Car crashes are the leading cause of both disability and death for children in the United States- and they are that way for a reason.
The video below is about a child who was 8 years old and was in a crash. He was in an adult seat belt and is now paralyzed from the neck down for the rest of his life because his seat belt was improperly positioned on his body. Something so simple as using a booster seat could have prevented his terrible disability.
http://www.boosttil8.org/
If you lost a child in a crash, you wouldn't think it is creepy any longer.
Contrary to the popular belief, it is not the shoulder belt that is dangerous. If it is resting on the child's neck, the only thing that is dangerous if the child places that shoulder belt behind their arm or underneath their back. The lap belt is very dangerous when on the child's stomach- it needs to be on their hips at all times while riding in the car.
@Jane Lookda: You are correct in that using a car seat when it is outgrown is not safe; however, there are some small children at age 6 who could still fit in a rear facing child seat by weight and height. For example, my cousin's child is 6 years old roughly 40 inches tall and 28 lbs. She is petite and she is healthy (as what her doc said), and she could fit in some rear facing convertibles.
Also, in the United Kingdom, the law says that children need to ride in a car/ booster seat until they are 12 years old or are 135 cm tall. The United States is very behind in car seat safety.
Edit again: I cannot believe that I recieved 4 thumbs down to my response. It all comes down to the survival of the fittest.