I think the other thing that plays into this (didn't we discuss this recently? I'm having some deja-vu going on here) is that women tend to have more responsibility for child care, so they are the ones picking up and dropping off at daycare, taking kids to after-school activities, leaving work midday to take kids to dentist and doctor appointments, etc. They are also the ones "on call" when a child gets sick and has to be picked up from daycare or school during the day. I commute by bike one or two days a week, but for a long time I didn't bike commute at all because every single day from 3:00 - 4:15 I had to make a run to my son's school (15 miles away from work), take him to his after-school program (7 miles from his school), then go back to my office (another 16 miles). DH was an hour away at his new job. Now DH is working from home most days, so I'm able to manage the commute by bike, but it'll never be a full time thing until the kids are out of the house.

One related thing I've noticed--when I get up at 5:30 to run or ride, women outnumber men by about 4 to 1, at least among runners (and generally among people out at that time of day at all for exercise--I only see one or two other cyclists when I'm out in the early morning, and the occasional dog walker). In the evening, it's more evenly split, and at night (when *I* walk my dog) it's all men running, never women.