Just like people, pets need to be restrained while in the car. Depending on what kind of pet you are traveling with, you'll need to use either a carrier or a harness to keep it properly restrained. A carrier should be used for cats and other small animals that cannot easily be harnessed. The carrier should be large enough for your pet to sit and lie down. The Humane Society of the United States, or HSUS, recommends selecting a carrier with smooth edges, opaque sides, a grille door and ventilation holes on four sides. Secure it in your car with the seat belt (through the handle or around it) or in the cargo area of an SUV or minivan. It is not safe to place a pet carrier inside a trunk because there is not enough air circulation and it can get dangerously hot very quickly on a warm day.