Here are some relevant points:
The central point of the report is that at every transition — from a rear-facing harnessed seat to a forward-facing harnessed seat to a belt-positioning booster — children lose some protection. So the pediatricians recommend that kids remain in each stage for as long as their car seats allow.Here is a very frightening video that shows just what happens to children in a crash: The Importance of Rear-Facing (skip to 1:30 for crash test images).
"Every parent wants their kids to achieve things as fast as they can," said Dr. Ben Hoffman, a University of New Mexico associate professor of pediatrics who helped write the new policy. "That's fantastic for developmental milestones or for school. But for child passenger safety, that's the wrong attitude to have."
My friend Angela also has a recent post on rear facing.