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 ten miles 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 until it reaches its northern terminus with I-5 near Dunnigan.
For most of its length, I-505 is a four-lane freeway, 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. However, I-5W and most of the other Interstates around the country with directional suffixes were eventually renumbered or eliminated, sans I-35E and I-35W in Texas and Minnesota. 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.