Illinois 251 runs along an old alignment of US51 that was before I-39 was built. IL251 currently runs from near Kappa (just minutes north of Normal) to less than a mile from the Wisconsin border near South Beloit.
Intersections from north to south:
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IL173 at its western end by Machesney Park
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Business US20 in downtown Rockford
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US20 by Rockford
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IL72 near Davis Junction
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IL38 joins from the east
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IL38 leaves to the west at Rochelle
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I-88 near Rochelle
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US52 joins from the west
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US34 while US52 is multiplexed with IL251 at Mendota
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US52 breaks off to head easterly again
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US6 in Peru
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I-39/ US51 IL71, and IL351 at its southern end
IL18 near Lostant
IL17 joins from the east at Wenona
IL17 leaves to the west, just south of the town of Wenona
IL116 joins from the east
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IL116 leaves to the west
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US24 at El Paso
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IL251 ends at I-39/US51 near Kappa.
It passes through these towns: S. Beloit, Roscoe, Machesney Park, Loves Park, Rockford, Rochelle, Mendota, Peru, Tonica, Lostant, Wenona, Rutland, Minonk, El Paso, Kappa.