are narrowed and shifted toward the outside of the roadway beneath the Happy Canyon Road bridge.
Happy Canyon Road travelers will continue to use the existing bridge to access to/from and over I-25.
Nighttime I-25 full closures for girder installation; traffic will be detoured via the off- and on-ramps.
Intermittent nighttime I-25 lane and ramp closures.

See map below for traffic configuration in place through Phase 1.

Project Phasing

Diverging Diamond Interchange Design

A Diverging Diamond Interchange (DDI) is an innovative approach to traffic design, selected through a joint effort among the City of Castle Pines, Douglas County, the Colorado Department of Transportation (CDOT), and the Federal Highway Administration (FHWA). These interchanges excel at managing heavy directional traffic, ensuring smoother flow, reducing congestion in high-volume areas, and improving safety and efficiency.

Diverging Diamond Interchange (DDI) - a highway interchange configuration where the two directions of traffic on the cross street (Happy Canyon Road) temporarily cross to the opposite side of the road, allowing vehicles to make left turns onto the interstate (I-25) without crossing oncoming traffic.

Left-turn movements are typically the riskiest and least safe movements through an interchange. With a DDI, these dangerous T-bone accidents are eliminated by removing the typical left turn movement.

Benefits of a diverging diamond:

Reduces delays using a two-phase signal as opposed to a traditional four- or six-phase signal
Requires four fewer lanes while accommodating greater traffic volumes
Reduces conflict points at intersections
Improves pedestrian crossing safety with easier access to medians
Increases left-turn capacity
Eliminates wrong-way entry to ramps
Costs less than a typical free-flow interchange that utilizes unidirectional flyover ramps

How to drive a diverging diamond interchange:

You can view a variety of video renderings on the City's YouTube Channel to watch alternate views of the DDI in operation and learn how to drive through the new interchange design.

Watch this video from the Colorado Department of Transportation for more information on driving through a DDI.

Funding and Timeline

Project Background

Background and History

The City of Castle Pines' Comprehensive Plan is another step in the City's ongoing efforts to build and maintain a balanced, unified, and sustainable community.

In 2015, Douglas County initiated the Happy Canyon Road / I-25 Improvement Project to identify long-term interchange improvements, which included 2030 and 2040 traffic projections, alternatives analysis, and development of conceptual design plans for various interchange alternatives. Revised traffic modeling was necessary to update the Castle Pines Parkway & Happy Canyon Road Final System Level Study (Aug 2009).

In 2022, the City of Castle Pines elected to move forward with managing the Happy Canyon Interchange Project and advance improving the connection from I-25 to both Lagae and Happy Canyon roads. This connection and roundabout were completed in summer 2024. The City worked closely with the County, CDOT, and FHWA to finalize the System Level Study that evaluated the existing and future traffic operations utilizing 2050 traffic projections.

Background on Funding

The Happy Canyon Interchange Project was identified in the Denver Regional Council of Governments’ (DRCOG) 2050 Metro Vision Plan. Federal funding through DRCOG, matched with City and County funds, supported early planning, environmental review, and design.

Construction costs are anticipated to be $39.3 million. The City of Castle Pines and surrounding developments' contributions total $12.8 million. Douglas County contributed $15.3 million from its current Transportation Capital Improvement Program 5-Year list of priority projects. Lastly, the City secured $11.2 million from CDOT to replace the existing Happy Canyon bridge over I-25 as part of the overall interchange improvement project.

Interchange Design Selection

Using CDOT's Policy Directive 1601 Interchange Approval Process, several strategies were developed that facilitate mobility options to help reduce the number of single-occupant vehicle trips. This is accomplished by promoting and implementing strategies, programs, and alternative modes of travel, including carpooling, walking, teleworking, transit, and biking.

Utilizing 2050 traffic projections, the City and its consultants identified four alternatives (design options) to improve the interchange. The four interchange alternatives evaluated include:

Tight Diamond with Traffic Signals at the I-25 Ramps
Tight Diamond with Roundabouts at the I-25 Ramps
Single Point Urban Interchange (SPUI)
Diverging Diamond with Traffic Signals at the I-25 Ramps

The Diverging Diamond Interchange (DDI) was selected as the preferred alternative to move forward through the National Environmental Policy Act (NEPA) clearance process and into preliminary and final design.

The Tight Diamond Interchange options studied cannot provide acceptable operations for the forecasted 2050 traffic volumes, meaning that this option would not provide traffic alleviation. Although the SPUI can operate with the 2050 volumes, it is estimated to cost $10 million more than the DDI. It was determined that the DDI alternative will better accommodate the 2050 traffic projections than the Tight Diamond with Roundabouts at the I-25 Ramps alternative, which was previously determined to adequately handle the 2040 traffic projections.

Approval of the surrounding developments has been contingent on interchange improvements and incorporates connections with the surrounding regional road network, such as Crowfoot Valley and Hess roads.

Happy Canyon Interchange Resource Documents

Stay Informed About the Project:

Sign up for The Source community e-newsletter.
For answers to questions, email or call the project team: HCInterchange@castlepinesco.gov or call the project hotline at 720- 689-3590‬‬.
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