Sock loves the car. She travels with her head out the window, has room to lie down and can get into the rear footwells. But one thing she is never allowed to do is travel or stay in the car unrestrained.
Dogs are about the same weight as small children. They are light enough to be thrown around severely in an emergency stop or crash. But also, dogs are unpredictable. Sock is usually relaxed, but more than once she's tried to jump onto my lap mid-highway. That's a real good way to have a bad accident.
It's cheap and easy to restrain a dog in a car without having to fork out for those metal cage things. Buy a soft harness (about $30) and a caribena (about $15). Then take the usual lead you use and create a loop in it about 50cm from the clip. Attach the lead to the harness (or collar, in a pinch) and then slip the loop onto the caribena. Click the seatbelt across into lock position, and then attach the caribena to the strap.
Make sure, when buying the harness, that you get one that will adjust small enough for your dog. Don't let the shop assistant sell you the wrong size - make them fit it properly in-store.