My normal everyday wear is a knee-length skirt, and I get into a car putting my bottom on the seat first, and then swinging both legs into the car, keeping them together so as to impede any view of my crotch. Getting out is the exact converse. Like Chloe,I think most girls do it that way too.
There's more of a question about how to get on and off a bike. For either sex out for a serious bike ride they'll be in bike gear - lycra shorts or tights, depending on weather, and so there's no trouble for either sex about cocking the leg over the saddle. There are ladies bikes with a low cross-bar that they can step over, but you see very few of those, and girls who ride bikes mostly do it in pants of some sort. For me if I'm in a skirt and have to go a short journey and dont want to change, I've got a problem. Cocking my leg over is, after all, a gesture similar to the one used by dogs the world over for a different purpose, and I cant help a brief view, and if there's someone around who I have to be gentlemanly with, I arrange to get on out of sight. When I'm on the bike, though, a knee-length skirt so long as it isnt too tight is OK to ride short distances.