After the wheelchair is secured to the floor of the vehicle, the next goal is to keep the wheelchair rider in the wheelchair. Most vehicle-related deaths and injuries happen in secondary impacts such as when the occupant hits the inside of the vehicle or is ejected from it.
Only the bony parts of the body can withstand the forces exerted against the body by the occupant restraint. A 3-point occupant restraint (lap and shoulder belt) acts like the seatbelt in a car. The lap belt portion of the best goes low and snug across the pelvis. The shoulder belt portion comes from the vehicle wall and crosses the collar bone and breast bone to anchor near the hip. If the belts do not cross bony areas, significant injury to soft tissue can occur.
It is important to use both the lap and shoulder belts of the restraint. All vehicles should have a means to modify the origin point of the chest strap on the vehicle wall. The adjustability of this origin point allows the shoulder belt to fit either a tall or short person.
Just because a wheelchair has a "seatbelt" with an automotive or airline style buckle does not mean that this belt is a crash-worthy safety belt. Most positioning or postural belts are NOT strong enough to handle crash forces.
The following videos demonstrate the contribution of an occupant restraint.