SBI Route 143 ran from
Benton to
Harrisburg; this was replaced by
Illinois Route 34 when it was extended north in 1937. That same year, IL 143 was applied to a former lettered route,
Illinois Route 127A from Highland to Illinois 127. In 1950, it was extended west to Edwardsville. In 1964, it was extended west again to Alton, replacing
Illinois Route 159. There have been no changes since. In 1987, the alignments of
Illinois Route 3,
Illinois Route 100, and Illinois Route 143 were switched to their current routes in the Alton area with the opening of the extension of the Homer M. Adams Parkway and the construction of Madison Avenue in Wood River. IL 143 was put on the new Madison Avenue alignment and routed on the Berm Highway, while IL 3 was put back on an older alignment through East Alton and the Homer Adams Parkway (later taking over the old IL 100 alignment north of Godfrey to Grafton), and IL 100 was put on then-unmarked River Road from Alton to Grafton. The old alignment of IL 143 (Alton-Edwardsville Road) became unmarked. == Major Intersections ==