As always, I do suggest you consult my pages first:
Originally, there was also an I-5W. This routing dates back to the original definition of I-5 in 1947. At that time, I-5 was defined to run along the present-day Route 99 routing from N of Los Angeles to Sacramento. I-5W was defined to run along a routing that corresponds to present-day Route 120, I-205, I-580, I-80, and I-505. US 50 was multiplexed on the I-580 section. The route was resigned to the present-day route numbers in 1964 as part of the regularization of state and legislative route numbers. Note that the CalTrans history shows that I-505 and I-580 were approved as interstate the same time as I-5 in 1947, but that I-205 wasn't defined until 1957, when the West Tracy bypass was constructed. However, it appears the three-digit routes were not signed until 1965. Perhaps this was done to avoid confusing the travelling public, as the interstate signage was new (and before 1965, coexisted with the pre-1964 route signage). According to Calvin Sampang, one issue of California Highways and Public Works has a picture showing an I-5W shield on a segment of present-day I-580.
A proposal unearthed by Richard Moeur from the AASHTO files indicates that, at least in 1957 and 1958, there was at least a proposal for an I-5E. These proposal had I-5 running along the traditional alignment (Route 99 in 1957; "Westerly Alignment" in 1958) until either Modesto (1957) or Tracy (1958). The route then split, with I-5W going off as described above, and I-5E continuing along present Route 99 (1957)/I-5 (1958) into Sacramento. Evidently, AASHTO liked the routings, but didn't like I-5E, because that was never signed.
If you think about it, the suffixed routes could not survive the great renumbering, due to the way legislative routes were defined. This is why in 1964 they become 505 and 580.
As for the AASHTO decision, again, the information is on my pages from the history of the interstates in California, produced by Caltrans:
http://www.cahighways.org/itypes.htmlRoutes submitted June 27, 1945
I-5
1: Mexican border to Oregon state line (via Route 99)
Westside Freeway Relocation: May 1957
14-Aug-1957 I-5, I-5E Tentatively Approved
08-Nov-1957 Proposed as I-11
07-Aug-1958 Back to I-5
10-Nov-1958 Approved as I-5, I-5E (Sacramento)
1 Caltrans documents show this as being approved in 1947, although later documents the number having been approved as of October 1956. In a letter dated November 8, 1957, G.T. McCoy of the Department of Highways recommended that I-5 become I-11, to allow assignment of I-3 and I-5 to the Bay Area (for the San Francisco Bay circumferential routes, which later became I-280 and I-680). However, by August 1958, the Department of Highways had gone back to recommending I-5. Other letters objected to proposed I-5W through the Bay Area, because the suffix would be confusing. However, the original assignments did result in I-5 and I-5W, although some maps show I-5 and I-5E.
I-505
4 I-80 to I-5, Vacaville to Dunnigan
14-Aug-1957 I-5W Tentatively Approved
08-Nov-1957 Proposed as I-7
01-Apr-1958 Proposed as I-115
07-Aug-1958 Proposed as I-5W
10-Nov-1958 Approved as I-5W
01-Jul-1964 Renumbered as I-505
I-580
4 I-80 to I-5, Oakland to Modesto
(Oakland adjustment, MacArthur Freeway, Sept. 1995)
(Tracy adjustment, November 1957)
14-Aug-1957 I-5W Tentatively Approved
08-Nov-1957 Proposed as I-72
07-Aug-1958 Proposed as I-5W
10-Nov-1958 Approved as I-5W
01-Jul-1964 Renumbered as I-580
4 Originally, these routes were numbered as I-5W. In the map that accompanied the November 1957 letter proposing I-76, I-505 is shown proposed as I-7, and I-580 is shown as I-72.