Here is the answer to the question about U.S. 40 in the subject issue.
The so-called 1928 Northern Route of the Lincoln Highway followed U.S. 40 from Sacramento to Oakland. It is interesting, however, that from November 11, 1926 until June 8, 1931 (effective January 1, 1932), U.S. 40 was still officially routed through Benicia and Martinez. Some of the route books for that period, including a July 1928 AAA Transcontinental Guide, showed the Vallejo routing for U.S. 40 after the Carquinez Bridge opened on May 21, 1927. A December 1928 State Map, a 1929 Texaco Map and both the 1927 and 1929 AASHTO log, however, still show the U.S. 40 routing through Martinez.
On June 8, 1931, AASHTO approved a change in U.S. 40 in California, as follows:
From Cordelia to Vallejo and State [Legislative] Route 14 via Napa Ye [sic], shall be known as Temporary U.S. 40. When the new section of road from Cordelia to Vallejo via American Canyon is constructed the signs shall be changed to that route and designated U.S. 40. [This is the present day path of I-80.]
Today this route can be found by taking SR 12 west (Jameson Canyon Road) to the old Napa Wye or “Y”, turning south on SR 29 (Napa-Vallejo Highway) then into Broadway and Alameda streets in Vallejo.
The delay until 1931 of the “official” relocation of U.S. 40 to the Vallejo route could have been due to several factors, including: LRN 74, which became SR 29 when the routes were signed in 1934, was not defined until 1931 and, in the early days of the U.S. Numbered System, U.S. Highways could not be routed over a privately owned toll bridge, such as the Carquinez