As part of a resigning project which commenced in 2016, references to I-80 Bus on the western segment were being removed, subsequently leaving that portion only signed as US 50.

[5] The I-80 Bus/US 50/unsigned I-305 segment then runs approximately 5.63 miles (9.06 km) from I-80 west to SR 99 southeast of Downtown Sacramento.

Unlike the US 50 segment of I-80 Bus, California Streets and Highways Code § 351.1 mandates that "Route 51 shall be signed Interstate Business Loop 80".

[3] At the interchange southeast of Downtown Sacramento, I-80 Bus turns north onto unsigned SR 51 near its southern end.

A partially built portion of a never completed replacement freeway for SR 51, in the median of I-80, now serves as parking and access for the northernmost three stations (Watt/I-80, Watt/I-80 West, and Roseville Road) on the Sacramento Regional Transit District's light rail Blue Line.

If this replacement freeway had been completely built as originally planned, I-80 would have continued south following the railroad tracks going through the Ben Ali neighborhood of North Sacramento, crossed over SR 160, and joined with the Elvas Freeway portion just north of A Street.

The State Division of Highways (predecessor to Caltrans) constructed Sacramento's freeways system incrementally from the 1940s to the 1970s.

[12] The Sacramento City Council voted in September 1979 to delete the I-80 bypass freeway from the Interstate System.

Unlike most business routes in California, which run along locally-maintained streets through a downtown area, I-80 Bus was not assigned to the pre-freeway alignment of US 40, but to a freeway.

Caltrans does not normally use the I-80 Bus designation, except for signage and other related concepts like Cal-NExUS exit numbers (which are continuous along the business loop).

[16] Under the California Streets and Highways Code § 351.1, "Route 51 shall be signed Interstate Business Loop 80".

[17] Despite Caltrans's official signage and reporting practices, mapmakers may still show the I-305 and SR 51 designations, as well as the I-80 Bus/US 50 concurrency on the western segment.

The SR 99 concurrency, running along US 50 and I-5 to northern Sacramento, is also not officially designated by Caltrans, but mapmakers will still also often show it as such.