The Potette ($10) and Potette Plus ($16) portable potties are a godsend. When you hear the call from a tiny voice in the back seat, just pull over. The folding potty is always at the ready. The “Plus” model also works as a toddler seat on an adult toilet.
Disposable liners, sort of like a plastic shopping bag with a maxi pad in the bottom, fit around the frame and go straight into the trash when you’re done. (I make sure it always has a liner in place so it’s ready to go.) Liner refills ($5/10) are available but I’ve heard of moms using disposable grocery bags with paper or paper towel inside.
Mine stays in the back of my SUV, but I’ve heard of parents taking it with them and offering it instead of public toilets. One friend’s child insisted on using only the “car potty” and seemed to enjoy the privacy of the garage. That’s a must-have item in my book!