[1] Although this route may appear to be a derivative of US 40, it does not follow any of the old US 40 alignment except for a portion of the Historic National Road in Fayette County from US 40 to Vandalia.

In 2007, IDOT decommissioned the portion of Route 140 from near the Bond and Fayette County line at Mulberry Grove, east to Vandalia.

While that section of highway is no longer marked for Route 140, the state still maintains its Historic National Road signage.

The highways run along the northern border of East Alton before entering Bethalto, where they meet Illinois Route 255.

The highway runs through a rural area of Madison County, where it passes through the community of Meadowbrook and then intersects Illinois Route 159.