There is a lot of debate on what is the safest way to secure your pup in the car. Car seats and crates keep your dog in one place and will protect your dog in a minor accident. The crate can go forward or sideways and you can use the seat belt to wrap around it and secure it in place.
In a major car accident, a dog can be thrown around in a crate. I think of it like being in a falling elevator. Car seats tether the dog, but still allow movement.
The safest way to travel would likely be a harness that is akin to a seatbelt for humans. Something like this:
Dog Car Harness » Comfort Ride Travelin' Dog Car Harness | PetSmart