From St. Louis, first take I-55/70, 64 or Highway 40 & 44 to get to the Poplar St. Bridge. Go over the Mississippi River into Illinois and follow I-55/70 to Exit 6. Turn right onto Highway 111 South. At Collinsville Rd., make a left.
Collinsville Rd. is where you’ll find both Woodhenge and Monks Mound, while the Interpretive Center is about 1.5 miles south.
Cahokia is free to visit. The Interpretive Center is free as well, though on their web site they suggest a donation of $7.
With the exception of major holidays, the Interpretive Center is open Wednesday through Sunday from 9:00 – 17:00.
Tip: Eat something in St. Louis before your departure. The town near the mounds has few options besides a Waffle House, and it’s not the type of area you want to stick around in.
If you don’t have your own rental car, taking a taxi from St. Louis is another option. Just be aware that there’s also a town called Cahokia around 10 miles away. The Cahokia Mounds are located in Collinsville, not Cahokia.