Finished up the first road blog which was on I-205:
http://surewhynotnow.blogspot.com/2017/11/november-bay-trip-part-1-interstate-205.html
It's a lot easier to see why US 48 was designated after driving I-205 and I-580. Short route aside the corridor is a hugely important one connecting the Bay Area to San Joaquin Valley...which was much better served when US 50 was extended IMO.
It seems the decision to extend US 50 over what was then US 48 had two rationales applied: the first was that when the Bay and Golden Gate bridge plans were finalized in the very early 1930's the Division of Highways proposed eliminating the E/W split of US 101 from San Jose to, respectively, ferry terminals in Oakland and San Francisco (the ferries rejoined the route at the Sausalito ferry terminal). There was at the same time a desire to extend US 50, which originally ended at the corner of 16th & L streets in Sacramento, all the way to the S.F. Bay; the rather clumsy multiplex SW on US 99 to Stockton was instituted, along with the subsumption of the former southern iteration of US 99W (today's I-5 south of Stockton and CA 120 east through Manteca); it then replaced US 48 to Hayward, the original western terminus of that route, and then used the original 101E alignment to reach the ferry terminal at the foot of Broadway in Oakland, a configuration that lasted a few years until the Bay Bridge was opened, when it was rerouted to, along with US 40 coming in from the north, the eastern approach to the bridge before crossing the Bay and, for a while, terminating at Bryant and 10th Streets (the latter US 101) in San Francisco. In short, the US 48 designation and signage didn't last long enough to make an impression on the driving public.