Car safety features

What are seat belts designed to do in a crash?

Keep you inside the vehicle and reduce the risk of colliding with the steering wheel, dashboard, or windshield.

How do seat belts reduce injury?

They restrain occupants and increase the time taken for their momentum to change during a crash, reducing the average force on the body.

What is the physics principle behind seat belts?

Impulse: impulse equals the change in momentum and also equals average force multiplied by collision time. Increasing collision time reduces average force for the same change in momentum.

What do adjustable upper belts do?

An adjustable upper belt lets you change the position of the shoulder strap to accommodate a person’s size. This feature may encourage passengers to wear their belts, since it increases shoulder belt comfort.

What does a seat belt pretensioner do?

Pretensioners retract the seat belt to remove excess slack, almost instantly, in a crash.

Why must you still adjust your seat belt snugly if it has a pretensioner?

Pretensioners are not powerful enough to pull you back into your seat in the event of an impact.

What do energy management features do?

Energy management features allow seat belts to “give” or yield during a severe crash to prevent forces on the shoulder belt from concentrating too much energy on your chest.

What are load limiters and tear stitching?

“Load limiters” are built into the shoulder belt retractor and/or “tear stitching” in the webbing that causes the seat belt to extend gradually.

How do energy management features reduce injury?

They allow the belt to extend gradually, increasing the time over which the occupant’s momentum changes and helping limit the force on the chest.

What are rear center seat lap/shoulder belts especially beneficial for?

Older children and children in booster seats who are often seated in the rear center position.

What do front air bags do?

Depending on the speed at impact and the stiffness of the object struck, front air bags inflate to prevent occupants from hitting the dashboard, steering wheel, and windshield.

What do side air bags do?

Side air bags reduce the risk that occupants will hit the door or objects that crash through it.

Why do air bags not eliminate the need for seat belts?

Maximum air bag effectiveness depends upon seat belts, which help keep you in place should a collision occur.

How can you reduce the risk of injury from a front air bag?

Buckle your seat belt and keep about 10 inches or more between your breastbone and the air bag.

Why should children age 12 and under sit in the rear seat?

Children can be killed or seriously injured by an air bag.

When should a rear-facing child seat not be used in the front seat?

Never use a rear-facing child seat in the front seat of a vehicle equipped with a front passenger air bag unless the air bag is off.

What is head injury protection?

Foam or other energy absorbing material under the trim of the vehicle interior and is likely to be invisible to vehicle occupants. Some vehicles have head air bags.

What are head air bags designed to do?

Both types of air bags are designed to help protect occupants from injuries caused when their head strikes the upper interior of a vehicle.

When do head air bags deploy?

All head air bags are designed to deploy in side impacts, while some are also designed to deploy during rollovers.

What do head restraints do?

Head restraints are extensions of the vehicle’s seats that limit head movement during a rear-impact crash, thus, reducing the probability of neck injury.

Why do head restraints reduce neck injury?

They limit the backward movement of the head relative to the body during a rear-impact crash, reducing the movement that can strain the neck.

Which head restraints generally provide the best protection?

Dynamic head restraints provide the best protection.

What does an antilock brake system (ABS) do?

An antilock brake system (ABS) prevents a vehicle’s wheels from locking during “panic” braking, which allows the driver to maintain greater steering control — a key factor in avoiding a collision.

How does ABS help maintain steering control?

It prevents the wheels from locking, helping the tyres maintain the friction needed to steer while braking.

Does ABS guarantee you can avoid a crash?

No. You still may lose control when driving at excessive speeds or when using extreme steering maneuvers.

What is the difference between four-wheel and two-wheel ABS?

Four-wheel ABS monitor and control all the wheels of the vehicle, while two-wheel ABS only monitor and control the rear wheels of a vehicle.

What does brake assist do?

Brake assist senses emergency braking by detecting the speed or force at which the driver presses the brake pedal and boosts the power as needed.

How can brake assist reduce stopping distance?

It can reach the braking force needed to activate the ABS more quickly and easily, potentially reducing overall stopping distance by eliminating the delay caused by not braking hard enough or soon enough.

What does traction control do?

Traction control systems improve vehicle stability by controlling the amount the drive wheels can slip when you apply excess power.

How does traction control work?

The system automatically adjusts the engine power output and, in some systems, applies braking force to selected wheels during acceleration.

What does all-wheel drive do?

All-wheel drive distributes power to both front and rear wheels to maximize traction.

Does all-wheel drive prevent wheel slip?

Unless combined with traction control, all-wheel drive systems do not prevent the drive wheels from slipping when you apply excess power during acceleration.

What does electronic stability control do?

It senses when a vehicle is starting to spin out (oversteer) or plow out (understeer), and it automatically applies the brake to a single wheel.

What is the limitation of electronic stability control?

It can’t keep a vehicle on the road if its speed is simply too great for the curve and the available traction.

How does vehicle weight affect crash safety?

Crash data show that heavy vehicles offer more protection than light vehicles with the same safety equipment, particularly in two-vehicle crashes.

What is a crumple zone and how does it reduce injury?

A crumple zone is an energy absorbing material used in the body of a car. It prolongs the time over which the vehicle changes momentum, which minimises the force of impact and reduces injury to passengers.

What is the physics principle behind crumple zones?

Impulse: increasing the time taken for momentum to change reduces the average impact force, for the same change in momentum.

What should you check when test-driving a car for safety?

Check that the seat belt fits well and is comfortable. Check that head restraints, roof structures, and windshield designs don’t interfere with your ability to see clearly. Try to conduct your test drive in the evening so you can check the visibility provided by the headlights.