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Chapter 6 - Physical Forces Influencing Driver Control

When you get behind the wheel, there are physical forces that can influence your control of your vehicle. These include speed, traction, and the force of impact from a crash itself.

As you increase your speed, the amount of kinetic energy also increases.

Speed Control (Acceleration, Deceleration, etc.)

According to the National Safety Council, your chance of dying in a crash doubles for every 10 mph that you travel above 50 mph. This is because of the increase of kinetic energy as your vehicle gains speed. Kinetic energy, or energy of motion, is the energy that an object, such as your vehicle, has when it moves. As you increase your speed, the amount of kinetic energy also increases - exponentially. For example, when you double your speed, the amount of kinetic energy quadruples. Thus if you were traveling at 30 mph and then decided to accelerate to 60 mph, the amount of energy in a crash would be four times greater! Here are some quick facts about energy and speed:

• A vehicle's kinetic energy doubles when its weight doubles.
• When the weight of a vehicle doubles, it needs about twice the distance to stop.
• A vehicle's energy of motion is proportional to the square of its increased speed. For example:
  • When the speed of a vehicle doubles, it needs about four times the distance to stop.
  • When the speed of a vehicle triples, it needs about nine times the distance to stop.

Effects of Acceleration

Increasing the speed of your vehicle will make it more difficult for you to drive safely as your overall control of your vehicle decreases. At higher speeds, your margin for error is smaller. When you accelerate, you are increasing the vehicle's energy. The faster you drive, the less time you have to react to hazards in the road around you. Also the faster you drive, the less time you have to react because your stopping distance is increased. Driving at high speeds reduces your ability to safely negotiate curves or roadway obstacles due to the increased difficulty in steering. Your vehicle responds differently at faster speeds, so you must adjust the way you handle your vehicle, whether it is to accelerate, decelerate or steer. In addition, acceleration decreases your vehicle's fuel efficiency. It also increases engine and tire wear.

Effects of Deceleration

When decelerating or decreasing the speed of your vehicle, the energy your vehicle exerts also decreases. At lower speeds, you have a larger margin for error, and you will need less time and distance to stop. As you decelerate, you must adjust the way you handle your vehicle as it will respond differently at lower speeds. Decelerating will give you more time to react to hazards, a shorter braking distance, better steering control, and an overall increase in control of your vehicle. It can also increase fuel efficiency and decrease wear to the engine and tires.

Traction (Friction, Stopping Distances, Centrifugal Force, etc.)

On slick roads, such as those you encounter during rain or snow, it will take you longer to stop your vehicle.

Traction is the gripping power, or friction, between a tire and the surface of the roadway. You rely on traction to maintain control of your vehicle. Traction is affected in one way or another by friction, stopping distance, and centrifugal force. An increase in any or all of these factors will decrease the amount of traction that your vehicle will have, and thus the amount of control you have over your car.

Friction

When you have to stop while driving, you are relying on friction. The amount of friction you have is dependent on your brakes, tires, road surface and speed.

Stopping Distances

Do you know how long it takes for your vehicle to stop? The speed you travel is a key factor. The greater the speed you travel, the longer it will take for you to stop. But speed is not the only factor that affects stopping distance. When you encounter a hazard, you must react to it. Your reaction time is the distance your vehicle travels in the time it takes for you to identify a threat, react by applying the brakes, and move your foot to the brake pedal. No matter how quickly you think you can react, you still need time to respond to a situation and then to stop.

The average driver in a passenger car traveling at 20 mph in ideal conditions will take approximately 22 feet to react to a situation, then 23 feet to apply the brakes, for a total distance of 62 feet traveled. However, the time and distance needs increase considerably at higher speeds. At 60 mph, the average driver will take about 66 feet to react, then 272 feet to apply the brakes, for a total of 272 feet traveled. That's almost the length of a football field! The chart below shows the approximate total distance it will take to stop from various speeds (again, distances are approximate and assume ideal laboratory conditions):

Speed (in mph)	Reaction Time (in feet)	Estimated Stopping Distance (in feet)	Total Distance Traveled (in feet)
20 mph	22 feet	23 feet	45 feet
40 mph	44 feet	81 feet	125 feet
60 mph	66 feet	206 feet	272 feet
80 mph	88 feet	368 feet	456 feet

On slick roads, such as those you encounter during rain or snow, it will take you longer to stop your vehicle. The same goes if the brakes or tires are worn.

Centrifugal Force

Centrifugal force is the force that pushes a vehicle away from the center of the road in a curve. This can affect your traction as it may lift the tires on one side of your vehicle off the road, depending on the speed at which you travel and where the center of gravity is located in your vehicle. Vehicles with a high center of gravity, such as SUVs, may roll over as a result of this force.

Hydroplaning

As discussed in the last chapter, hydroplaning is a condition where the tires of a moving vehicle ride on the surface of water on the roadway. The water on the roadway may have a mixture of oil and water, which makes the road even more slippery. When hydroplaning occurs, the tires lose contact with the pavement and lose traction. In addition to loss of vehicle control, hydroplaning increases your vehicle's stopping distance and decreases its capability to turn. Hydroplaning also makes it more difficult to accelerate and decelerate.

Force of Impact (Momentum, Kinetic Energy, Inertia, etc.)

These factors will affect vehicle handling:

1. Inertia. The law of inertia, also known as Newton's first law of motion, says that an object will remain at rest or in motion in the same state unless acted upon by some outside force.
2. Kinetic Energy. Referred to above as energy of motion, this is the energy that an object, such as your vehicle, has when it moves.
3. Centrifugal Force. This is the force that pushes a vehicle away from the center of the road in a curve.
4. Gravity. This is the force that draws objects to earth. Gravity affects uphill and downhill motions.

The faster you drive, the less time you have to react to hazards in the road around you.

Every time you increase one of these factors, you also increase the force at which you will hit an object. An increased force of impact also means an increased risk of injury or death in a collision. When you increase any of these factors, the result is:

• An increase in stopping distance.
• A reduction in vehicle control.
• An increase in the force of impact.

These factors affect the force of impact:

1. The speed at which you travel. When you increase the speed of your vehicle, you also increase the force of impact when you hit an object.
2. The total weight of the vehicle, including passengers and cargo. When you increase the weight of an object (such as your vehicle or the object you are about to hit), you increase the force of impact.
3. The distance your vehicle has to travel between the time you apply the brakes and impact. When the distance you travel before you stop increases, you increase the time you have to slow down before impact, thereby decreasing the force of impact. This decrease in the force of impact also decreases your risk of injury or death in a collision.

An increase in speed and weight will:

• Increase your vehicle's stopping distance.
• Reduce your control of the vehicle.
• Increase the force of impact.

The First Second of Impact

A crash can take place in a matter of seconds. Some experts determined that the following would happen to a driver if a vehicle traveling at 55 mph were to crash into a fixed object, in the first second of impact:

1. First tenth of a second: The front bumper and grill collapse.
2. Second tenth of a second: The hood scrunches up, rising and striking the windshield. The rear wheels, still spinning, lift from the ground. The fenders begin wrapping themselves around the object. The car's frame stops, but the driver and the rest of the car is still going 55 miles an hour. Instinct causes the driver to stiffen his legs against the crash, and they snap at the knee joint.
3. Third tenth of a second: the steering wheel starts to disintegrate, with the steering column pointing straight at the driver's chest.
4. Fourth tenth of a second: The front two feet of the vehicle is wrecked, while the rear end still moves at 35 miles per hour. The driver's body is still traveling at 55 miles per hour.
5. Fifth tenth of a second: The steering column punctures the driver's chest, and blood rushes into his lungs.
6. Sixth tenth of a second: The driver's shoes, despite being tightly laced, are ripped off his feet. The brake pedal breaks off. The car frame buckles in the middle. The driver's head smashes into the windshield. The rear wheels, still spinning, fall back to earth.
7. Seventh tenth of a second: Hinges rip loose, doors fly open and the seats break free, striking the driver from behind. At this time, the driver is already dead.

The above assumes these two things: the vehicle has no air bags (or they failed to activate), and the driver did not buckle up. If that driver had been properly secured by a seat belt, he would not be hurtling toward the steering column at dangerously high speeds and thus remain safe.

The crumple zone collapses upon impact, cushioning the passenger compartment.

Energy-Absorbing Features

You may not know it, but there are more safety features in a vehicle than just seat belts and air bags. Several vehicle components must be designed to meet Federal Motor Vehicle Safety Standards (FMVSS). How important are they? According to a December 2004 NHTSA report, these features mandated by the FMVSS saved more than 20,000 lives on American roads in 2002. Vehicle safety features that absorb the energy from a crash include:

• Crumple zones (the front and rear crash areas).
• Energy-absorbing bumpers.
• Side door beams.
• Reinforced windshield (windshields are specifically designed to reinforce the roof so it does not collapse).
• Padded dashboard.
• Head restraints.
• Air bags.
• Safety belts.
• Antilock brakes.

The crumple zone collapses upon impact, increasing the vehicle's stopping distance and cushioning the passenger compartment by absorbing the energy. Windshields keep you protected inside your vehicle. Seat belts keep you from being thrown around inside the vehicle, as well as from being ejected. Frontal air bags keep you from hitting the dashboard, and side air bags keep you protected from the sides. Head restraints keep your head from snapping back. These are just a few of the many design features in your vehicle that help to decrease the force of impact and increase your control over the vehicle as well as the vehicle's performance. The decreased force of impact works as a cushion that protects you in a collision.