Thanks to Adam Froehlig for providing this scan from the Bureau of Public Roads' (precursor to FHWA) Yellow Book of proposed Interstates.
A little background - the Interstate system had been conceived before the eventual 1956 signing of the plan; and general highway proposals were drawn up immediately after the war.
Here, we see the current I-280/680 beltway(including I-780, which was a former routing of 680), I-580, and I-80. But within San Francisco, there are some definite routings (although no numbers are shown):
1. I-80 - Starting at what appears to be I-280 in the plan (CA 1 in real life), continuing eastward on the Western Freeway corridor, then the short multiplex with US 101 on the Central, and finally the then-US 40/50 Skyway.
2. I-280: From San Jose to the current CA 1 north split, no real changes; but at the split, instead of following former CA 82 and 87, it continued north on the 19th Avenue CA 1 corridor to the current US 101/CA 1 interchange.
3. I-480: The Embarcadero Freeway, plus Doyle Drive (the 101 freeway east of CA 1).
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