|Robert T. Monagan Freeway|
I-205 highlighted in red
|Defined by Streets and Highways Code § 505|
|Maintained by Caltrans|
|Length||12.973 mi  (20.878 km)|
|History||State highway in 1910|
|West end||I-580 near Tracy|
|East end||I-5 near Lathrop|
|Counties||Alameda, San Joaquin|
Interstate 205 (abbreviated I-205) is a short auxiliary Interstate Highway that connects Interstate 5 with Interstate 580 in the San Joaquin Valley in the U.S. state of California. 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.
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 Federal Highway Administration.  It is officially designated as the Robert T. Monagan Freeway after the California legislator who represented the area from 1961 to 1973. 
When the Department of Engineering laid out the initial state highway system after the state's voters approved a bond issue to pay for it in 1910, they included Route 5, connecting Santa Cruz and Oakland with Stockton via Altamont Pass.  San Joaquin County paved the portion near Tracy with asphalt with their own bond issue, passed in 1909, and the state later resurfaced it with concrete.  In addition, the new concrete road bypassed Banta, which the old county road had passed through via Banta Road, F Street, and Grant Line Road. Otherwise, the road was relatively direct, coming down from Altamont Pass onto Grant Line Road, following Byron Highway into Tracy, and leaving east and northeasterly on 11th Street to the San Joaquin River at the Mossdale Crossing.    The Lincoln Highway Association chose this route in 1913 for their transcontinental highway,  where it remained until the Carquinez Bridge opened in 1927, creating a shorter route via Vallejo.  In 1926, the American Association of State Highway Officials designated the Stockton-Bay Area route as US 48,  which was absorbed by an extension of US 50 by the early 1930s.  
A 1938 four-lane bypass of the old road around Altamont Pass   was extended east to Tracy as a four-lane expressway on November 16, 1954.   By then, the entire route between the Bay Area and Stockton was four or more lanes, following the present I-580 (eastbound lanes where they separate ), I-205, 11th Street, and I-5 from Livermore through Tracy to Stockton.  During early planning for the Interstate Highway System, the main north-south route through California  (now I-5) was to use SR 99 through the San Joaquin Valley; a connection to the Bay Area split near Modesto and roughly followed US 50.  The Bureau of Public Roads approved a move to the proposed Westside Freeway in May 1957, and in November they added a North Tracy Bypass that would connect I-5 and I-580.  Construction began in the late 1960s,   incorporating part of the 1954 expressway and a new alignment bypassing Tracy to the north, and the $14 million road opened to traffic on December 21, 1970.  (A short piece at the west end, including the bridge over the California Aqueduct, was upgraded several years earlier when I-580 and I-5 to the south were built. )
Since 1970, I-205 has seen few changes. The largest have been widening from four to six lanes west of Business 205 in 1999,  and converting two diamond interchanges to partial cloverleafs — Grant Line Road in about 1997  and Mountain House Parkway in 2007 (including ramp meters).   As of Summer 2009, I-205 has been widened to six lanes (three in each direction) for its entire route. Also, the 1970s concrete pavement has been resurfaced with fresh asphalt from the junction of I-5 to due east of the 11th street connector ramp.
Caltrans has plans to improve merging distances by constructing auxiliary lanes between the interchanges, and to add new interchanges at Lammers Road and Paradise Road.   Caltrans is also planning to add one HOV lane in each direction between I-580 and I-5. 
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).  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.
|||0.21||I-580 west ( Arthur H. Breed Jr. Freeway) – San Francisco||Westbound exit and eastbound entrance; western terminus; I-580 east exit 65|
|||1.38||2||Mountain House Parkway|
|Tracy||R3.37||4||Eleventh Street ( I-205 Bus.) – Tracy||Eastbound exit and westbound entrance; I-205 Bus. unsigned; former US 50 east|
|R5.29||6||Grant Line Road ( CR J4) / Naglee Road|
|R7.01||8||Tracy Boulevard ( CR J13)|
|||R13.40||I-5 north – Stockton||Eastbound exit and westbound entrance; eastern terminus; I-5 south exit 458B|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
Interstate 205 Business (I-205 Bus.) is a locally maintained business route. It locally follows 11th Street, the historic four-lane alignment of U.S. Route 50, through Tracy. The route begins at a split with I-205 west of the city. After passing through downtown Tracy, it curves northeast at a junction with former SR 33, which has been truncated to the south at I-5. The final stretch of I-205 Bus. runs diagonally to a merge with I-5, which comes from the south and continues northeast along the former US 50 alignment. The east end of I-205 Bus. is about 3⁄4 mile (1.2 km) southwest of the end of I-205; normally I-205 Bus. would return to I-205 at both ends, but here I-205 and I-205 Bus. both end at I-5. 
- California Department of Transportation. "State Truck Route List". Sacramento: California Department of Transportation. Archived from the original ( XLS file) on June 30, 2015. Retrieved June 30, 2015.
- Google Maps street maps and USGS topographic maps, accessed February 2008 via ACME Mapper
- Oakland Tribune, Creative solution OKd to unlock I-205 gridlock, September 30, 2005
- Les Mahler, Oakland Tribune, Commuters to get break with I-205's expansion, August 1, 2005
- California State Legislature. "Section 250–257". Streets and Highways Code. Sacramento: California State Legislature. Retrieved June 9, 2017.
- Federal Highway Administration (March 25, 2015). National Highway System: Tracy, CA (PDF) (Map). Scale not given. Washington, DC: Federal Highway Administration. Retrieved September 15, 2017.
- Natzke, Stefan; Neathery, Mike & Adderly, Kevin (June 20, 2012). "What is the National Highway System?". National Highway System. Washington, DC: Federal Highway Administration. Retrieved July 1, 2012.
- California Department of Transportation; California State Transportation Agency (January 2015). 2014 Named Freeways, Highways, Structures and Other Appurtenances in California. Sacramento: California Department of Transportation. pp. 84, 256. Archived from the original (PDF) on May 30, 2015. Retrieved May 30, 2015.
- Howe & Peters, Engineers' Report to California State Automobile Association Covering the Work of the California Highway Commission for the Period 1911-1920, pp. 11-16
- Ben Blow, California Highways: A Descriptive Record of Road Development by the State and by Such Counties as Have Paved Highways, 1920 ( Archive.org or Google Books), pp. 104-105
- Automobile Club of Southern California, Automobile Road Map of California, 1917: shows the route via Banta
- Official Automobile Blue Book, Volume Eight, 1918, pp. 75-77: describes the route via Banta
- Rand McNally & Company, San Francisco and Vicinity, 1926: shows the more direct bypass of Banta Archived December 31, 2007, at the Wayback Machine.
- New York Times, Lincoln Highway Route Announced, September 14, 1913, p. C6
- Kevin J. Patrick and Robert E. Wilson, Indiana University of Pennsylvania, Lincoln Highway Resource Guide Archived 2014-10-24 at the Wayback Machine.: Chapter 17 Lincoln Highway in California, August 2002
- Bureau of Public Roads & American Association of State Highway Officials (November 11, 1926). United States System of Highways Adopted for Uniform Marking by the American Association of State Highway Officials (Map). 1:7,000,000. Washington, DC: U.S. Geological Survey. OCLC 32889555. Retrieved November 7, 2013 – via University of North Texas Libraries.
- Rand McNally & Company, California, 1933 Archived January 6, 2008, at the Wayback Machine.
- Division of Highways, Los Angeles and Vicinity, 1934
- "New Altamont Pass Route Will Open Tomorrow". Fresno Bee. August 3, 1938.
- United States Geological Survey, San Jose (scale 1:250000), 1947
- Daily Review (Hayward), New Highway 50 to Tracy Will Open Tomorrow, November 15, 1954
- California Department of Transportation, Index to California Highways and Public Works, 1937-1967, June 1997, p. 73
- H.M. Gousha Company, California, 1955 Archived May 25, 2011, at the Wayback Machine.
- "California Public Relocation Resource". California Relocation Division. 2007-08-13.
- Public Roads Administration, National System of Interstate Highways, August 2, 1947
- California Department of Transportation, State Highway Routes: Selected Information, 1994 with 1995 revisions, pp. 16, 234
- Modesto Bee and News-Herald, San Joaquin's Multimillion Road Jobs Zip Along Toward Finish, June 27, 1967
- Modesto Bee and News-Herald, Highway Delay Could Hit $38 Million in SJ Work, September 19, 1969
- Modesto Bee and News-Herald, $14 Million North Tracy Bypass Will Open for Traffic Tomorrow, December 20, 1970
- California Department of Transportation (July 2007). "Log of Bridges on State Highways". Sacramento: California Department of Transportation.
- California Department of Transportation, Interstate 205 Widening from I-5 to 11th Street in Tracy, accessed February 2008
- Stockton Record, Mountain House Parkway work, January 27, 2007
- David Siders, Stockton Record, S.J. carpool plan talks surface, October 17, 2007
- Cheryl Winkelman, Oakland Tribune, Cure for I-580, I-205 in the works, November 26, 2007
- Mike Martinez, Tri-Valley Herald, Cranes won't halt work, February 27, 2007
- California Department of Transportation, California Numbered Exit Uniform System, I-205 Eastbound and I-205 Westbound, accessed February 2008
|Wikimedia Commons has media related to Interstate 205 (California).|
- California @ AARoads.com - Interstate 205
- Caltrans: Route 205 highway conditions
- California Highways: I-205