Colorado State Highway 128

The route begins in the west at a junction with SH 93 roughly thirteen miles south of Boulder. From there, the road proceeds eastward into the northern portions of the Denver metropolitan area. The road passes through portions of the cities of Superior, Broomfield, Northglenn, and Westminster and crosses SH 121 before being split by a 3.8 mi (6.1 km) section of concurrency with U.S. Route 287 (US 287) and again resuming its course towards its eastern terminus at exit 223 of I-25 in the city limits of Westminster.

A brief gap between the junction of SH 128 and SH 121 and the junction of SH 121 and US 287 was closed with the construction of a bridge over US 36 to SH 121. This connector began construction in 2009 and finished in 2018, at a cost of $90.1 million.^[2] There was a five-year hiatus in construction from 2010 to 2015 during negotiations with BNSF.^[3] Following the completion of construction, the total end-to-end length, including the section that is nominally concurrent with US 287, is 14 mi (22.5 km).

The route was established in the early 1920s, when it began at Federal Boulevard and ended at SH 185 (now deleted). A section from US 85 to US 6 was added by 1939 and deleted by 1949. The west terminus was extended from US 36 to Indiana Street in 1965 and to its current terminus at SH 93 two years later.^[4]