To reduce the force experienced by the passenger you need to extend the time for a passenger to stop in a collision. As force is the change in momentum divided by time.
![momentum triangles](https://www.tutormyself.com/wp-content/uploads/momentum-triangles.jpg)
![car saftey](https://www.tutormyself.com/wp-content/uploads/car-saftey.png)
To reduce the force experienced by the passenger you need to extend the time for a passenger to stop in a collision. As force is the change in momentum divided by time.