Car Safety

When the velocity of a car changes, that of the people inside it has to change too. During normal driving, the force to accelerate the driver and passengers comes from the seat; it pushes the person forwards to cause an increase in speed and the friction force between the person and the seat is sufficient to slow the person down or cause a change in direction.

If there is not enough friction, a passenger appears to be ‘thrown’ in the opposite direction to the change in velocity.

During hard braking or in a collision, friction is not enough to match the car’s deceleration. Viewed from the outside of the car, a passenger appears to have been thrown forwards. Despite what the television advertisements say, this is not the case! The photograph shows what can happen to a passenger in a collision. Seat belts prevent this from happening. However, a rigid seat belt that caused a passenger to decelerate at the same rate as the car could prove fatal in a head-on collision.

To be effective a seat belt should:

• restrain the passenger and prevent collisions with the inside of the car
• allow the passenger to come to rest over a longer time period than that taken by the car.

Excessive force from a seat belt can break the ribs and damage internal organs.

To achieve these conflicting aims, seat belts are designed so that they stretch sufficiently to allow the passenger to carry on moving for a short time after the car has stopped, but not so much that would result in the passenger hitting the windscreen or the seat in front.

Seat belts can still inflict severe injuries during a collision. The amount of space to allow stretching, particularly in front of a driver, is limited so the restraining force from the belt can be large. There is also a design conflict in deciding on the width of the belt; wide belts exert less pressure than narrow ones but are less comfortable to wear.

There is less space in front of the driver than there is in front of the passengers because of the steering wheel. In modern cars this is designed to collapse on impact.

Airbags are designed to restrain a driver and passengers without any risk of causing physical damage to the person. The bag only operates during a rapid deceleration such as a head-on collision. This releases a gas which causes the bag to inflate and surround the driver or passenger like a cushion. The force exerted on the person is similar to that from a seat belt, but because it is over a much larger area the pressure is smaller.

Airbags are mis-named as they do not use air. An inert gas such as nitrogen or argon is used to inflate the bag within 0.02 s of a collision.

Modern cars, and railway carriages, are also designed so that parts of them crush on impact. Although the part of the vehicle containing passengers is rigid to give them protection, the front and rear are crumple zones.

As the name implies, the crumple zone should squash during a collision. This has two effects:

• it increases the time it takes to stop the vehicle, reducing the deceleration and the force
• it absorbs much of the kinetic energy of the moving vehicle, so that it does not bounce backwards, causing further injury to passengers.