Cats are easily restrained in pet carriers, which is the safest method of all. The real challenge is restraining large dogs. My dogs fit easily in my car, but no crate that fits them would fit in my car so they really need a seat-belt type restraint. It's definitely something that has yet to be invented. Here are the challenges as I see them:

* prevent the animal from becoming a projectile
* prevent the animal from being hanged or otherwise injured by the restraint itself
* allow the animal a small amount of freedom to move around so that it can stand, turn around and lay down
* prevent two or more animals from getting tangled with each other