Both SR 51 and I-305 (the eastern and western segments of I-80 Bus, respectively) are part of the
National Highway System, Currently, I-80 Bus is the only Interstate business route in California to exist as a freeway.
US Route 50/Interstate 305 segment The western section of I-80 Bus begins in
West Sacramento at
I-80, where I-80 leaves the West Sacramento Freeway onto the Beltline Freeway, which travels north of the Bryte neighborhood, over the Sacramento River, and into the Natomas area. This interchange in West Sacramento is also the current west end of
US 50, The I-80 Bus/US 50/unsigned I-305 segment then runs approximately from I-80 west to
SR 99 southeast of
Downtown Sacramento, though only US 50 is signed in this portion. Beginning in 2016, signs along the western segment were being updated to remove references to I-80 Bus and instead sign the route only as US 50. However, Caltrans still lists this segment under the I-80 Bus exit list instead of the US 50 one, and mapmakers to this day may continue to sign both the eastern and western sections as part of I-80 Bus.
State Route 51 The unsigned
State Route 51 (
SR 51) is defined as follows in sections 351 and 351.1 of the
California Streets and Highways Code: 351. Route 51 is from
Route 50 in Sacramento to Route 80 east of Sacramento. 351.1. Notwithstanding Section 640, Route 51 shall be signed Interstate Business Loop 80. Despite the legal definition, exit numbers assigned along SR 51 start at 6 instead of 1 or 0, treating I-80 Bus as one continuous route. At the interchange southeast of Downtown Sacramento, I-80 Bus turns north onto unsigned SR 51 near its southern end. US 50 continues east, SR 99 heads south, and the unsigned I-305 ends. The elevated freeway carrying I-80 Bus east of downtown is between 29th and 30th streets, and an older section beginning at A Street and continuing northeast was originally known as the Elvas Freeway (originally signed as
US 99E). On this section of freeway, I-80 Bus crosses the
American River before its northeast-bound merge with the North Sacramento Freeway (the northern portion of
SR 160 toward downtown). The North Sacramento Freeway, originally a portion of
US 40, originally ran west from Marconi Avenue to an intersection with Del Paso Boulevard. The section from
Arden Way eastbound back to Marconi Avenue is now part of the currently named Capital City Freeway (while the SR 160 section retains the original freeway name). The original North Sacramento Freeway connected to the Roseville Freeway at the curve north of Marconi Avenue (commonly known as the Marconi Curve, where the freeway once ended at Auburn Boulevard). At the east end of I-80 Bus, the Roseville Freeway continues northeast to
Roseville as I-80, while the short unsigned
SR 244 heads east to Auburn Boulevard. I-80 heads in both directions around the north side of Sacramento. 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. ==History==