At the race track, where drivers challenge the limits of man and machine and often crash when they exceed them, one type of wreck causes the most seasoned observers to gasp in horror: the driver's-side T-bone.
In a hit to the front or the rear of a race car or a street car, several feet of structural metal and mechanical components collapse, absorbing much crash energy before it reaches the passenger compartment. Front or rear parts rarely intrude into the passenger compartment. But on the side of any vehicle, mere inches of metal and plastic stand between the people inside and the point of impact outside, and it's far more likely that the door and the other object in the impact — be it a telephone pole or another car — will smash through to the passenger compartment.
So racing veterans consider the driver's-side T-bone one of the worst accidents in motorsport: in the blink of an eye, the body of a driver with very bad luck can end up sharing its seat with the tortured and twisted side structure of its own car and with the front end of the other car. Street drivers face the same risk crossing intersections.
In the 1970s, the government issued Federal Motor Vehicle Safety Standard 214, the first regulations requiring all passenger cars sold in the United States to sustain a certain amount of side-impact force before bending into the passenger compartment. Automakers installed horizontal steel beams inside car doors to strengthen them. In the 1980s, the government updated FMVSS 214 by adding minimum performance requirements for protection of the thorax and pelvis in a 33.5-mph impact. The new rules took effect from 1994 to 1998 for passenger cars and from 1995 through 1999 for light trucks and sport-utility vehicles. To meet the new rules, automakers began adding padding and side airbags that pop out of the door or outer edge of the seat.
But even as it phased in the new regulation, the National Highway Traffic Safety Administration began considering yet another update. Why? The door-reinforcing beams reduced the number of injuries in side impacts, but did not reduce deaths, according to data from the federal Fatality Analysis Reporting System. Side impacts were still killing about 9,800 people and injure more than a million every year. In fact, NHTSA found that the beams did reduce the number of deaths in one-car accidents, such as a car crashing into a tree or pole, but not in wrecks involving two or more vehicles. That's because the beams helped reduce head injuries in single-car impacts but not in multi-vehicle crashes.
In single-vehicle impacts, the roof rail and sill at the bottom of the door work with the side-impact beam to resist intrusion; all three absorb energy before and during bending. But in multi-vehicle impacts, the entire force of the impact may be focused on the door, rather than on the door and surrounding structures.
Making matters worse is the proliferation of sport-utility vehicles and other light trucks, which boosted the odds that a car will be hit by a truck. More than 43 percent of the deaths and 37 percent of the serious injuries in side impacts occur in accidents in which a truck is the striking (or so-called bullet) vehicle, according to government statistics. Fatalities caused by light trucks hitting cars doubled from 1980 to 1996. So NHTSA questions whether its latest side-impact regulation offers enough protection in an increasingly truck-populated nation.
"Our fleet is quickly becoming a fleet where half the vehicles are light trucks and vans," Tom Hollowell, chief of NHTSA's crashworthiness division, says of the vehicles on American roads as the 20th century closes. "When we put the crash test in place, there were less than 20 percent light trucks. The last few months they've been over half of new-vehicle sales. So we do need to change testing so that it's more representative of today's fleet."
Meanwhile, a new head-injury rule may cover some of the areas side-impact rules miss. The head-injury rule basically requires automakers to shield the upper interior — the inside roof rails and the stiff pillars that support the roof, windshield and backlight — so that a head hitting it in a 15-mph impact won't be fatally injured. Since occupants' heads are more likely to hit the upper interior on the side than in the front, automakers and safety engineers are devoting lot of time and effort to protecting the head inside the vehicle, and to keeping it inside the vehicle, in side impacts.
While side-impact collisions cause about 25 percent of all injuries to car occupants, they cause more than 35 percent of serious and fatal injuries. And more than half of the serious and fatal injuries in side-impact collisions are to the head.
In many cases, automakers are combining side and head protection. The thorax-only bag of the first generation of side airbags is giving way, on some models, to seat-mounted head-and-thorax bags. On others, the thorax bag works with an inflatable curtain that drops from the roof headliner to cover the windows and side pillars or with an inflatable tube that stretches diagonally across the side windows and pillars from the base of the windshield to the roof behind the driver's head. Over the next two to three years, automakers will put inflatable curtains, which resemble small inflatable rafts, by one supplier alone on at least 45 car and truck models.
For the engineers who design safety equipment, side impacts present a much greater challenge than front impact.
The system that controls the front airbags has little less than 30 milliseconds — thirty thousandths of a second — to sense an impact, decide whether to deploy the airbags, and inflate them by the time the force of the impact throws the driver and passenger inside against the steering wheel, dashboard or windshield. In a front impact, the crash stops the forward movement of the vehicle. But the flesh-and-blood bodies of the occupants keep going at the pre-impact speed until they are stopped, by seat belts and airbags if they have them, by the interior or windshield if they don't.
Thirty milliseconds may not seem like much, less than a blink of the eye. But it's more than triple the time airbag systems have to respond to a side impact. Safety engineers aim to get that time down to five milliseconds, but today's side systems are operating at closer to 10 milliseconds.
Side-impact airbags serve a somewhat different function than front airbags. They are designed to come between the occupant and intruding metal and glass, rather than to stop and cushion a person's forward acceleration. In other words, in a side impact, the side of the car — and the leading edge of the bullet car — move very rapidly toward and into the person inside, rather than the other way around.
The duty of side airbags is "filling the gap between you and the striking vehicle as quickly as possible," says NHTSA's Hollowell. Since the hips are closer to the point of impact than the head, side airbags fill first at the bottom and then fill up to protect the torso and then, in some cases, the head. As with other new technology, side airbags first appeared in large and luxury vehicles, which need them less than small cars because their doors are thicker and larger. As prices fell and production volumes grew, automakers began putting side bags on smaller cars.
Even if there is no intrusion — if the striking car or stricken pole doesn't force its way into the passenger compartment — side crashes stress the human body with forces it wasn't designed to bear, and bend and twist it in way it was not designed to bend. For example, protection of the brain is heavier in front than on the side, and the spring-like materials that support and protect the internal organs tear more easily in side impacts.
While seat belts can't prevent that sideways twisting and bending, they do moderate it. They also keep crash-test dummies — and people — from being thrown across the vehicle and slamming into other people or hard surfaces. Just as important, they help side airbags do their job by keeping people in the position the bags protect.
With or without intrusion, another major threat in side-impact crashes is the thick, roof-supporting vertical piece of metal between the doors. It is called the B-pillar, and as a part of the basic vehicle structure, it is much harder than the door. To protect occupants' heads, automakers cover the B-pillars with padding and plastic materials that absorb energy by crushing or use inflatable protections such as the curtains and tubes. A 1997 Insurance Institute for Highway Safety crash test of the then-new BMW Head Protection System — an inflatable tubular restraint — on the 5-series model showed it reduced the force sustained by the crash dummy's head from nearly five times the fatal level to substantially below it. In a 20-mph pole-intrusion side crash, the dummy in the car without the HPS suffered an impact with a 4,720 head-injury criterion rating. The rating for the dummy in the car with the HPS was only 620, which indicates far less chance of serious injury or death. Government standards require a rating of less than 1,000, but automakers tend to want a rating substantially lower.
Unlike ordinary side airbags, inflatable curtains and inflatable tubular restraints also offer protection in an arena that has automakers increasingly concerned: Rollover.
Although practically any vehicle can be made to roll over in the right circumstances, vehicles with a high center of gravity, such as today's preternaturally popular sport-utility vehicles, have a greater propensity to tip over onto their sides and roofs. And although rollover can occur without an impact, a sudden, heavy push from the side can definitely help increases the chances.
The biggest danger in rollover is ejection of all or part of the human body. A head or arm that comes through the window as the vehicle is rolling onto it will almost certainly be gruesomely crushed. While ejection caused 26 percent of the vehicle-occupant deaths in all accidents, the ejection rate in fatal crashes was twice as high for people in light trucks as for people in cars. And sport-utilities had the highest rollover rate of any passenger vehicles involved in fatal crashes: 36 percent, compared with 24 percent for pickups, 20 percent for vans, and 15 percent for cars.
Safety engineers provide three tools to prevent ejection: seat belts, inflatable curtains and tubes, and advanced window glazing. Automakers, anxious to protect their proverbially profitable sport-utility sales without neglecting buyers of their other products, are rapidly adding the inflatable curtains or tubes and upgrading seat belts to include pretensioners, which yank up belt slack in an impact to hold the occupant securely in his seat. And advanced window-glazing, which was designed to make it harder for thieves to break into cars and trucks, turns out to also help prevent ejections in rollovers, says NHTSA's Hollowell.
As rally drivers know, it is not necessary to be hit by anything to roll over. Enough of a sideways slew at a high enough speed can leave the side of the vehicle facing in the direction of forward movement — and tip it over. But most rollovers off the race track occur after an impact. Safety engineers are hard at work on sensors that can measure the difference between the horizontal angle of the vehicle and the angle of the ground beneath it — or the difference between the angle at which the vehicle will slam back down onto its feet and the few degrees more tilt that mean it will roll over.
This is one of the biggest challenges safety engineers face.
When rollover sensors are perfected and put into production, inflatable curtains or tubes linked to them will stay inflated longer than they do in today's vehicles. But in the meantime, both still provide the best ejection-prevention available. Both already stay inflated far longer than a standard air bag, and both remain in place after deflating. That means regardless of how long after the initial impact rollover occurs, the heavy fabric of the curtain or tube will help keep people inside.
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