Colorado State Highway 66

State Highway 66
Map of northern Colorado with SH 66 highlighted in red
Route information
Maintained by CDOT
Length:	22.7 mi (36.5 km)
Major junctions
West end:	US 36 / SH 7 southeast of Lyons
	US 287 in Longmont I-25 / US 87 near Longmont
East end:	US 85 in Platteville
Location
Counties:	Boulder, Weld
Highway system
Colorado State Highways Interstate U.S. State Scenic ← SH 65 SH 67 →

State Highway 66 is a 22.7 mi (36.5 km) long east-west state highway in Boulder and Weld counties in Colorado. The highway extends from the foothills of the Rocky Mountains at a junction with U.S. Highway 36 southeast of Lyons, proceeding east through the northern edge of Longmont, to Platteville where it ends at U.S. Highway 85. Although a number of western states retain their "state highway 66" as the decommissioned U.S. Route 66, the "Mother Road" did not run through Colorado, and State Highway 66 has no connection to the famed Historic Route 66.

Modern SH 66 begins as a continuation of Ute Highway, southeast of Lyons at a signalized T intersection where U.S. Highway 36 turns south towards Boulder on North Foothills Highway. From here, the route heads eastward along Ute Highway through open grassland, passing several small reservoirs and streams. Near a junction with CR 37, the road passes McCall Reservoir, Independent Reservoir, and Burch Lake before passing by McIntosh Lake. It passes through the northern edge of Longmont, where it meets US 287. Continuing eastward, the land on the side of the road transitions to farmland. After crossing the Boulder/Weld county line, the road heads east in a straight line, crossing Interstate 25 at Exit 243 near Mead, continuing on to Plattville which it enters on Justin Avenue, crossing Business US 85 at Main Street before ending at U.S. Highway 85.