I often exceed 40 mph on descents. My personal maximum was when drafting behind an RV coming down Fremont Pass in CO, going 50+. I didn't quite have the nerve to pass.
Riding the California coast, a day biker passed me and my wife near the top of one of the headlands. We then blew by him on the descent. When he caught up again on the next hill, we described the ride as a runaway freight train.