Interstate Highways and U.S. Highways are assigned at the national level. Interstate Highways are numbered in a grid—even-numbered routes are east–west routes (but the lowest numbers are along Mexico and the Gulf of Mexico), and odd-numbered routes are north–south routes (with the lowest numbers along the Pacific Ocean). US Highways are also numbered in a grid—even numbered for east–west routes (with the lowest numbers along Canada) and odd numbered for north–south routes (with the lowest numbers along the Atlantic Ocean). There are 21 Interstate highways in California, ranging from Interstate 5 to Interstate 980. There are seven current U.S. Highways including U.S. Route 6 and U.S. Route 395.
State Route 52 (SR 52) is a state highway in the US state of California in San Diego County, that extends from La Jolla Parkway at Interstate 5 (I-5) in La Jolla to SR 67 in Santee. It is a freeway for its entire length, and serves as a major east–west route through the northern part of the city of San Diego. The road connects the major north–south freeways of the county, including I-5, I-805, SR 163, I-15, SR 125, and SR 67. SR 52 passes north of the Rose Canyon Fault before traversing Marine Corps Air Station Miramar. East of Santo Road and west of SR 125, the highway goes through Mission Trails Regional Park, a large open space. SR 52 is also known as the Soledad Freeway and the San Clemente Canyon Freeway. Plans for a route between La Jolla and Santee date back from 1959. Construction began in 1966 with the I-5 interchange to La Jolla, and the San Clemente Canyon Road serving as an early predecessor. The freeway was complete all the way to I-805 in 1970, and to Santo Road east of I-15 in 1988. However, the road east of there faced delays from environmentalists over the endangered Least Bell's Vireo songbird, which faced habitat destruction, as well as those concerned with the destruction of homes and businesses for the right of way for the freeway. The extension to Mission Gorge Road opened in 1993, and opened all the way to SR 125 in 1998. Funding issues delayed the completion of the entire route to SR 67 until 2011, over fifty years after construction began; until then, the city of Santee faced traffic snarls. A widening project was completed in 2007 between Santo Road and Mast Boulevard; further widening has been put on hold due to state budget concerns.
The four-lane Foresthill Bridge was built in 1971 to replace the low crossing of the American River on State Route 49 in preparation for the construction of the Auburn Dam. The dam was canceled for environmental reasons, but the bridge, the tallest in California, remains along the local Foresthill Road.