Rear-Facing is Safest
Rear-facing provides a superb way to travel for infants and toddlers. Statistically, it reduces the risk of death by 71%. Why? The most common type of crash is the frontal crash. In a frontal crash, the entire back of a rear-facing car seat absorbs crash forces, protecting the child's head, neck and spine. In the less common, but more injurious side impact crash, the rear-facing car seat again protects the head, neck and back. Since there's almost always an element of forward motion in a side impact—such as when a vehicle is going straight through an intersection when it's struck in the side by a red-light runner—a rear-facing seat does a better job of keeping a child's head contained within the safety of the seat.
What about an older rear-facing child's legs—won't they be broken by the back seat? Maybe, but it's unlikely. Again, statistically, the most common type of crash is the frontal crash. Physics tells us that everything will keep moving toward the point of impact, including the child's legs which will fly up. Broken legs are much easier to treat than a broken neck, which is a real risk if a child is turned forward before the neck bones have hardened and the ligaments have developed enough to withstand crash forces.