^Westbound trailblazer at Illinois Route 1 in Gallatin County, March 2023.
Illinois Route 13 is mainly an east-west route in southern Illinois. It runs west from Shawneetown, on a north-south segment of the Ohio River, west through Marion and Carbondale; then it turns northerly out of Murphysboro along Illinois Route 127 for a long distance before redirecting northwest out of Pinckneyville to end by East St. Louis. I don't know why the northwest-southeast segment from the Metro East to Pinckneyville is even the same number as the east-west section from Murphysboro east to Shawneetown, but I guess we just ended up with a route having multiple personalities.
Centreville is where Illinois 13 gets its start heading eastward, from Illinois 163, a north-south route that seems to have little purpose besides giving Illinois 13 a western endpoint; Illinois 157's northbound side is concurrent with Illinois 13's initial eastward pathing: