Using vehicle restraints has saved thousands of lives throughout the world over the past 52 years. Around 1968, seatbelts were first installed in new vehicles. Before that time, they could be installed from a Sears or Montgomery Ward seatbelt kit by drilling holes in the floorboard and attaching washers, bolts, and nuts to the strap assemblies. Seatbelts were designed to do two key things for passengers when in an accident: keep passengers from tossing around inside a vehicle and keep passengers inside a vehicle if doors/windows flew open.
Modern vehicle restraint systems include lap and chest straps, which are designed to spread the forces of an accident’s impact over more areas of the torso. Along with headrests and airbags, the seatbelt system helps protect the more vulnerable head, neck, and spine areas where permanent nerve damage can occur. 
Forces involved in a collision are reduced when occupants are held in place and come to a stop over a few feet of distance as a vehicle hood and engine area crumple to a stop. Without restraints, an occupant would travel through the cab at full speed and strike the windshield, steering wheel, dashboard, or other areas, which by then have stopped over that distance. Then, instead of stopping over the few feet, their head and body would stop in an inch or less or fly through the broken glass in the windshield. The second scenario is more often fatal. 
These stopping distance differences result in higher or lower deceleration forces within the head and torso. When compared with gravitational forces from falls, vehicular deceleration forces can be very high. 
At 20 mph, a vehicle is traveling at the same speed as an object (or person) falling a distance of 14 feet when it reaches the ground. If a person lands in a foot of mud, their landing may be messy but OK. If they land on concrete, the result could be death on impact. Similarly, a restrained passenger in a car striking a tree at 20 mph may stop in one foot of crumpled-hood distance, at 14 G’s of deceleration force across their lap and chest. If unrestrained, however, they will hit the hard windshield at 20 mph, the same as a concrete sidewalk at 20 mph. The stopping force on his head within that short one inch stopping distance (or less) is 169 G’s of deceleration force. 
Fatality rates in 1968 were around 4.0 per 100 million miles of travel. Now, rates are closer to 1.1. Keep wearing your seatbelt!