Wearing a seat belt correctly will reduce the amount of injuries in a crash
"But that is the key, seat belts must be worn properly. Your lap belt should be securely wrapped around the waist, resting on bone. If it is pulled across the soft part of your stomach, it could penetrate the skin and even drive into your body in an extreme accident, causing injury or even death.
If your seat belt has a shoulder strap, it should rest securing on the front of your body and on the collar bone. If it is placed on the neck, it could be a fatal consequence in an accident, or in a more minor collision."
In the pictures to the right you can see the position of the lap and shoulder belt on the girl is lower on her body than the boys belt is on his body below.
Notice the shoulder strap is on her shoulder away from her neck and the lap belt is low on her hips. This shows the correct way of wearing a seat belt for maximum safety in a collision.
The hip bones and shoulder bones are some of the stronger bones in the body, able to withstand excessive pressure in a crash.
This boy on the other hand is wearing a seat belt with the shoulder strap very close to his neck, and as stated in the quote above, this could cause the strap to cut into his neck in a collision causing severe injuries.
Also notice his lap belt comes a bit higher on his stomach. As the quote says, this is a soft area of the body that isn't protected, so it could cause serious abdominal injuries by the belt being forced into the abdomen when the body is pushed forward in a crash.
* This is also why parents are now warned to keep their children in a car booster seat until they are about 4'9" and between 8-12 years old.