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Adams County, Thornton plan lane additions at 128th and Riverdale to ease school‑time congestion
Summary
Engineering staff recommended adding a southbound right‑turn slip lane and an eastbound left‑turn lane at 128th Avenue and Riverdale Road to reduce peak delays by ~39–45%. Estimated design cost ~$113,000 and construction ~$582,000; Thornton agreed to a 50/50 cost split and to lead project delivery; staff recommended an IGA and budget amendment to fund 2026 design.
Adams County Public Works and City of Thornton staff presented a joint proposal to address recurring congestion and safety concerns at the intersection of 128th Avenue and Riverdale Road near Riverdale Regional Park and Riverdale Ridge High School.
Traffic counts and modeling showed excessive delays (level‑of‑service failures) at peak times, including school pickup and drop‑off. Engineers recommended a modest capital fix: add a southbound right‑turn slip lane and an eastbound left‑turn lane to separate turning traffic and improve flow while retaining stop control for the intersection.
Modeling indicates morning delay could fall from about 90 seconds to under 60 seconds (roughly a 45% reduction) and afternoon queues could fall from about 72 seconds to about 39 seconds (a ~39% reduction). Staff estimated design at about $113,000 and construction at roughly $582,000 (including a 20% contingency); total outlay just under $700,000.
City of Thornton agreed to lead design and construction and proposed a 50/50 cost share; staff recommended a formal intergovernmental agreement (IGA), amending the 2026 budget to fund Adams County’s half of design (about $56,500) and planning for 2027 construction funding.
Commissioners supported the partnership and asked staff to coordinate timing with summer events at Riverdale Regional Park and to pursue grant funding (e.g., CDOT/NAP) for longer‑term improvements such as a roundabout. Staff will prepare an IGA and return final cost and schedule details for board action.
