I participated in the Pedal Hilton Head charity ride. Talk about flat! My average speed jumped by about 5 MPH, and I was barely pushing any watts.
The one climb on the route was up and over a bridge. About 1/4 mile long, and a 6% grade. It was funny hearing the "locals" complain about including the bridge on the route as I was passing them. Riding in the Upstate where flat roads are few and far between (and not very long) the bridge was a piece of cake.