A.W.
Having a potty for the car is such a good idea, and it has saved my three-year-old twin boys from accidents many times (like when we go to a park that doesn't have a toilet close by). I would recommend simply getting a basic potty (it helps if you have a minivan or SUV so you can put it in the back, and your daughter can go in the relative privacy of your car). I put two plastic shopping bags in the potty (double bagging helps prevent leaks), so that when my kids are done using it, I can simply tie it up and throw it away (I do this mainly because I don't think most people would appreciate seeing me dump urine on the ground, understandably). Little kids have even smaller bladders, so I think an "emergency" potty for the car is a must.