The HB has CA 123's south end at San Pablo Ave. in Oakland, where it crosses under I-580 west of exit 19B. That's what I had thought, based on CA 123's legislative definition ("Route 123 is from Route 580 at San Pablo Avenue in Oakland to Route 80 in Richmond at Cutting Boulevard", quoting
Section 423 of the California Streets and Highways Code). Based on that, the one time I know I traveled CA 123, I took I-580 west to exit 19B, following one of I-580's frontage roads about 0.35 mile west to its intersection with San Pablo Ave.
Upon further review (including neroute2's usaca peer review for CA 123 -- BTW, this is the last leftover peer review item), it seems that Caltrans has CA 123's connection to I-580 following a different path, between I-580 exit 19A and San Pablo Ave. via MacArthur Blvd.'s frontage roads. This is supported by:
-- Caltrans' Postmile Query Tool, which also indicates that CA 123 doesn't extend south of MacArthur Blvd. on San Pablo Ave.
-- Caltrans' bridge log for CA 123, showing a bridge at Peralta St., which does not cross over or under San Pablo Ave., but does cross over MacArthur Blvd. (and intersects its frontage roads)
OSM follows the routing via MacArthur Blvd. Google Maps doesn't, it has CA 123 following San Pablo Ave. south of MacArthur to I-580 exit 19B. Other online maps are ambiguous, or don't show route numbers at all.
Signage on the ground doesn't point to one routing or the other. CA 123, like other routes in California's densely-populated urban areas, has few route markers. There are no CA 123 markers at or near I-580 exits 19A or 19B. There is one on the
westbound MacArthur Blvd. service road at its intersection with San Pablo Ave., indicating that a right turn puts you on CA 123 northbound. However, that doesn't mean much, since you can't turn south onto San Pablo, or continue straight ahead to the service road west of San Pablo.
Caltrans might've fudged the legislative route 123 definition because there are no ramps between the San Pablo/580 intersection and I-580 to the west, nor are there direct ramps to or from that intersection and I-580 to the east. One can follow the 35th St./36th St. frontage roads about 0.35 mile to make the latter connection, as I did. But the MacArthur Blvd. connection to 580, serving traffic to/from WB 580 and I-80, is shorter at about 0.26 mile.
