US Route 40 closely parallels the route taken by Interstate 70 across Illinois, connecting directly from St. Louis, Missouri, to Terre Haute, Indiana. This runs along an east-northeast angle from the St. Louis area through places like Vandalia and Effingham, to cross the state border into Indiana near Terre Haute. One of the first major highway corridors across the United States was the National Road, and US Route 40 very closely follows the original route of that road.
US Route 40 crosses into Illinois from downtown Saint Louis, Missouri, using the Poplar Street Bridge that is shared with Interstates 55 and 64...
US Route 40 crosses into Illinois from downtown Saint Louis, Missouri, using the Poplar Street Bridge that is shared with Interstates 55 and 64...
At East Saint Louis, an interchange along Interstate 55 and US Route 40 trades Interstate 64's eastern continuation for a new concurrency with Interstate 70...
Interstate 55, Interstate 70, and US Route 40 run together through several more interchanges (for details, see the page for Interstate 55), but near Troy, US Route 40 splits away and heads east as an Illinois surface road for the first time...
On the east side of Troy, US Route 40 meets the eastern end of Illinois 162:
As US Route 40 approaches the village of St. Jacob, it passes under Illinois 4, but a connector road in the northeast quadrant of the crossing provides a link...
In Highland, Illinois, Illinois 143 joins US Route 40 from the northwest...
Soon after that, Illinois 160 also joins US Route 40 and Illinois 143 east...
The triple concurrency doesn't last long, though, and still within the town of Highland, Illinois 160 splits from Illinois 143 and US Route 40 to head south again...
After US Route 40 and Illinois 143 have gained a couple miles of distance away from Highland, the two routes split up...
Just north of the split away from Illinois 143, US Route 40 joins Interstate 70 east...
In Pocahontas, US Route 40 splits from Interstate 70 to head east as a surface road once again...
In Greenville, US Route 40 crosses Illinois 127...
At Mulberry Grove, US Route 40 meets the eastern end of Illinois 140...
On the west side of Vandalia, US Route 40 crosses over to the south side of Interstate 70, with an interchange...
Still in Vandalia, US Route 40 is joined by Illinois 185, which comes from the northwest:
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Just north of downtown Vandalia, US Route 40 and Illinois 185 both join US Route 51 south:
All three routes leave Vandalia crossing the Kaskaskia River, and US Route 51 promptly splits from US Route 40 and Illinois 185:
About five miles east of Vandalia, Illinois 185 splits from US Route 40 to go southeasterly again:
Immediately after Illinois 185 reestablishes its independence, US Route 40 crosses another interchange with Interstate 70:
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After passing through St. Elmo, US Route 40 meets Illinois 128, which joins from the north:
At Altamont, Illinois 128 splits from US Route 40 to head south a bit more...
After passing under Interstates 57 and 70 with no interchange, US Route 40 meets Illinois 33, which joins US Route 40 eastward, at a point in Effingham that is also the southern end of Illinois 32 (turning westerly onto Fayette Avenue here will provide access to Interstates 57 and 70):
In Effingham, US Route 45 joins US Route 40 and Illinois 33 eastward...
After about three blocks, US Route 45 splits from US Route 40 and Illinois 33 to head north again:
After just another three blocks in Effingham, Illinois 33 splits from US Route 40 to head southeasterly:
In Greenup, US Route 40 crosses Illinois 130, two blocks south of Illinois 121's southeastern terminus:
In Casey, US Route 40 crosses Illinois 49:
US Route 40 then crosses Illinois 1 in Marshall...
Just over one mile west of the Indiana border, US Route 40 joins Interstate 70 eastward once again...
This short concurrency with Interstate 70 carries US Route 40 over the Indiana state line, to eventually split up and head northeastward toward downtown Terre Haute...
