CDOT outlines I‑25 safety upgrades, warns median transit station could cost $50–70 million

Colorado Department of Transportation staff on Tuesday walked Thornton leaders through the status and schedule for the I‑25 safety and operations project covering US‑36 to 104th Avenue and asked the city to help refine alternatives ahead of a June public meeting and the state’s 10‑year plan update.

CDOT Region 1 transportation director Jessica Micklebust told council staff and RTD are trying “to build the right thing on opening day,” using limited strategic funds while completing environmental review and final design. CDOT currently holds a placeholder of $110 million in strategic funds for the corridor but staff said the full project will likely exceed that amount and will require a mix of state strategic dollars, toll revenue from CTIO, grants and local or developer partnerships.

Why it matters: I‑25 through Thornton carries heavy ramp weaving, increasing crash risk and congestion. CDOT presented alternatives intended to reduce rear‑end and sideswipe crashes, improve traffic flow and preserve transit operations while keeping construction readiness compatible with the department’s fiscal plan.

What CDOT presented

- Corridor segmentation and timeline: CDOT split the corridor into south, center and north segments and said it is in NEPA (environmental review) and design, aiming to finalize technical reports and a draft environmental assessment by September 2025. CDOT targets public engagement in late June and to be construction‑ready in 2027 if funding is available.

- South and north segments: Potential widening, added shoulders and a collector‑distributor (CD) road to separate heavy ramp traffic (notably traffic from I‑270 and US‑36) from mainline flows. CDOT staff said modelling (microsimulation) will guide whether a CD road or climbing lane or both best relieve weaving and maintain speeds.

- Center segment and transit conflict: The corridor’s “center” section around the Thornton Park‑and‑Ride presents the most complex tradeoffs because express buses (Route 120X) now use slip ramps and park‑and‑ride lots that have dropped in utilization (RTD data cited a roughly 90% decline in parking‑lot utilization from historic levels). CDOT said the preferred operational fix would be a median station aligned with the express lane that would avoid buses crossing multiple general‑purpose lanes to reach slip‑ramp stops.

Costs and tradeoffs

CDOT engineers warned that a median station would be expensive and require major bridge work. Staff said the median station alternative would require a longer span at the 80th Avenue bridge and could cost about $50 million to $60 million to construct; modifying the replacement bridge so it did not preclude a future median station would add roughly $20 million or more in early widening and pier offset costs.

Funding and next steps

CDOT emphasized the project will require a mix of strategic (10‑year plan) dollars, CTIO/toll revenue and competitive grants. Micklebust said the department is updating its 10‑year plan (the plan released July, finalized December 2025 timeframe) and needs a defined footprint and cost estimate by mid‑2025 to place the project competitively. CDOT listed key upcoming outreach events: a statewide telephone town hall May 22 and a Region 1 planning meeting June 11, with a project‑specific public meeting targeted for late June or July.

Transit service and RTD

RTD staff said ridership and travel‑pattern changes (remote work, fare structure changes and RTD network changes such as the A‑Line/peaks) have reduced demand at the Thornton Park‑and‑Ride. RTD reported that Route 120X carries a small fraction of its peak historic ridership and said the agency is considering service changes including hybrid routes or starting points at multiple park‑and‑ride lots and better local connections to the A‑Line as alternatives to a large median station. RTD is launching a new comprehensive operations analysis next year to reevaluate network design for the period after 2027.

Council questions and next steps

Council members asked for follow‑up on technical details such as whether proposed Colorado 7 intersection improvements include dedicated turn signals and for confirmation on 104th Avenue redesign elements. CDOT agreed to provide written clarification after consulting its traffic and safety staff. CDOT asked Thornton to engage in upcoming regional outreach: department staff said citizen participation will be important to retain the project’s competitive position in the 10‑year plan.

What remains undecided

The council did not take a vote. The key outstanding policy choice remains whether to proceed with a center median transit station (high cost, high infrastructure impact) or to invest in local transit connections, bridge replacement and other, lower‑cost alternatives that CDOT and RTD said may better match “opening‑day” demand.

Ending: CDOT plans a second public outreach meeting in late June or July and asked Thornton to provide comments for the 10‑year plan update, which will determine when a construction tranche of funds could be available.