I would defiantly recommend a crate. I once read a story about a lady who crashed on the motorway. She had her dogs in the boot behind a guard, thinking that they were safe and secure. Unfortunately, when she crashed her car, the windows in the boot smash and the dogs got loose on the motorway. If I recall correctly, one of them was killed

A crate is the safest option. Or ever better, a dog cage made to fit in the back of your car. Again, I heard about a lady coming back from a dog show. Her car was involved in an accident and her super strong dog cage saved her and her dogs lives. The car roof collapsed under the impact of the crash, and the dog cage stopped her being crushed to death.

Worth every penny IMO.