Illinois Route 255 (IL 255), also referred to as the Alton Bypass, is a northwesterly extension of Interstate 255 (I-255) in southwestern Illinois in the St. Louis metropolitan area. IL 255 starts at I-270 in Pontoon Beach and ends at U.S. Route 67 (US 67) in Godfrey, at a total length of approximately 23.3 miles (37.5 km). It was constructed in four segments opening from 1998 to 2012 at a total cost of $165.2 million.

Interstate 255 (I-255) is a bypass route of I-55 in Greater St. Louis. Along with I-270, it forms a loop around the central portion of the bi-state metro area; a majority of I-255 is located on the Illinois half. It shares its southern terminus with I-270 at the junction with I-55; I-270 and I-255 go to the west and east, respectively. U.S. Route 50 (US 50) joins I-255 at Lemay Ferry Road. It crosses the Mississippi River on the Jefferson Barracks Bridge, enters Illinois, and then turns northeast. There is a junction with I-64, where US 50 breaks off and goes east, and then further north there is a junction with I-55 again, I-70, and US 40, all of which are all cosigned together. The next junction is another one with I-270, which is the northern terminus of I-255. After that, the limited-access freeway continues on as Illinois Route 255 (IL 255). I-255 is 30.82 miles (49.60 km) in length.