California State Route 166

Route 166 starts off in Guadalupe in northwestern Santa Barbara County and heads east towards Santa Maria, the largest city on its eastern journey. It then joins with U.S. Route 101 for the last few miles in Santa Barbara County before crossing the Santa Maria River and splitting off in San Luis Obispo County. For the next 75 miles (121 km), SR 166 crosses the Santa Barbara/San Luis Obispo county line a total of five times. This stretch follows the Cuyama River through a canyon separating the Sierra Madre Mountains from mountains in San Luis Obispo County, and then opens out into the Cuyama Valley, passing cattle ranches, going through the Russell Ranch Oil Field, and passing Aliso Canyon Road, the turnoff to the South Cuyama Oil Field. On the north during this stretch is the mile-high Caliente Range, which contains Caliente Mountain, the highest peak in San Luis Obispo County.

After going through the towns of New Cuyama and Cuyama, the highway meets SR 33 north of Ventucopa. SR 33 and SR 166 merge until reaching Maricopa, where SR 166 heads due east for its last 20 miles (32 km), intersecting with I-5 about 9 miles (14 km) north of the Grapevine. SR 166 ends at SR 99 in Mettler, and it is the last exit for both I-5 and SR 99 southbound before they merge near Wheeler Ridge.

The western terminus of Route 166 snakes through the Guadalupe-Nipomo Dunes, which end at the Pacific Ocean

In Kern County, Highway 166 is known as the Maricopa Highway. West of Maricopa, where it skirts Santa Barbara and San Luis Obispo counties, it is called the Cuyama Highway. In the cities of Santa Maria and Guadalupe, it is known as Main Street.

SR 166 is part of the California Freeway and Expressway System,[2] but is not part of the National Highway System,[3] a network of highways that are considered essential to the country's economy, defense, and mobility by the Federal Highway Administration.[4] The route is eligible for the State Scenic Highway System,[5] but it is not officially designated as a scenic highway by the California Department of Transportation.[6] From US 101 to SR 33, SR 166 is known as the "CHP Officers Irvine and Stovall Memorial Highway". In February 1998, a large storm swelled the Cuyama River and caused it to wash out a section of the highway. Officers Britt Irvine and Rick Stovall were responding to an early morning call about a truck accident when their CHP cruiser drove off the washed out section.[7]

Before 1964, the portion of SR 166 merged with SR 33 was part of US 399.

Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment that existed at the time, and do not necessarily reflect current mileage. R reflects a realignment in the route since then, M indicates a second realignment, L refers an overlap due to a correction or change, and T indicates postmiles classified as temporary (for a full list of prefixes, see the list of postmile definitions).[1] Segments that remain unconstructed or have been relinquished to local control may be omitted. The numbers reset at county lines; the start and end postmiles in each county are given in the county column.

County	Location	Postmile [1][8][9]	Exit [10]	Destinations	Notes
Santa Barbara SB 0.00-90.99[N 1]	Guadalupe	0.00		West Main Street	Continuation beyond SR 1
		0.00		SR 1	West end of SR 166
	Santa Maria	7.87		SR 135 (Broadway, US 101 Bus.)
		8.93 88.60[N 1]		US 101 south / Main Street	Interchange; west end of US 101 overlap
		8.93 88.60[N 1]	West end of freeway on US 101
		89.69[N 1]	172	Donovan Road
		90.75[N 1]	173	SR 135 south (Broadway, US 101 Bus. south) – Santa Maria
San Luis Obispo SLO 0.00[N 1]-22.89	​	0.81[N 1] 13.51	East end of freeway on US 101
	​	0.81[N 1] 13.51		US 101 north / Cuyama Lane	Interchange; east end of US 101 overlap
Santa Barbara SB 22.89-R34.99	No major junctions
San Luis Obispo SLO R34.99-R51.09	No major junctions
Santa Barbara SB R51.09-R70.14	No major junctions
San Luis Obispo SLO R70.14-4.95[N 2]	​	74.72 2.80[N 2]		SR 33 south – Ojai, Ventura	West end of SR 33 overlap; former US 399 south
Kern KER 0.00[N 2]-24.62	Maricopa	R11.56[N 2] 0.01		SR 33 north (California Street) – Central District, Taft	East end of SR 33 overlap; former US 399 north
	​	14.86		Old River Road – Old River
	​	22.80		I-5	Interchange; I-5 exit 225
	Mettler	24.62		Mettler Frontage Road West – Mettler
		24.62		SR 99 – Los Angeles, Bakersfield	Interchange; east end of SR 166; former US 99; SR 99 exit 3
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus

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Wikimedia Commons has media related to California State Route 166.

• California @ AARoads.com - State Route 166
• Caltrans: Route 166 highway conditions
• California Highways: SR 166