The southern terminus of I-505 is at I-80 next to the Nut Tree Airport in the Nut Tree area in Vacaville. From there, it travels north about 10 miles (16 km) through rural areas near the western edge of the Sacramento Valley before reaching Winters. I-505 then skirts the eastern city limits of Winters, intersecting with State Route 128—the only exit in Winters. After leaving that city, the Interstate then proceeds north through rural areas again for about 20 miles (32 km) until it reaches its northern terminus with I-5 near Dunnigan.
For its entire length, I-505 is a four-lane freeway (two lanes in each direction) with a maximum speed limit of 70 mph (110 km/h), which is typical for rural Interstates in California.
What is now I-505 was originally conceived as part of a loop Interstate with a directional suffix, I-5W.[5] However, I-5W and most of the other Interstates around the country with directional suffixes were eventually renumbered or eliminated, except I-35E and I-35W in Texas and Minnesota. I-69 segments I-69W, I-69C, and I-69E in southern Texas have since been designated as well along with proposed suffixed segments for future extensions of I-14 and I-27 to follow. The former route of I-5W now corresponds to I-580 from I-5 south of Tracy to Oakland, I-80 from Oakland to Vacaville, and I-505 from Vacaville to I-5 near Dunnigan.
The northernmost section of I-505, between SR 16 in Madison and I-5 near Dunnigan, opened in August 1977.[6]
^California Department of Transportation (October 2014). "Log of Bridges on State Highways". Sacramento: California Department of Transportation.
^"Article 2 of Chapter 2 of Division 1". California Streets and Highways Code. Sacramento: California Office of Legislative Counsel. Retrieved February 6, 2019.
^Federal Highway Administration (March 25, 2015). National Highway System: California (North)(PDF) (Map). Scale not given. Washington, DC: Federal Highway Administration. Retrieved July 29, 2017.
^Natzke, Stefan; Neathery, Mike & Adderly, Kevin (June 20, 2012). "What is the National Highway System?". National Highway System. Washington, DC: Federal Highway Administration. Retrieved July 1, 2012.