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City presents preferred diverging‑diamond design and $42M estimate for Happy Canyon interchange; public meeting March 31
Summary
City staff presented the Happy Canyon Interchange update, recommending a diverging‑diamond interchange (DDI) with signal control as the preferred alternative to avoid backing traffic onto I‑25; project planning includes environmental reviews, a March 31 open house and a roughly $42 million cost estimate with CDOT, Douglas County and developer contributions under discussion.
Joel Gribbenic, the city’s project lead, told the Castle Pines Council the Happy Canyon Interchange project—originally advanced by Douglas County and now led by the city—aims to improve safety, reduce queues that can back onto I‑25 and provide multimodal connectivity including a multiuse trail for bicyclists. “We determined that the diverging diamond with traffic signals is the preferred alternative,” Gribbenic said, pointing to traffic modeling and queue‑management as controlling criteria.
Gribbenic explained the alternatives analysis: the project team used DRCOG travel demand modeling and traffic simulation to test interchange types and measure whether off‑ramp queues would back onto the interstate. He said a single‑point urban interchange (SPUI) could also meet traffic volumes but would require a substantially larger, more expensive bridge (roughly 20% higher cost) because of longer bridge spans. The DDI, he said, reduces signal phases and increases left‑turn capacity by allowing the bridge to serve as left‑turn queue storage.
On environmental and permitting work, Gribbenic listed required studies: noise, threatened and endangered species, wetlands, water resources, noxious weeds, hazardous materials and cultural/archaeological resources. Those assessments will be documented and, where necessary, mitigated for review by CDOT and Federal Highways as part of clearance and approval processes.
The staff schedule includes a public open house March 31 (library, 4:30–6 p.m.), preliminary design later in 2025 and final design in early 2026 with tentative construction start in mid‑2026. Gribbenic said the project budget is currently estimated at about $42,000,000; funding sources discussed include Douglas County contributions, developer reimbursements, potential CDOT participation (the city is asking CDOT to contribute about $6.5 million to replace CDOT’s existing bridge), federal BUILD or earmark opportunities and inclusion in future DRCOG TIP cycles.
Gribbenic described a phased construction approach possible with the DDI’s two separate bridges, which would allow building a new bridge without taking the existing bridge fully out of service. He said the project team will pursue public outreach and a contractor‑led public relations program during construction to limit community impacts.
Council members discussed the advocacy role of elected officials for DRCOG TIP and federal funding cycles. Council member Mulvey offered to prepare talking points and a concise one‑pager for electeds to use when engaging peers and legislators; Gribbenic agreed staff could prepare such materials. No formal council vote occurred at the study session; the presentation concluded with council thanks for the update.
