The Science of Safety

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Simple physics can demystify car accidents
Regardless of what a driver does to get into a pickle, what happens next is generally predictable. Sir Isaac Newton mapped it all out in the 17th century. Newton's three laws of motion (slightly modified for a 21st century teeming with cars and trucks) tell us that out of control vehicles in motion continue moving until they hit something. In any collision, the rate of deceleration is directly proportional to the impacting force and inversely proportional to the weight of the vehicle and its contents. Every force applied to a colliding vehicle results in expensive damage plus an equal and opposite force---and damage applied to the object it hits.

We know that last law all too well from experience, but let's see how Newton's principles can shed light on specific safety situations.

Law enforcement agencies persistently remind us that "Speed Kills!" Actually, none of the above three laws support that familiar refrain. While excess speed may precipitate a loss of control, it's the slowing down, not the speed, that gets you. The human body can only tolerate so much deceleration before sustaining injury. Bump your head into a wall at walking speed and you'll be bruised; smash your noggin into a tree at downhill-skiing velocity or into the windshield pillar during an accident and a concussion, a skull fracture, and neurological damage are the result. Exceed 40g of deceleration and all bets are off: only supremely fit humans can survive 40 times gravitational force for more than a few milliseconds.

Whether or not the driver is fully in control, as long as there are no radically abrupt changes in velocity, there's no injury. If the rear wheels slip wide or the front of the car veers off the intended path, as long as there's nothing to hit and nothing to trigger a rollover, all that's likely to get bruised is the driver's ego.

Impact Speed Does the Damage
But sometimes a collision is unavoidable. This is where velocity is a critically important factor. The kinetic energy of the vehicle and its contents rises with the square of its speed. In other words, the energy of a car traveling 60 mph is four times higher than the energy of the same vehicle traveling 30 mph. Before you can step out of the vehicle to assess collision damage, all of the kinetic energy must be dissipated. Applying the brakes or skidding the tires converts some of your momentum to heat and noise energy, which causes no injury. The rest becomes the force mentioned in laws two and three (above), which crumples fenders, inflicts injury, and raises your insurance premiums.

Fortunately, the crush zones built into modern vehicles are very effective at absorbing energy and softening the slow-down process to protect occupants from too-abrupt deceleration. There are also new technologies like that available on the new Lexus GS sport sedan, which has a radar-based system that senses an inevitable collision and automatically applies the brakes to reduce the vehicle's kinetic energy before impact. This helps reduce accident severity, particularly when the driver fails to properly respond to the situation.

Big Cars vs. Little Cars
What about the notion that heavier cars fare better in collisions than lighter vehicles? According to law two, the heavier the striking object, the more gradually it slows down. Less deceleration means a lower likelihood of occupant injury, but there can be mitigating factors that can un-do the benefits of greater weight. The effectiveness of crumple zones and safety systems can be more critical than vehicle weight.

Keeping the Passengers in Position
What about the role of seatbelts, airbags, and other energy-absorbing interior materials? Newton's first law also applies to occupants. After the vehicle makes contact with a phone pole or another vehicle, the interior compartment begins stopping while occupants tend to keep right on cruising for a few milliseconds. This culminates in the "second" collision your high school driver's Ed instructor warned you of. Without seatbelts and/or airbags to arrest motion and absorb bodily kinetic energy, occupants will crash into the steering wheel, instrument panel, seat back, or roof pillar with sufficient force to cause severe injury. Crashing into a fixed barrier at 30 mph imposes the same kinetic energy on an occupant as falling from a two-story building.

While a simple lap belt helps stop the lower half of your body, lap and shoulder belts significantly reduce the likelihood of contact with the interior. According to the National Highway Traffic Safety Administration (NHTSA), buckling up reduces the chances of death in a collision by 75 percent. Belts stretch and their anchors are engineered with give to soften restraining forces, thereby avoiding or at least minimizing belt-induced injury. But in more severe collisions, belts alone won't stop occupants from making contact with interior furnishings such as the steering column, windshield pillars, and dash. Airbags save the day by absorbing additional deceleration energy. According to NHTSA, seatbelts are currently saving well over 10,000 lives per year while airbags kept 8,000 U.S. drivers off the fatality rolls between 1991 and 2001.

Staying Right-Side Up is Critical
One-third of the current highway deaths are attributable to rollover accidents. SUVs figure prominently in the statistics because of the relationship between their ride height and track width.

SUVs ride higher to provide the more commanding view of the road that many drivers appreciate and to clear ruts and rocks in the event they venture off-road. Elevating the vehicle also raises its center of gravity (cg), an imaginary point around which the vehicle's mass is evenly distributed.

All vehicles roll (lean outward) on their suspension systems during cornering maneuvers. The higher the cg, the more the tendency for a large roll angle. In extreme conditions, a precipitous roll angle results in the inside tires losing contact with the pavement. In two specific scenarios, this roll motion can continue until the vehicle rolls over.

The first is the high-speed lane-change maneuver that most every driver must use now and then to avoid colliding with something in the road. It's a quick twitch of the wheel one way to avoid the object followed by reverse steering to return to the original lane of travel. In some vehicles, the resulting rocking motion destabilizes the vehicle. One or more wheels may briefly lift into the air during the second half of the maneuver. Or the rear wheels may lose their grip with the road and begin slewing sideways. Either action tends to alarm the driver, sometimes prompting steering and/or braking inputs that compound the situation.

According to NHTSA, in 95 percent of the rollover accidents, the vehicle is "tripped" by wheel contact with a curb or shallow ditch. The mechanism at work here is what engineers call an "overturning moment." The magnitude of the overturning moment is directly proportional to the mass of the vehicle, the height of its center of gravity, and the lateral (sliding) speed resulting in contact with the curb or ditch. Obviously, a lightweight vehicle with a low center of gravity is the most resistant to rollover.

There are readily available technologies that remedy the above rollover scenarios. Electronic Stability Control is very effective at helping the driver maintain control during emergency maneuvers; that means the likelihood of sliding sideways in an ESC-equipped vehicle is greatly reduced. In addition, some systems automatically apply the brakes any time a risky cornering threshold is exceeded. This diminishes the roll and rocking motion of the vehicle to the extent that loss of control, a sideways slide, and tip-over are less likely.

Sir Isaac Newton was too busy defining the laws of physics to imagine that we'd be motoring around bumping into each other centuries after his day. But Newton's definitions are all encompassing. The most rudimentary grasp of physics is enough to understand what happens when drivers lose control and become accident statistics.

by Editors /

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