Like the eastern end of Interstate 580, the northern end of I-505 is located in a sparsely populated area in the Central Valley. Traffic counts along the route are some of the lowest of any California Interstate, with just 10,700 vehicles per day (vpd) recorded by Caltrans at the north end in 2015. Completion of the SR 113 freeway between I-80 at Davis and I-5 at Woodland in 1990 may have deferred some of the I-505 traffic headed to communities northeast of Vacaville along Interstate 80.
Interstate 505 was signed as Temporary I-505 between Vacaville to Dunnigan until 1972. The route consisted of a two-lane road prior to that time, with two short freeway segments serving interchanges with SR 16 and SR 128. Staged construction through 1978 upgraded the remainder of I-505 to freeway standards. Completed first was the section from SR 16 to SR 128, followed by I-5 south to SR 16, and SR 128 south to I-80.1
The state of California entered a franchise agreement with the California Toll Road Company (CTRC) to develop the Mid-State Tollroad along the planned SR 84 and SR 239 corridors between I-680 at Sunol and SR 4 at Antioch. This included a connection to the I-580 / 205 interchange near Tracy via future SR 239. An extension of the toll road included a northern segment into the Sacramento / San Joaquin River Delta.2
Had this extension been constructed, potential corridors included possible connections with the southern terminus of Interstate 505, the SR 113 freeway near Davis, or a route along SR 84 toward Sacramento. However due to environmental concerns north of Antioch, costs of bridges spanning the San Joaquin and Sacramento Rivers, and political opposition, none of these proposals came to fruition.3 Caltrans eventually deleted portions of the project, including the extension north to Solano County, in 1993.2
The rest of the proposed toll road remained in question, having been suspended due to political considerations. The project web site referenced a $600 million, 40 mile or more four-lane toll road winding northeast from I-680 near Sunol to SR 4 near Antioch.2