Your pet should travel in a carrier. I often see loose cats in the car and while it may be nice for them to snooze in the back window, such freedom is quite hazardous. Why?
Well, a loose animal in the car can run under your foot and hit the gas pedal or the break pedal at a crucial moment. (This recently happened to a neighbor of mine whose dog jumped on the gas pedal and caused a rear end collision.)
Also, your pet could decide that they want your attention during a critical driving situation -- does your cat head butt you to show affection? Or perhaps like to jump on your shoulders? What if this happens and distracts you for a moment as you are in the middle or preparing to pass some traffic on a two lane highway? Or perhaps you hear your cat make a noise and look around to make sure its ok. Such momentary distractions could have very serious consequences for you and your pet.
Some roads involve tolls or perhaps you need directions in a strange area -- opening your window gives your cat the opportunity to escape. That's the same when you stop for that restaurant break and open the door.
And what happens if you are in an accident? What protection does your cat have? If it is not in a carrier, it has none -- it also becomes an object flying around that can prevent you from regaining control of the situation and making the accident even worse. If the accident is serious enough, you could be unconscious and your cat could again escape or it could be thrown out of the car and killed.
The safest way for your cat to travel is in a carrier -- and you should seat belt the carrier into the back seat preferably with a strap around the seat across the front of the carrier to anchor it. The better the construction of the carrier, the better protection for your pet. Line it with diapers to ensure any accidents are absorbed away from your cat. The best ones we've found are bed pads and chair pads for people available at health supply stores.
A litter pan with a lid lets you offer your cat a bathroom break. Some cats will use them, some prefer to wait until the journey ends. A Rubbermaid storage box works really well.
There is a set of things that you want to have handy while traveling
in the car and then there is a set of things you will need when you get
to your destination. A bag with pockets works well. Pack it
so things you need while traveling will be easy to access.