Interstate 238 (I-238) is a short auxiliary route of the Interstate Highway System in the San Francisco Bay Area, California, United States. Part of State Route 238 until 1983, and still considered the same route (part of Route 238) by the California Department of Transportation (Caltrans), it connects Interstate 580 in Castro Valley (where SR 238 continues south) with Interstate 880 in San Leandro. Despite running in an east–west direction, it is signed north–south, as the rest of Route 238 is more north–south than east–west.
The number does not follow established rules for numbering Interstates, as there is no Interstate 38. Of the nine possible numbers ending in 80 (as it connects two auxiliary routes of Interstate 80), two—180 and 480—were in use by State Routes (the latter an Interstate until 1968), and the seven others were used by other Interstates. (I-880 was designated at the same time as I-238; it too could only be an auxiliary route of I-80.)
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