That's a very good question. Unfortunately, I don't have the answer because I don't really race. I've done a couple of triathlons where my average was 19 mph for 14 miles and I had a lot of people passing me, and I passed quite a few myself. I had also swam 1000 meters just before getting on the bike and had the run to think about.
A local racing team here does a weekly time trial series where the median speed seems to be about 22-23 for a 10 mile time trial. The high and low range is about 18 to 27 mph. These time trials are open to anyone who wants to come and the course is about as flat as flat can get.
As I said, it's a really good question. I'm curious about the answer myself. A better way to phrase it, however, might be what's usually the average speed in a peloton of cat. 5 racers.