Ive raced in a couple events that supports women racers.
http://thundervalleyracing.com/Karen is a fantastic person and racing with her is an amazing experience. All the people in the group are interested in eachother, recognizing eachother and helping eachother. The whole group becomes like family. That never happens with other groups. In other events, people are so into themselves.
At any rate, the number of women in the racing events Ive been in has been pretty constant. Thats to say about 1 in 15 drivers are women. Of course, its a lot more in Karen's organized events. But many of the women have been doing it for a long time and are very competitve. I'd say there would be more women drivers at a professional level if they got into racing at a younger age. It seems to me, most women need to be introduced to the sport, while more men enter on their own interest.