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6.50 miles ; from I-680 in Benicia to I-80 in Vallejo. It was approved as an interstate highway on Sept. 15, 1955, but as part of I-680. In July 1973, non-chargeable miles were added to California's system to allow 680 to be routed north along CA 21 toward Sacramento; the leftover portion of I-680 became Interstate 780.  Signing for this was updated around 1976.
See also: I-780 (Dan Faigin)