I've stitched in the car once when I was on a serious deadline and had to get something done as it was a gift for the person at the other end of the road trip. Talk about pressure! Luckily it was all blackwork, so relatively easy, and only one color.
I'd suggest if you're going to stitch in the car, make it something that is on a higher count fabric - I wouldn't try to stitch over 1 on 28 count for example. That and pick a pattern that is fairly simple - no frequent color changes or confetti - LHN patterns come to mind because they're small, don't use too many colors and are fairly straightforward - lots of straight lines. I'd also use a smaller hoop/frame than usual simply because you don't have that much space and don't want to be banging elbows with the person sitting next to you. And look up and out often! I don't get motionsick at all, but I found that when I was stitching in the car that I had to look up every now and then or I started to feel a bit off.. Use the biggest needle you can for the peice you're stitching so that just incase you do drop it, it's less likely to stab you compared to a tiny pointy 28. And that said, make sure you do take a couple of extra needles just to be safe! Nothing like having your cross stitch with you, being bored and wanting to stitch and not having a needle because you lost it. Not that that's ever happened to me.

Oh.. and prep your floss before hand - dealing with bobbins or skeins is a hassle in a small space - I like to make up a project card with lengths of all the colors I need cut in advance so that I have all my floss ready to stitch. It ends up something like this:
Made simply from a sheet of cardstock folded in half, and then the ends are folded inwards and holes punched down that smaller flap to attach the floss to. This way for storing, I can fold the entire thing in half and tuck the floss inside so that it doesn't rub against anything and get worn and frayed. I do this every time I start a new project simply because it makes life so much easier than dealing with bobbins constantly.