A study entitled "Mortality of front-seat occupants attributed to unbelted rear-seat passengers in car crashes, "found the risk of death was 5 times higher than if the back-seat passengers were in seat belts.1   In other words, if rear seat belts had been used almost 80% of deaths of belted front seat occupants could have been avoided.  The study looked at 103,590 front seat passengers who were injured in car to car crashes in Japan.  In frontal crashes the risk of death was increased 6 times for the driver and 7 times for the front seat passenger.  Rear-end crashes did not increase the risk of death of front passengers even if the rear seated passengers were unbelted.

COMMENT:  In some states in the United States and most countries worldwide only the front seat passengers must wear a safety belt.  The force of a body moving forward at any commonly driven speed is immense and potentially lethal. When you are the driver and responsible for your passengers' safety, don't forget a 150 pound Grandma moving at 60 mph toward the front seat can cause quite a lot of damage – make sure everyone is properly belted.

 1) Ichikawa  M. Mortality of front-seat occupants attributable to unbelted rear-seat passengers in car crashes. Lancet. 2002 Jan 5;359(9300):43-4.