Route 110, consisting of State Route 110 (SR 110) and Interstate 110 (I-110), is a state and auxiliary Interstate Highway in the Los Angeles metropolitan area of the US state of California. The entire route connects San Pedro and the Port of Los Angeles with Downtown Los Angeles and Pasadena. The southern segment from San Pedro to I-10 in downtown Los Angeles is signed as I-110, while the northern segment to Pasadena is signed as SR 110. The entire length of I-110, as well as SR 110 south of the Four Level Interchange with US Route 101 (US 101), is the Harbor Freeway, and SR 110 north from US 101 to Pasadena is the historic Arroyo Seco Parkway, the first freeway in the western United States.

Greater Los Angeles is the most populous metropolitan area in the U.S. state of California, encompassing five counties in Southern California extending from Ventura County in the west to San Bernardino County and Riverside County in the east, with Los Angeles County in the center, and Orange County to the southeast. The Los Angeles–Anaheim–Riverside combined statistical area (CSA) covers 33,954 square miles (87,940 km2), making it the largest metropolitan region in the United States by land area. The contiguous urban area is 2,281 square miles (5,910 km2), whereas the remainder mostly consists of mountain and desert areas. With a population of 18.4 million in 2024, it is the second-largest metropolitan area in the country, behind New York, as well as one of the largest megacities in the world.