Illinois Route 251
|Maintained by IDOT|
|Length:||135.32 mi (217.78 km)|
|Existed:||Early 1970s – present|
|South end:||I-39 / US 51 in Hudson|
| US 24 in El Paso
I-39 / US 51 in LaSalle
US 6 in Peru
I-80 in Peru
US 52 in Troy Grove
US 34 in Mendota
US 52 in Mendota
US 30 in Compton
I-88 / IL 110 in Rochelle
US 20 in Rockford
|North end:||US 51 / IL 75 in South Beloit|
Illinois Route 251 is a north–south state road that runs on the former alignment of U.S. Route 51 before Interstate 39 was built in central Illinois. It runs from U.S. 51 at the border with Wisconsin to I-39 and U.S. 51 south of Kappa. Illinois 251 is 135.32 miles (217.78 km) long.
Illinois 251 runs parallel to Interstate 39 and U.S. 51 for most of the length of I-39 in Illinois. Like Illinois Route 351, it can be considered a spur, loop or alternate of its parent route, U.S. 51. Illinois 251 terminates about 10 mi (16 km) north of the southern terminus of I-39.
The road parallels the east bank of the Rock River through Rockford, where the highway goes north on Eleventh Street, west on Harrison, north on Kishwaukee, west on Walnut then split between northbound Third and southbound Second through downtown Rockford. After donwtown, both directions take the southbound lanes' Second Street name as a divided urban expressway (also badged as Martin Luther King Jr. Memorial Drive) for a couple of miles north of downtown Rockford. It stays Second Street through Loves Park, Machesney Park and Roscoe until its north endpoint at US 51 and Illinois 75 in South Beloit
Illinois 251 was established in 1984 when first segment of I-39 opened between Rockford and Bloomington. In 1992, the final segment of I-39 opened north of Bloomington. The remaining stretch of Former U.S. 51 became Illinois 251. The initial designation of I-39 was U.S. Route 51 until 1992.