The sign is actually posted on Illinois Highway 158, not on the interstate. If you take I-64 east to the Highway 158 exit, the sign is posted just before the I-64 Westbound exit. This is the overpass where they were killed. The sign couldn't be placed on I-64 as it's a federal roadway, so the State put it as close on a state highway as they could.
Next goal - renaming the interstate between Mascoutah and Highway 158 after them. It just so happens I'm going to meet with the Secretary of Transportation next week in Chicago, so I'll ask him how I go about getting the highway named for them.