state route 21 closely followed route of current interstate 680. began in san jose heading northeast i-280 , 101 , closely paralleling sr 17 (now i-880) until reaching fremont. there, sr 21 headed northeast toward pleasanton , intersected i-580. sr 21 headed northwest concord , martinez, california, intersecting sr 24 , sr 4 before reaching carquinez strait. in benicia, sr 21 intersected sr 141 (now i-780) , ended in fairfield @ i-80.
the bureau of public roads approved urban routes of interstate highway system on september 15, 1955, including loop around san francisco bay, numbered i-280 , i-680. i-680 built along sr 21. first piece of i-680 freeway built, other pre-existing nimitz freeway, in late 1950s, along sr 24 overlap between north main street in walnut creek , monument boulevard in pleasant hill. southerly extension, bypassing downtown walnut creek south main street, opened on march 22, 1960, connecting sr 24 freeway oakland. in next decade, freeway completed vallejo south sr 238 @ mission san jose, , roadway north benicia fairfield, became remaining piece of sr 21, upgraded freeway standards. sr 21 ceased exist in 1973 , deleted california legislature in 1976.
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