Lately, there have been reports of pets lost after a vehicle accident because they were roaming free inside. Pets should be restrained either in a crate (best option) or with a seat belt made specifically for dogs. Accidents can happen. When a vehicle gets hit, even a slight bump will make your pet go flying. If they can, they will bolt. Do yourself a favor and take the extra steps to restrain your pets when traveling.
Whole Dog Journal has a great article about this subject: “Crates or Seat Belts, People!”