It's an odd one, this. The usual shortcomings of anecdote and observation apply, whilst the evidence is simultaneously striking and inconclusive. That we have a culture that rewards assertiveness in men and punishes it in women seems obvious, and it is a small leap from this observation to conclude that, in general, we produce men who tend to be more assertive than their female counterparts. I have my own stock of anecdotes from teaching cycling to adult beginners that would support this in a cycling context. But then it's a commonly-held view that cycling on the road is an unusually dangerous activity, and one might therefore expect that women who do it regularly are a self-selecting sample of the more adventurous and assertive sort. Take a workplace analogy - it would be odd to argue, with everything we know, that women in managerial roles are less assertive than their male counterparts - in fact we might expect them to be (if anything) more assertive, because they have more barriers to overcome than their male equivalents, and are judged more harshly on their perceived weaknesses.
I'm just thinking aloud, and not really getting anywhere near a conclusion, but I think DZ might have hit obliquely on something when he mentioned aggression. I don't experience an enormous amount of unrestrained aggression on the roads, but there's a certain undercurrent of hostility, or perhaps just resentment, that surfaces often enough to be of concern. Going back to my first point, people find assertiveness in women threatening when the same trait in men would go unnoticed. I'm probably at the more assertive end of the scale of cyclists of both sexes, and I think that this mostly gives me an advantage in traffic. But just now and again I have a hunch that a driver is seriously pissed off not just because a cyclist has passed him, but because a woman has passed him. The two things are so ridiculously intertwined as to be inseparable, but we might just be talking about factors that combine to produce a flashpoint.