Illinois Route 100
|Maintained by IDOT|
|Length:||159.09 mi (256.03 km)|
|Existed:||1924 – present|
|South end:||US 67 in Alton|
| I-72 / US 36 in Bluffs
US 67 in Meredosia
US 67 / IL 103 in Frederick
US 24 / US 136 in Lewistown
|North end:||IL 78 in Dunfermline|
|Counties:||Madison, Jersey, Greene, Calhoun, Pike, Scott, Morgan, Cass, Schuyler, Fulton|
Illinois Route 100 (abbreviated IL-100) is a 157 mile (251 km) highway in the southwest Illinois. It generally parallels the Illinois River. Starting in downtown Alton, it trends northward to Buckheart Township near Canton. It makes up much of the Illinois River Road, a U.S. National Scenic Byway. Illinois 100 is 159.09 miles (256.03 km) long.
Route description 
The southern end in Alton is at US 67 where Broadway, Landmarks, and Piasa Streets come together. It follows the east bank of the Mississippi River and Illinois River through Grafton until Hardin, where it crosses the Joe Page Bridge across the Illinois River. A portion of the Great River Road was on an old railroad alignment, which you can see parts of it just north of Alton. During periods of high water, this highway is susceptible to flooding.
North of Hardin, the highway follows the west bank of the Illinois River until the bridge near Florence. From that point on, IL 100 follows mostly farmland on much higher ground until Beardstown, where it will cross the river a third and final time. It will again follow the west bank of the Illinois River until US 136. After meeting with US 24 in Duncan Mills, IL 100 and US 24 is cosigned until Lewistown. North of Lewistown, IL 100 heads in a north-east alignment until its northern terminus with IL 78 near Dunfermline.
Illinois River Road 
Illinois Route 100 overlaps the Illinois River Road from Grafton to Detroit.
IL 100 originally went from Godfrey from the intersection at Godfrey Road and Homer M. Adams Parkway (IL 111/US 67) to Grafton on the current IL 3. In 1987, IL 3, 143, and 100 alignments were changed to their current routes in the River Bend (Illinois) area when the Homer M. Adams Parkway Extension was opened. On June 28, 2012, the Florence Bridge over the Illinois River was closed for at least 9 months by IDot due to difficulties with the lifting mechanism that allows barge traffic to pass under the bridge. Inspections suggested that continued use of the lift span could harm the structural integrity of the bridge. As barges need to pass under the bridge, the lifting span was raised up and the bridge was closed. Motorists wishing to cross the river must use the signed detour utilizing Illinois Route 106, US Route 54, and Interstate 72/US Route 36. Plans have not been made whether to repair, replace, or permanently close the bridge.
Major intersections 
||Alton||0.00||0.00||US 67||Southern terminus of IL 100|
||Grafton||IL 3 south||Northern terminus of IL 3|
|Richwood Township||IL 16 east||Eastern end of IL 16 concurrency|
||Hardin||IL 16||Western terminus of IL 16|
|Kampsville||IL 96 north||Southern terminus of IL 96|
|IL 108 east||Western terminus of IL 108|
||Detroit||IL 106 west||Western end of IL 106 concurrency|
||IL 106 east||Eastern end of IL 106 concurrency|
|I-72 / US 36|
||US 67 south / IL 104 east||Souhern end of US 67/IL 104 concurrency|
|IL 104 west||Northern end of IL 104 concurrency|
||Beardstown||IL 125 east||Western terminus of IL 125|
||US 67 north / IL 103 west||Northern end of US 67 concurrency; eastern terminus of IL 103|
||Isabel Township||US 136 east||Eastern end of US 136 concurrency|
|US 24 west||Southern end of US 24 concurrency|
|US 136 west||Western end of US 136 concurrency|
|Lewistown||US 24 east / IL 97 south||Northern end of US 24 concurrency; southern end of IL 97 concurrency|
|IL 97 north||Northern end of IL 97 concurrency|
|Dunfermline||IL 78||Northern terminus of IL 100|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|