That's really more dependent on the race than which catagory. A Cat 5 road race can be 25+ mph average if it's flat with a tail wind. In general cat 4/5 races are around 20-25, for shorter road races and criteriums, in my personal experience. Cat 1/2's are in the pro speed range, maybe 30+ mph. Speed, however, really is relative and highly variable. And remember that "average" speed means half the time it's faster and half the time it's slower.