The route begins in the west at a junction with SH 93 roughly thirteen miles south of Boulder.

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.

This gap is due to be closed in 2018 by a mile-long connector that began construction in 2009;[2] there was a five-year hiatus from 2010 to 2015 during negotiations with BNSF.

[3] When construction is complete, the total end-to-end length, including the section that is nominally concurrent with US 287, will be 14 mi (22.5 km).

[4] ^ a: The 3.8 mi (6.1 km) long section of concurrency with US 287 is not maintained with state funds and therefore not considered to be part of SH 128 by the Colorado Department of Transportation.