Although the I-238 goes in an east–west direction from Castro Valley to San Leandro, Caltrans officially signs it as a north–south freeway since the rest of SR 238 is more north–south. The southern terminus of I-238 is at its interchange with I-580 and SR 238 in Castro Valley. From there, it enters into the southern portion of the census-designated place of Ashland, running parallel to its border with Cherryland. Then after entering San Leandro, I-238 ends at its northern terminus with I-880. I-238 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. I-238 and I-880 are used as an alternate truck route between Castro Valley and Oakland; trucks over are prohibited through the latter on I-580.
Numbering
The number does not follow established rules for numbering Interstates, as there is no I-38. As it connects two auxiliary routes of I-80, it would normally use a three-digit number ending in 80, but of the nine possible numbers, two were in use by State Routes, and the remainder were already in use by other California auxiliary routes.
History
The section of road that is now I-238 had no signed number before the 1964 renumbering; it was pre-1964 Legislative . The segment from what is now I-580 to I-880 was built as a freeway in 1956. The rest of SR 238 south to I-680 in Fremont was also planned to be upgraded to a freeway, but after it was unsuccessfully submitted to the Interstate Highway System in October 1968, and after several lawsuits, it was cancelled in 2003. When present, I-880 was added to the Interstate Highway System as a renumbering of part of California State Route 17|, the short piece of connecting I-880 to I-580 was also added; both were non-chargeable routes. Both numbers—I-238 and I-880—were approved by the American Association of State Highway and Transportation Officials on July 7, 1983. The interchanges with I-580 and I-880 were rebuilt from 1988 to 1994, in part to add missing ramps between I-238 and I-880 towards the south. Prior to the completion of the ramps, access was provided by Hesperian Boulevard. On July 7, 1983, while approving the designation, AASHTO said: On July 27, 1983, Caltrans responded: With the decommissioning of in 1991, the "480" designation was once again made available. However, there has been no push since then to renumber I-238 to I-480. In September 2006, a project began to reconstruct the entire length of I-238, including a reconfigured interchange with I-880 and an added travel lane in each direction. Additionally, almost all of the bridges and overpasses were replaced with new ones meeting current earthquake resistance standards. The project was completed in October 2009, six months ahead of schedule.
