As evident by my Oh, The Places That I Pump post, there are very few places where I won't pump anymore. My living room floor, a closet at work, my car.
Yes, my car.
This started when I was nursing my older daughter. I pretty much pumped anytime my husband was driving. Why not? I know I won't need to or be able to feed the baby right then and I am not doing anything else.
Then I started doing it when I was driving. It is actually easier than it sounds. I have a car adapter for my pump and I plug it in and hook up before I go anywhere. Once the pump is started, it is easy to sort of slouch and keep the pump in place.(This is probably how my chiropractor stays in business.) I can generally pull my shirt down over the pump parts or I will wear a sweater I can close over them. I pump for 15 minutes and disconnect. I put the milk bottles in the cup holders and then transfer them to a larger bottle for storage when I reach my destination.
To make it easier, you could use 8 oz bottles. That might eliminate some slouching. If you are in a smaller car, I recommend using a nursing cover while pumping. In our vert small, manual transmission Saturn, I feel a little public and often take extra precaution to cover up.
I am busy. Busy at home and busy at work. Sometimes pumping before I leave in the morning is necessary but impractical. I also don't want to have to pump right as I arrive at work for the day. Pumping in the car gives me the ability to get that morning pump when it is most convenient.