Illinois 15 is a long east-west route, almost spanning the entire state. It runs from East St. Louis to Mt. Carmel, on the Wabash River.
Intersections from east to west:
IL15 starts at no route in particular, but on Missouri Ave. in East St. Louis
Next, IL15 forms the northern end of IL163 at Alorton
I-255 at Alorton (from here on, IL15 forms an expressway)
IL157 at an interchange
IL13**
IL158 at Belleville*
IL159 at Belleville*
Then IL13 joins from the northwest*** (Now IL15 is no longer an expressway)
IL13 breaks off at Freeburg
IL4 joins IL15 east at Fayetteville
IL4 goes off south again near St. Libory
IL153 at that route's northern end
IL160 at that route's southern end
IL127 at Nashville
US51 joins from the south
Soon after, US51 breaks off and heads north near Ashley
I-57, muliplexed with I-64, meets IL15 near Mt. Vernon
In Mt. Vernon, IL15 meets IL37
IL242 at its northern end in Wayne City (in Wayne County, too)
US45 joins from the north
Then US45 breaks off to head southerly again at Fairfield
IL15 joins IL130 north for one block in Albion
IL15 breaks off of IL130
IL15 follows IL1 south for about 1/2 mile in Mt. Carmel
Then IL15 breaks off and heads southeast across a bridge over the Wabash River. It becomes Indiana Route 64, which I believe is poorly numbered. Interstate 64 is just minutes south of it; that's 2 routes with the same number! I wouldn't blame anyone for being confused.
It passes through these towns: East Saint Louis, Alorton, Belleville, Freeburg, Fayetteville, St. Libory, Nashville, Ashley, Mount Vernon, Wayne City, Fairfield, Albion, Mount Carmel.
*- This indicates that IL15 meets this route with an interchange.
**-Eastbound traffic on IL13 can't turn onto IL15 westbound, and eastbound traffic on IL15 can't get onto IL13 westbound. (But who wants to go east and turn around the other way, anyway?)
***-This is like the intersection with 2 stars; traffic going east on IL13 can't access IL15 westbound, and vice versa.