Interstate 205 (I-205) is an east–west auxiliary Interstate Highway in the San Joaquin Valley in Northern California. It runs from I-5 west to I-580. Along with those highways, I-205 forms the north side of a triangle around the city of Tracy. The route provides access from the San Francisco Bay Area to the northern San Joaquin Valley. When I-205 opened in December 1970, it replaced 11th Street, which passed through downtown Tracy, as part of the primary all-land connection between the Bay Area and Sacramento until the Carquinez Bridge opened in 1927, and carried the Lincoln Highway and later U.S. Route 50. 11th Street is now signed as Interstate 205 Business. Route description Interstate 205 begins at the bottom of I-580's eight-lane descent from Altamont Pass into the San Joaquin Valley. Here I-580 turns southeast to a junction with I-5, paralleling the California Aqueduct and Delta-Mendota Canal along the foothills, while I-205 continues east as a six-lane roadway, immediately crossing both waterways. The first interchange is with Mountain House Parkway, formerly Patterson Pass Road, which serves the planned community of Mountain House. Next is a split with Business 205, a business loop that follows 11th Street through Tracy. As I-205 curves east-northeast and back east through the northern part of Tracy, it has interchanges with Grant Line Road (County Route J4 towards Antioch), Tracy Boulevard (County Route J13 through downtown Tracy), and MacArthur Drive. After several miles without an interchange, the highway ends at a merge with I-5, where traffic can continue northeast to the junction with SR 120 near Manteca and then east on SR 120 towards Yosemite National Park or north on I-5 towards Stockton. As it connects to I-580, I-205 is a frequently-congested major commuter route to the Bay Area. Signs on eastbound I-580 instruct travelers to take I-205, SR 120, and SR 99 to reach Modesto instead of using the direct, but non-freeway, route SR 132. I-205 also serves to connect the Bay Area with popular weekend destinations such as Yosemite, Reno, and Lake Tahoe. The Altamont Corridor Express provides commuters with an alternate route over Altamont Pass to San Jose and San Francisco, the latter through a transfer to BART. I-205 is part of the California Freeway and Expressway System, and is part of the National Highway System, a network of highways that are considered essential to the country's economy, defense, and mobility by the