Source: December 31, 2016 Interstate Route Log and Finders List
I-238 California Annual Average Daily Traffic
SR 185 Mission Bl
SR 185 Mission Bl
I-880 Nimitz Fwy (16.70)
Source: Caltrans, Traffic Operations Program - Traffic and Vehicle Data Systems 
The northern portion of the proposed Mission Freeway in 1971.
The west end of California 238 originally consisted of just a wye interchange with California 17. Note also that California 92 was proposed to run east from Hayward directly to Interstate 580. This was never built.
Looking at the south end of the proposed Mission Freeway through Fremont in 1971.
The Mission Freeway would have tied into Interstate 680 at its turn south from Sunol Grade. Grading from the planned interchange remains visible along I-680 today.
Interstate 238 is a short connector between Interstates 880 and 580. Signed north-south in conjunction with California 238, the freeway portion travels east-west. This is due to the original Mission Freeway plans, which called for California 238 to continue southeast as a freeway bypass of Hayward, then terminate at Interstate 680 just southwest of the Sunol Grade.
There is a section of Interstate 580 that prohibits trucks in Oakland. Interstate 238 provides part of a truck route between I-580 at Castro Valley and the San Francisco-Oakland Bay Bridge (Interstate 80) in conjunction with the Nimitz Freeway (Interstate 880) through Oakland.
Unlike most other three-digit Interstate routes, Interstate 238 has no parent route. The reason why I-238 exists as an Interstate Highway is due to the timing of its acceptance into the Interstate Highway System; at that time, all "branch" routes of Interstate 80 (180 through 980) were reserved for other routes. The route upgraded a preexisting stretch of California 238 and AASHTO approved the designation on June 20, 1983. The same meeting included the renumbering of Interstate 180 across the Richmond-San Rafael Bridge as an extension of Interstate 580. Considerations to number SR 238 as I-180 however would conflict as California does not duplicate its state routes (SR 180 refers to a long east-west state route through Fresno).
Interstate 238 was designated in 1984 as a result of California State Assembly Bill 2741, which modified the route numbering for California 17 and Interstate 580 in the state highway system. This legislation introduced I-238 and Interstate 980 in Oakland, as well as provided for an extension of I-580 to Marin County and the creation of a new Interstate 880 between San Jose and Oakland. Sign changes due to AB 2741 were implemented along all affected routes by 1985.
Originally California 238 was to continue as a freeway south from Interstate 580 through Hayward, but that plan was shelved indefinitely due to residential opposition and environmental concerns. A court decision in 2003 ended the plans for the recommended Mission Freeway along a green route, with subsequent plans calling instead for the upgrade of Mission Boulevard into an expressway, with potential interchanges at key locations. Stub ramps, since demolished, were previously located along I-680 west of Exit 15 (Washington Boulevard) for the unconstructed freeway.
Southern Terminus - Interstate 580 - Hayward, California
Perspective from Interstate 238 south
Interstate 238 south defaults onto Interstate 580 east beyond the southbound off-ramp to California 238 (Foothill Boulevard). The cloverstack interchange between the two freeways includes wide grassy areas originally reserved for the freeway continuation south to Hayward. Photo taken 03/27/16.
A loop ramp (Exit 14) follows for Interstate 580 leading northwest to Oakland. I-580 connects the San Francisco Bay area with the Central Valley at Tracy. Photo taken 03/27/16.
Interstate 238 ends as I-580 turns east through Castro Valley for the climb east to Dublin Canyon. Exit numbers along I-238 reference the mileage of SR 238 south to Interstate 680 at Fremont. Photo taken 03/27/16.
Historical Perspective from Interstate 238 south
Replaced button copy signs posted at the southbound beginning of California 238 (Foothill Boulevard). SR 238 leads 14 miles south to Fremont. Photo taken 12/27/01.
Perspective from Interstate 580 west
Interchange sequence sign displaying I-238 to I-880 on westbound Interstate 580. The left lanes of I-580 west default onto Interstate 238 north at Exit 34 in 1.75 miles. I-238 and I-880 provide an alternate route to I-580 for the San Francisco-Oakland Bay Bridge and Interstate 80. Photo taken 11/26/04.
The forthcoming cloverstack interchange joins Interstates 580 and 238 with California 238 (Foothill Boulevard) to the southeast. There is no access to SR 238 south from I-580 west however. Strobridge Avenue (Exit 35) and Mattox Road instead provides a surface route to Foothill Boulevard south. Photo taken 11/26/04.
Westbound Interstate 580 prepares to exit, leaving the left lanes for northbound Interstate 238. Note lack of control cities and cardinal directions on overhead signs. Interstate 580 serves eastern Oakland and is the more direct connection to the East Bay Cities of Walnut Creek and Concord, by using California 13 (Warren Freeway) north to California 24 east (via the Caldecott Tunnel). Photo taken 11/28/04.
Three lanes of Interstate 580 veer northwest toward Oakland as the mainline defaults onto I-238 north to San Leandro. Previously there were only two through lanes for Interstate 238, but the Alameda County Transportation Sales Tax eventually funded widening of the freeway to three lanes per direction. Photo taken 11/26/04.
Perspective from California 238 (Foothill Boulevard) north
California 238 separates from Mission Boulevard and follows Foothill Boulevard north from Jackson Street to the junction with Interstates 238 and 580. The state route veers away from Foothill Boulevard onto the northbound lanes of the Mission Freeway stub in this scene. Photo taken 12/27/01.
SR 238 splits into ramps for I-580 west (using the Mission Freeway northbound roadway), I-580 east and I-238 south. Photo taken 12/27/01.
California 238 transitions to Interstate 238 south to San Leandro and I-880 as motorists depart for I-580 east to San Ramon Valley and ultimately the Central Valley and Interstate 5. Photo taken 12/27/01.
Perspective from Interstate 580 east
Advancing south from the 163rd Avenue on-ramp, Interstate 580 (MacArthur Freeway) next meets Exit 34 for both Interstate 238 north and California 238 north. I-238 is unmarked in this direction, with signs referencing only SR 238 or the connection to I-880 (Nimitz Freeway). Photo taken 08/22/03.
A two-lane off-ramp (Exit 34) departs Interstate 580 south for California 238 south to Hayward and I-238 north to San Leandro. I-580 swings east to overtake the ending I-238 through Castro Valley. Photo taken 08/22/03.
Exit 34 carries two lanes south directly to California 238 and Foothill Boulevard. This would have been the Mission Freeway mainline had it been constructed south to Hayward. A single lane ramp otherwise turns west onto Interstate 238 to I-880 for motorists headed south to San Mateo Bridge (SR 92) or San Jose. Vidcap taken 04/93.
Northern Terminus - Interstate 880 - San Leandro, California
Perspective from Interstate 238 north
Sound walls separate Interstate 238 from residential areas west of the Mission Boulevard (SR 185) ramps. The freeway continues west 0.75 miles to the flyover (Exit 16A) for Interstate 880 south. Photo taken 12/27/01.
This scene looked at the original four-lane freeway of I-238 over Ashland Avenue. The forthcoming exit for I-880 was expanded to two lanes by 2009. Photo taken 12/27/01.
A high speed flyover shuttles motorists onto Interstate 880 (Nimitz Freeway) south as the remainder of I-238 north continues to I-880 north. Photo taken 12/27/01.
Two local exits depart from Interstate 238 north for San Lorenzo and south San Leandro prior to the merge with I-880 north. Photo taken 12/27/01.
Perspective from Interstate 880 north
One mile south of the two-wye interchange (Exit 31A) with Interstate 238 south on Interstate 880 north. Photo taken 10/00.
Exit 30 leaves I-880 (Nimitz Freeway) north for Hesperian Boulevard and San Lorenzo just ahead of the off-ramp for Interstate 238 south. I-238 references the I-580 south control points. Photo taken 10/00.
Perspective from Interstate 880 south
A two lane off-ramp (Exit 31) departs Interstate 880 (Nimitz Freeway) south in 0.4 miles for Interstate 238 south. I-238 provides a connection to I-580 south for Stockton and Fresno in the Central Valley. Photo taken 03/27/16.
Exit 31 forms a collector distributor roadway through the parclo interchange with Washington Avenue. An unmarked ramp precedes the southbound ramp for I-238 from I-880 south. Photo taken 03/27/16.
A loop ramp adds traffic from Washington Street and San Leandro as Interstate 238 south passes below the I-880 mainline. I-238 ventures east through Ashland to merge onto I-580 south through Castro Valley. Photo taken 03/27/16.