Castle Pines, Douglas County and CDOT are rebuilding the Happy Canyon Road interchange at I-25 as a Diverging Diamond Interchange (DDI). It's anticipated to start fall 2026 and finish mid-2028, and the project team says it's still waiting on final approvals from CDOT.
What's changing
The interchange redesign is a joint effort between the City of Castle Pines, Douglas County, CDOT and the Federal Highway Administration. Per the project page, the work breaks into three phases:
- Phase 1: Construct a new bridge structure over I-25 north of the existing bridge. Happy Canyon Road traffic stays on the existing bridge during this phase.
- Phase 2: Demolish the existing bridge and build the southern bridge structure, after Happy Canyon Road traffic has moved onto the newly built northern bridge.
- Phase 3: Tie in new ramps to I-25 and the bridge structures on the east and west sides of I-25.
The stated purpose is to "improve safety and address rising traffic volumes," and to improve "traffic operations, connectivity, and regional mobility."
Timeline, and it's still "anticipated"
The project page lists an anticipated start of fall 2026, adding that "more specific information will be shared as it becomes available," and an anticipated completion of mid-2028. The project page says the DDI design "is being evaluated," and the project team says it's waiting on final CDOT approvals before those dates firm up.
What drivers should expect
The project page says "the majority of interchange reconstruction will take place outside of the roadway" to limit disruption, but it does list specific impacts to plan around:
- Intermittent ramp closures — first the northbound I-25 on-ramp and the southbound off-ramp, later the southbound on-ramp and northbound off-ramp, and more intermittent ramp closures during Phase 3
- Nighttime I-25 closures for bridge demolition and for girder installation
- I-25 lanes narrowed and temporarily shifted during construction
- Various lane closures on I-25 and Happy Canyon Road for finishing work
Who's paying for it
Construction costs are anticipated at $42 million total:
- $14.5 million from the City of Castle Pines and surrounding developments
- $16.5 million from Douglas County, from its Transportation Capital Improvement Program 5-Year priority list
- $11 million procured from CDOT
Stay updated
The project team can be reached at [email protected] or 303-705-0200, and the project page points to the City's The Source community e-newsletter for progress updates.
Related: What's Being Built in Douglas County — every tracked development and opening across the county. · Castle Pines — our city guide.
Source: City of Castle Pines, Happy Canyon Interchange project page. Checked Oct. 6, 2026.