Lay out the rules before you leave.
There are a few hard and fast rules of etiquette for ensuring a smooth road trip. Before starting the engine, inform your co-passenger to dispel any feuding down the road.
You will pull over to pee: The 10 minutes of driving you might save by forcing your friend/child/hitchhiker to pee in an empty Gatorade is more than offset by having to endure the funk of dried urine in your upholstery for eternity. When mother natural calls, it's best to listen.
The driver owns the radio: Passengers get two vetoes per three hours or 150 miles, whichever comes first. No exceptions.
B.O. is a no-go: A happy car is one that is clean-smelling. Forcing your friends to endure endless hours in a rolling Dutch oven thanks to your reeking body is a Geneva Convention-level offense. It's also deeply uncool.