We secure ourselves with seatbelts and our small children and babies with appropriate sized car seats. There are many brands of dog cages available through pet stores. When purchasing one for your dog, buy one that offers enough room for the dog to turn around and lie down, but also is small enough to offer comfort and security. Dogs are feral animals, that is, they are happy in a den. If caging your dog, make sure the cage is secure.
Cages can slide around and get projected from an open vehicle in an accident.
If your vehicle isn't large enough to accommodate a cage, then use one of the many seat belt adapters or harnesses that can be used to secure your dog. Save your dog from being tossed around in a car. The simple act of slamming on your breaks to avoid an accident could cause your dog to fall off the seat and possibly get injured. And make sure your dog has ID tags or discs. Here is a list of past health articles.