For women, hitchhiking seems to be more risky. But it is not really. Many men are reporting the same degree of violation as women. If most reported cases of murder or rape of hitchhikers involve women hitchhiking alone, it is because men are less likely to report rapes (as a product of homophobia in the world), and murders are not getting the label of "hitch-hiking-murders" as often with male victims.
Hitchhiking as a woman can be easier though. Often, women stop for other women or girls. Even families (or other usual-not-hitch-hiker-friendly car owners) stop to "save" women from a situation in which they think she could be in "danger". As a product of sexism in the society many people think that women are more safe to take up as hitch hikers. And, possibly as a result of sexism in the society, women tend to be more safe to take up as hitch hikers.
You can find a specific guide about women hitchhiking at girls.hitchbase.com!