When it comes to frontal crashes, rear-seat protection has not kept pace with front-seat safety systems and better restraint systems are required in the back, according to a new study from the Insurance Institute for Highway Safety.

The study evaluated 117 frontal crashes in which rear-seat occupants utilized seat belts but were either killed or seriously injured. In many of the cases, the back-seat passengers were injured more severely than the front-seat occupants, suggesting the restraints in the rear didn't perform as well as the ones in the front.

The study also examines the types of injuries belted passengers sustained in these collisions and IIHS is using the data to develop a new front crash test that will evaluate occupant protection in the rear as well as the front.

The most common type of injury, found in 22 of the injured back-seat occupants and 17 of the 37 fatalities, was to the chest. The second-most common injury type were head injuries, which were present in nine injured passengers and 18 fatalities.

The IIHS authors explain why riding in the back seat is less safe in the event of a head-on collision.

When a frontal collision begins, seat belts in the front seat tighten around the occupants, due to embedded devices called crash tensioners. The front airbags also deploy within a fraction of a second. In some cases, the side airbags may deploy, too.

The tightened belts and deployed airbags keep front-seat occupants safely away from the steering wheel, instrument panel and other structure when the vehicle stops abruptly, even if the force of the crash pushes that structure inward. To reduce the risk of chest injuries, these belts also have force limiters, which allow some webbing to spool out before forces from the belt get too high.

However, the situation in the back seat is more precarious. In the rear seat, side airbags protect passengers in a side crash—but there are no front airbags, and the seat belts generally lack crash tensioners and force limiters.

While intruding structure is typically not an issue in the back seat during a frontal collision, crash forces can cause a back-seat passenger to collide with the vehicle interior. Seat belts can prevent that, but, as the new study shows, seat belts without force limiters can inflict chest injuries.

IIHS says that there are a variety of solutions that could enhance back seat safety in frontal collisions ranging from belts with force limiters to inflatable seat belts to equipping rear seats with frontal airbags.

The Institute is confident that a crash test that evaluates rear-seat protection will prompt automakers to determine what combination of technologies works best to enhance back seat safety.