There are gaps in the alphabetic values to allow for future system expansion or for decommissioned routes.

The alphabetic naming suffixes are included as small letters on the bottom of route shields.

U.S. Highway 90 Alternate is an alternate route to U.S. Highway 90 in the U.S. state of Texas, running from west of Seguin east via Seguin, Gonzales, Hallettsville, Eagle Lake, Rosenberg and Sugar Land to northeastern Houston.

US 90-U is a business route from I-610 in Houston northeastward to FM 2100 in Crosby along portions of McCarty St. and the Beaumont Highway in northeastern Harris County.

[2] The portion of the business route between I-610 and Beltway 8 remained concurrent with US 90 until the completion of the current US 90 along the Crosby Freeway in January 2011.

US 90-Y continues east along Green Ave past FM 1006 and then turns north along Simmons Dr. returning to I-10 and US 90 at Exit 878.

On the other hand, U.S. 90 runs along surface streets through New Orleans, crossing the Mississippi on the older and narrower Huey P. Long Bridge.

The road today serves largely as a commercial corridor for the northern suburbs of Pensacola.