From St. Charles to Augusta (roughly 20 miles) the KATY trail gets so much use that it's been packed down until it's the next thing to black top. That section is definitely road-bike-able. Immediately past Augusta the trail gets noticeably more gravely. People have ridden road bikes for the entire length, but I think that I'd prefer tires in the 32 to 35mm range. There are some attractive areas between St. Charles and Augusta, mostly surrounding the HWY 40 bridge across the Missouri, but the first several miles from the St. Charles riverfront are, I think, pretty ugly. It's not my favorite ride and I only live a couple of miles from there.

If you start at the soccer park on the north end of St Charles, there are almost always road bikers heading out to ride the network of flat roads between the Mississippi and Missouri rivers. On a good Monday night at 6:00, about 100 road riders of all skill levels will go out for a 20 or 30 mile ride.