Taking your dog in the car should always be taken seriously. You have a legal responsibility to ensure that nothing causes or is likely to cause injury to your pet and that no unnecessary suffering is caused during the journey. An unrestrained dog loose in the car has the potential to cause serious injury or accident, either by distracting the driver or worse knocking the steering or getting into the drivers foot well. If you do have an impact, an unrestrained animal could suffer serious or fatal injury and potentially become a deadly projectile if situated behind a passenger.
The safest way to keep your animal safe is to use a dog crate in the boot or a dog seat belt harness if you don’t have room. Recent studies show some harnesses will fail during a crash so do your research and get one that has had proper testing. Other options are a dog net or fence that partitions off the rear of the car. This separates the animal from the driver and passengers but will not protect the dog in an impact.
Make sure you plan long journeys to allow for regular water and toilet stops for your dog. Go steady with your driving, especially with a dog that is not used to being in the car.