The highway extends from the foothills of the Rocky Mountains at a junction with U.S. Route 36 (US 36) and SH 7 southeast of Lyons, proceeding east through the northern edge of Longmont, to Platteville where it ends at US 85.

From here, the route heads eastward along Ute Highway through open grassland, passing several small reservoirs and streams.

The route was established in the 1920s, when it began at US 87 (now moved) in Longmont and headed west to Estes Park.

The section east of Longmont, which now extends to Platteville, was added in 1939, when the older portion of the route was paved.

In 2007 this remnant of SH 66 in Estes Park was turned back and it was truncated to its junction with US 36 southeast of Lyons.