Interstate 255

I-255, west of IL 3

I-255's counterclockwise terminus is at an interchange with I-55 and I-270 in Mehlville, Missouri. The beltway is designated as I-270 (with Kansas City as the control city) west of this interchange and as I-255 east (with Chicago as the control city) east of this interchange. The section of I-255 east from this interchange to the Jefferson Barracks Bridge was first built in the 1960s to carry U.S. Route 50 traffic across the Mississippi River. The roadway was upgraded between 2002 and 2004. Repairs included both new pavement and the replacement of the Telegraph Road bridge.

Once it crosses the Mississippi River and enters Illinois, I-255 is signed as a north/south highway. It is considerably newer than the Missouri section, having been constructed in the 1980s. At the Illinois Route 3 interchange, I-255 turns sharply to the north. It is briefly duplexed with Illinois 3 in Dupo. After Dupo, I-255 turns northeast, skirting Cahokia and East St. Louis. An interchange at Mousette Lane in Sauget was constructed in the 1990s and provides motorist access to GCS Ballpark (home to the Gateway Grizzlies baseball team) and the St. Louis Downtown Airport. Given its proximity to the airport, this section of I-255 features shorter light poles to better accommodate takeoffs and landings.

North of Sauget, I-255 traverses a portion of the Frank Holten State Recreation Area before an interchange with Interstate 64 west of Caseyville. This section of freeway opened in 1986.[2] Traffic can enter and exit I-255 at an interchange with Collinsville Road (formerly U.S. Route 40) just east of the Fairmont Park Racetrack.

I-255 meets I-55/I-70 at an interchange in Collinsville, its original intended terminus. North of this interchange, the control city for I-255 changes from "Chicago" to "To I-270". In the 1980s, I-255 was extended approximately five miles north to meet I-270 in Pontoon Beach, its current terminus. While the I-255 designation ends at the I-270 interchange, the freeway continues north and west (parallelling the bend in the Mississippi River) for approximately 23 miles as Illinois Route 255. The decision to not extend the I-255 designation to the new freeway reflects the fact that state and not federal funds were used to pay for construction.

I-255 route was proposed from I-55/I-244 near Green Park to I-55/70 west of Cahokia Mounds[3] near the intersection with IL 111 when plans surfaced in the 1950s Yellow Book. It was originally designed to be a four lane highway but was changed to six lanes.[4]

Since that routing was to go through the American Bottoms, archaeological investigations had to be conducted prior to any construction, which would later become known as the FAI 270 Series. When Cahokia Mounds was designated in the federal register, an alternative alignment for I-255 from I-64 to I-55/70 was selected.[5] This delayed construction of I-255 until the late 1970s, making it the last interstate highway in metropolitan St. Louis to be built.

Both the western (I-270) and eastern (I-255) portions of the interstate loop around St. Louis were originally designated as I-270. However, the route that Illinois selected ("Corridor 413"[6]) included an interchange in Pontoon Beach between the new beltway (running north and south parallel to the Mississippi River) and the existing beltway (running east and west to a terminus at I-55/I-70). As such, two intersecting interstate highways would have carried the same route number.[7] As such, Illinois' Department of Transportation (IDOT) concluded a different route number would be necessary for the eastern leg of the beltway. While I-870 was initially considered,[8] informal use of I-255 in contemporary local media coverage led to its widespread adoption. By 1980, IDOT announced that the eastern portion of the St. Louis beltway was to be designated as I-255.