America's highways are the safest in the world, thanks, in large measure, to devices that protect cars and their occupants in collisions with roadside hazards. Some of the most successful highway crash cushions were designed by WPI Provost John Carney, whose work has earned him an international reputation in automotive safety.
The slow-motion film shows a driverless automobile traveling along a paved track. The camera follows the car as it approaches the blunt end of a concrete median barrier. The car strikes the barrier head on, and like a dull knife, the thin concrete slab slices deep into the engine compartment as the car's front end explodes in a shower of metal and glass. Had this been a real accident instead of a crash test, the car's occupants would be dead.
"The idea was to create a forgiving train, so that in the event of a crash, the energy would be distributed evenly throughout the train, instead of having it concentrated on the impacting cars."
In the next film, a pickup truck speeds toward another median barrier at more than 60 miles per hour. This time, just ahead of the start of the concrete strip is a line of hollow, black plastic cylinders connected along their sides by metal cables. The truck strikes the first cylinder, which immediately flattens. In four tenths of a second, the entire 27-foot column of cylinders compresses to just three feet, absorbing the truck's kinetic energy and bringing it to a stop, its grill and hood slightly crumpled. Instrumentation in the truck shows that the occupants would have survived the crash with no serious injuries. After the crash, the cushion restores itself to its original shape, ready for the next wayward vehicle.
The life-saving line of plastic cylinders is the culmination of more than 25 years of research in impact mechanics and crash attenuation, work that has made John F. Carney III, WPI's provost and vice president for academic affairs, an internationally known authority on highway safety.
This sequence from a crash test of Carney's new reusable line of crash cushions shows how the plastic cylinders compress to absorb the energy of an errant vehicle (first two frames), then restore themselves to their original shape, ready for the next crash.
Currently chairman of the Transportation Research Board's Committee on Roadside Safety Features, an arm of the National Academy of Sciences and Engineering, Carney has conducted groundbreaking research in structural mechanics and roadside safety with support from the National Science Foundation, the Federal Highway Administration, and the departments of transportation in six states, among other organizations. The results have been published in over 140 research publications, and have been the subject of more than 100 presentations at national and international meetings and conferences.
But his greatest accomplishments may well be the many lives saved by the safety devices he's designed, devices that have come between motorists and such deadly hazards as abutments, bridge pilings and median barriers.
Carney says he didn't set out to devote his career to transportation safety. A native of Lowell, Mass., he received his bachelor's degree in civil engineering from Merrimack College and his master's and Ph.D. from Northwestern University, where he was supported by a National Defense Education Act Fellowship. He joined the engineering faculty at the University of Connecticut in 1966 and for a half decade conducted research in vibrations, elasticity and the stability of structures.
In the early 1970s, UConn's Civil Engineering Department got a phone call that changed Carney's life. It was from the Connecticut Department of Transportation (DOT), which had just suffered the loss of two highway maintenance workers, killed when a car ran into them as they were working alongside an interstate highway. The department wanted to find a way to prevent such tragedies in the future. With his background in structural mechanics, Carney seemed a logical choice to tackle the problem.
"As I started thinking about how to protect workers," Carney says, "I got interested in what happens to structures when loads are applied to them very rapidly - as when a car hits a barrier at highway speeds. Structural materials are sensitive to the rate at which loads are applied. The characteristics of steel, for example, change dramatically when loads are applied quickly."
For the Connecticut DOT, Carney designed a device made of four thin-walled mild steel cylinders attached to an aluminum plate on one end and the back of a highway service truck on the other. This truck-mounted attenuator (TMA) was designed to travel behind highway crews performing mobile operations, such as line painting, or act as a shield for crews engaged in temporary roadside work. The design was verified in a series of crash tests that proved that it could protect both road crews and drivers.
By the mid-1970s, the TMAs were put into service on Connecticut highways. "Now, 20 years later, Connecticut has about 100 of these in use every day," Carney says.
The success of the truck-mounted attenuator led Carney, with support from the Federal Highway Administration and the Connecticut DOT, to develop similar devices to protect errant motorists from roadside hazards. "Until about 1960," Carney says, "there was little national interest in highway safety. As far as many federal and state agencies were concerned, if the Śnut behind the wheel' was stupid enough to drive off the road, any adverse consequences were his or her problem. But it was about then that Congress became alarmed at the rate at which we were killing ourselves on the road and began providing research funds to address the growing fatality rate."
As interest in crash protection grew, the federal government began promulgating regulations that required the effectiveness of roadside safety devices to be demonstrated in controlled crash tests. Over the years, those regulations have expanded dramatically and today run to several hundred pages. Crash cushions, for example, must pass eight tests using different speeds, different vehicles, and different types of crashes (head-on, at a slight angle, and tangential). The tests must demonstrate that vehicle occupants would not be exposed to unsafe conditions in a crash.
"There are two basic requirements you have to meet," Carney says. "They are associated with the occupant impact velocity and the subsequent ridedown deceleration. If you are driving without a seat belt and hit a crash cushion, the vehicle will immediately begin to decelerate, but you will keep moving at your pre-impact speed until you hit the vehicle interior. If the velocity at which you hit is too high, there's a high probability that you'll be seriously injured or killed. Current regulations limit the occupant impact velocity to no more than 12 meters per second.
"Once you've hit the interior, you decelerate with the vehicle. If that deceleration is too severe, you'll die. Your brain is swimming in a fluid to protect it from sudden acceleration and deceleration, and it can withstand high G forces for short periods of time. In fact, the regulations allow an average 10 millisecond deceleration of 20 Gs, which is very high - much higher than what astronauts experience. In addition to these requirements, the regulations specify that the passenger compartment can not be crushed and that the vehicle may not roll over."
The first stationary crash cushion Carney developed for the Connecticut DOT was designed to protect vehicles from crashing into bridge piers or wide concrete barriers at exit ramps. Like the truck-mounted attenuator, it used steel cylinders that collapsed laterally on impact, absorbing the kinetic energy of a wayward car. Steel cross members were attached to the insides of the cylinders to stiffen them when the cushion was struck near the back or at an oblique angle, so as to safely redirect the vehicle. The 26-foot long, 12-foot-wide cushion employed 14 cylinders of various sizes arranged in the shape of an arrowhead.
For his next project, Carney tackled the growing problem of unprotected highway median barriers. "These barriers, sometimes called the concrete safety shape or New Jersey Barriers, are popular because they don't require any maintenance," Carney says. "But you have to terminate a run of median barrier at some point, and the blunt end is a severe hazard." Carney modified his wide highway cushion and produced an attenuator that uses a single line of steel cylinders of various widths.
Carney's early crash cushion designs have been used by state transportation departments around the country and overseas. In addition to being highly effective at saving lives and reducing injuries, they're relatively easy and inexpensive to build. But they do have an important disadvantage: they can be used just once. "Once a cushion is hit, it's out of commission," Carney says. "Often, it will sit for days, weeks - even months before it's replaced. A crash cushion that has not been refurbished won't work, and that's bad news for the errant motorist and a severe legal exposure for the agency responsible for the device."
For his next generation of crash cushions, Carney knew he would have to find a material that would provide the same level of protection as mild steel, but that was also capable of springing quickly back into shape. By this time, he was a professor of civil and environmental engineering at Vanderbilt University (where he would serve as associate dean for graduate affairs and, later, associate dean for research and graduate affairs). One day, during a meeting at the Tennessee Department of Transportation, he got a tantalizing lead.
"The head of maintenance for the state was talking about problems they were having with corrosion in submerged metal drainage pipes," he says. "Instead of replacing the pipes, they were lining them with high-molecular-weight, high-density polyethylene tubing. The intriguing thing was that these were pre-formed cylinders made in exactly the right diameters for crash cushions. Also, the material is relatively inexpensive - comparable in cost to that of steel cylinders."
Since nobody had ever considered using polyethylene pipes to dissipate energy, little was known about their deformation characteristics. To learn more, Carney obtained some small-diameter pipes and conducted preliminary lab tests. The encouraging results convinced him to apply for a grant from the federal Strategic Highway Research Program, which funded the project after the State of Washington agreed to pay half of the costs. Additional small-scale tests demonstrated that the polyethylene pipes had the characteristics Carney sought for his reusable crash cushions.
With these results in hand, he obtained funding from a consortium of states to develop a working device. Rather than moving on immediately to an expensive full-scale crash-testing program, Carney decided to refine his design through computer simulations using the tools of finite element analysis.
"A typical crash test at one of the few facilities in the U.S. capable of conducting them can cost $15,000 to $20,000," he says. "Using the standard trial-and-error approach, you may end up running 50 to 75 tests - at a cost that can easily exceed $1 million - to get a design that works, if you ever do, that is.
"Now, with sophisticated computer software and workstations, you can eliminate most of the trial and error and develop a design that is likely to pass the required eight tests the first time out. We did many scale model experiments along with the finite element simulations. When we were finally ready for full-scale crash tests, we were able to pass all eight tests with very few failures."
The finite element techniques Carney used to model crash cushions have proven useful in another transportation safety project. In 1980, while on a sabbatical leave at Cambridge University, he worked with a colleague who later established the British Advanced Railroad Research Centre with funding from British Rail. Knowing of Carney's success with roadside crash cushions, he asked him if would be interested in working on a British Rail initiative to develop safer passenger trains.
Carney began studying the mechanics of rail crashes and found that the design of passenger coaches was a major contributor to the injuries and deaths suffered in rail accidents. "Rail stock throughout the world tends to be built with a very rigid chassis," he says, "so the cars don't dissipate energy well. In the worst accidents, cars override their neighbors and break right through the skin of the other cars."
To prevent that kind of accident, Carney proposed the revolutionary idea of building sacrificial crush zones into the ends of rail cars. Using finite element analyses, he and his colleagues developed a variety of crush zone configurations and determined that a zone roughly 3 feet long at either end of the car can significantly reduce fatalities in rail accidents. "The idea was to create a forgiving train," Carney says, "so that in the event of a crash, the energy of the collision would be distributed evenly throughout the train, instead having it concentrated in the impacting cars."
British Rail recently spent 1.5 million pounds (about $3 million) to organize and conduct a full-scale crash test program that verified the results of the computer models. The concept has yet to find its way into actual rail cars, Carney says. Still, he is hopeful that it will be employed in new generations of rail coaches in Europe and the United States.
In his most recent research, Carney, who joined the WPI faculty in 1996, has revisited the problem that first brought him into the field of highway safety: the truck-mounted attenuator. With funding from the federal government and a consortium of state departments of transportation, he has developed a reusable version of the cushion he created for the state of Connecticut in the 1970s. Like his reusable crash cushions, the new TMA is made from polyethylene.
Thanks to his research and the work of many other researchers, entrepreneurs and transportation agencies, Carney says the state of American roadsides have changed dramatically since he first began designing crash cushions.
"The United States now leads the world in highway safety," he says. "Our highways have never been safer. But there is still much to be done. Despite the strides we've made, safety is still largely an afterthought in highway design. That's a battle we've been fighting for decades. It might well make a good subject for faculty research and student projects right here at WPI."
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