quick change

[Catrin]

If I can't make it to the showers at the state park in which I generally mountain bike before coming home, then I change in the bathroom. I keep a towel and washcloth with me where I can at least wipe the sweat off my body before putting on clean/dry clothing. I've seen women in the parking lot at the same park doing what you describe under a large towel.

I know another woman who purchased a long casual skirt that she uses for the same purpose that you use the towel for - and the advantage is it can't fall off

[OakLeaf]

In hot weather I put on a sarong. If I have a place to change (bathroom, woods, car) that's preferred, but if not, I can leave my sweaty bra on and change shorts underneath it. It's obviously not enough to wear when the temperature is below 85 or so though.

[ny biker]

First choice: find a bathroom. I often start my rides near an outdoor shopping center with a decent public restroom, and there's usually a convenience store or fast food place near the start point for other rides.

Second choice: change in the front seat of my car. It's not the easiest thing to do, but it works. I have several reflective sun shades that I use to cover the windshield and side window in case anyone walks by while I'm indecent.

(I won't change in a porta-potty -- they're too dirty and I worry I would accidentally drop something.)

[Crankin]

I've changed in the car with very little issue. I throw a beach towel over my lap, but if there's no one around, I have done it without the towel. Heck, one time I got drenched in a cold spring rain, before I had rain gear. This was before I lived in Concord, and we had driven here and parked smack in the center of town. DH put the heat on high and I stripped right in the front seat, with just a small towel to dry myself . I was definitely more worried that I had hypothermia.
When my son was racing, there were several racers (guys) who wore changing kilts, so they could stand up and change outside of their cars. They put the kilt on over their shorts, then pulled the shorts down and off, and then put on regular shorts.

[westtexas]

I just change on the side of the road, using my car as a block. There's no natural cover out west (tumbleweeds just don't cut it) and rarely a rest stop or porta potty. And the cars are going by so fast... no one has time to see much anyway. I don't worry much about the top - I leave my sports bra on and put on a dry t-shirt if it's going to be a long drive home, mostly for the fabric in my car's sake.

And you could always accost others nearby. Once I asked a nice older fellow if I could slip into his motorhome really quickly to use the potty. He totally let me and was really nice about it! (Probably unsafe but... I really had to go.)