Interstate 505 California
Interstate 505 in California connects I-80 in Vacaville with I-5 near Dunnigan. Generally rural, the freeway provides part of a long distance route linking the San Francisco Bay area with the Pacific Northwest. North from industrial park areas of Vacaville, I-505 stays east of the English Hills through a mix of ranch and agricultural areas to the outskirts of Winters, a city of around 7,000. Beyond there, the route bee lines north to SR 16 near the community of Madison, where sunflowers grace the area landscape, providing a truly magnificent site during the peak flowering period.
Crossing Cache Creek, Interstate 505 makes a gradual shift to the east, traversing farm land east of Hungry Hollow into the Dunnigan Hills. Emerging from the hills at Oat Creek, the final segment ties into I-5 amid agricultural land in northern Yolo County.
I-505 is unusual for a California Interstate in that it was not predated by a U.S. highway or state route, as most others in the state were. It was included as part of Interstate 5W until 1964. The planned route for I-5W would have followed:
- Interstate 580 west from I-5 near Vernalis to Hayward
- Interstate 580 northwest from Hayward to Oakland
- Interstate 80 northeast from Oakland to Vacaville
- Interstate 505 north from Vacaville to Zamora
Source: December 31, 2021 Interstate Route Log and Finders List
I-505 Annual Average Daily Traffic (AADT)
Source: Caltrans 2017 Traffic Volumes : Route 505-980
Interstate 505 was initially built as a two lane road, with two freeway sections: two miles outside Winters, with an interchange at SR 128, and one mile at the interchange with SR 16 near Madison.1
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 south end of Interstate 505, the freeway along SR 113 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 running northeast from I-680 near Sunol to SR 4 near Antioch.2
North End – near Dunnigan, California
South End – Vacaville, California
South End Throwback
I-80 used to overtake Monte Vista Avenue at a parclo interchange one half mile west of I-505. Monte Vista Avenue is the historic alignment of U.S. 40, and per USGS topos it was already replaced by a four lane bypass for U.S. 40 by 1953. This bypass is now I-80, and the former ramps pictured here were replaced with a new interchange for Allison Drive. Photo by Michael Summa (1976).
- California Highways (www.cahighways.org): Route 505.
- Mid-State Tollway.
http://www.dot.ca.gov/hq/ paffairs/about/toll/midstate.htmCalifornia Department of Transportation (Caltrans), project web site.
- Rouse, Joe.
Page updated February 6, 2020.