^Westbound trailblazer leaving Interstate 255 near Centreville, February 2023.
Illinois Route 15 runs east-west across the entire width of southern Illinois, from East Saint Louis through Mount Vernon to Mount Carmel on the Wabash River.  This closely parallels the Interstate 64 corridor.

The western end of Illinois 15 is in East Saint Louis, near the combination of Interstates 55 and 64...
At Alorton, Illinois 15 meets the northern end of Illinois 163:
Illinois 15 becomes a bit of an expressway entering the Belleville area, as it meets a full cloverleaf interchange with Interstate 255:
At Centreville, Illinois 15 has exits and onramps to Illinois 157 in a classic diamond configuration...
Reaching Belleville, Illinois 15 has a partial interchange with Illinois 13 (Illinois 15 west and Illinois 13 cannot access each other)...
Illinois 15 meets an interchange to Illinois 158, where the ramps meet Illinois 158 at a pair of roundabouts...
Illinois 15 then meets another interchange, to the main north-south route through Belleville: Illinois 159...
As Illinois 15 leaves Belleville, Illinois 13 joins from the west, retiring from being the local alternate to Illinois 15...
At Freeburg, Illinois 15 and Illinois 13 split up, with Illinois 13 taking the more southeasterly route...
At the small town of Fayetteville, Illinois, Illinois 4 joins Illinois 15 from the north...
At the next town, St. Libory, Illinois 15 decides to deviate from Illinois 4 to head due east...
Five miles east of St. Libory, Illinois 15 meets the northern end of Illinois 153:
Next, Illinois 15 meets the southern end of Illinois 160 (I don't know why Illinois Routes 153 and 160 aren't the same route)...
At Nashville, Illinois 15 crosses Illinois 127...
US Route 51 comes from the south to join Illinois 15 east into Ashley, Illinois:
Exiting Ashley, Illinois 15 continues east while US Route 51 splits to resume going northbound...
As Illinois 15 reaches Mount Vernon, it encounters a diamond interchange with Interstates 57 and 64, which are concurrent:
Illinois 15 acts as a pair of one-way streets in inner Mount Vernon, and they cross Illinois 37 in the center of town:
At Wayne City, Illinois 15 meets the northern extent of Illinois 242:
As Illinois 15 approaches Fairfield, US Route 45 comes from the north to join it eastward into town:
Then, while inside Fairfield, Illinois 15 sends US Route 45 to continue southward...
In Albion, Illinois 15 joins Illinois 130 north for one block:
Then, Illinois 15 leaves Illinois 130 to head easterly on that next Albion street to the north:
In Mount Carmel, Illinois 15 turns southerly onto Illinois 1...
After six blocks (from 9th Street to 3rd Street), Illinois 1 departs from Illinois 15, sending Illinois 15 toward the Wabash River:
Illinois 15 leaves Mount Carmel, and the state of Illinois, crossing the Wabash River into Indiana, where it becomes Indiana State Route 64...
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