Interstate 270 (I-270) makes up a large portion of the outer belt freeway in Greater St. Louis. The counterclockwise terminus of I-270 is at the junction with I-55 and I-255 in Mehlville, Missouri; the clockwise terminus of the freeway is at the junction with I-55 and I-70 north of Troy, Illinois. The entire stretch of I-270 is 50.59 miles (81.42 km).

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.