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Tire marks, dented metal help police experts solve car crashes

Tuesday, October 15, 2002

By Lillian Thomas, Post-Gazette Staff Writer

Tire marks, crumpled metal, and bodies and debris flung from vehicles are better at telling the story of a crash than most witnesses.

Police officers studying accident reconstruction survey the curvature of a skid mark to determine the speed of a vehicle in a collision yesterday at the former Greengate Mall in Hempfield. From left are Penn Hills police officer Bernard Sestili, Murrysville police officer Charles Tappe, state police Cpl. Fran Winkeler, and Murrysville police Sgt. Lee Wilhelm. (Martha Rial, Post-Gazette)

They speak the language of math and physics, though. That's why, when state police Cpl. Greg Sullenberger responds to a call for a fatal accident, he never travels light. Sullenberger, a collision reconstructionist, packs an electronic surveying station, cameras, a thermometer, mechanic's tools, tape measures, lumber-marking crayons, spray paint, tire gauges, a level, and a hunk of tire filled with concrete called a drag sled.

He and other collision reconstructionists -- there are 34 within the Pennsylvania State Police -- are called to the scenes of serious crashes, usually involving fatalities. They collect complex sets of measurements and data, then use mathematical formulas to transform them into speeds, distances and directions of travel.

Last week, several dozen reconstructionists honed their skills at a conference organized by Sullenberger, the state police collision investigation coordinator for Pennsylvania. He works out of the Southwest Training Center in Greensburg.

Those attending heard lectures on specialized topics such as yaws, the sideways slipping of a vehicle that happens when a driver wrenches the steering wheel around too hard. Then they went to the Greengate Mall parking lot and put cars into yaws, measuring tire marks, collecting other data and running the figures into formulas to calculate speeds. In all cases, they came within 1 or 2 mph of the actual speeds, which were determined with radar guns.

After the conference, Sullenberger explained how the process works in real life.

He usually arrives at an accident scene after emergency medical personnel and local police or state troopers have responded, taken care of injured victims and secured the scene. He walks the scene, looking at tire marks; the positions of cars, debris and bodies; the damage to the vehicles and people; the condition of the road; and other factors that could contribute to a crash, such as an obscured stop sign.

Sullenberger talks to witnesses and people involved in the accident if they're there. But those in the best position to describe the crash often get it all wrong.

"Usually I put much more credence and weight to the evidence, to applying my mathematics and physics and coming up with an answer," he said. That's partly because interested parties -- say, a drunken driver who ran a stop sign -- might try to shape their stories to hide their culpability. But often it's witnesses genuinely trying to help who cause problems.

"Witnessing a collision is a stressful event for a citizen. Sometimes people will miss details. And sometimes people will consciously or subconsciously try to help you. Almost every investigator has had someone say, 'I heard the crash, then I looked up and saw this vehicle coming this way and that one coming that way.' Well, obviously if they heard the crash first, they couldn't see the vehicles as they headed toward each other."

The witnesses trying to "help" by filling in blanks, or substituting their own guesses for what they actually saw and heard, slows down the investigator by sending him or her down the wrong path.

Tire marks, on the other hand, have no agenda.

Sullenberger's surveying equipment is used to make extensive measurements of those marks, as well as distances between vehicles, objects and people at the scene. The measurements must be exact to within a fraction of an inch.

"Most people think any black mark on the road is a skid mark," he said, but they are broken down into numerous subcategories, such as skids, scrubs, scuffs and yaws, each of which is analyzed differently.

Sullenberger shoots still photos and often makes videos as well, using the camera to show what each driver would have seen, for example. He checks air pressure, tread depth and condition of the tires.

He examines the brake and turn signal light bulbs if they were hit in the collision. He can often determine whether they were on or not at the moment of impact.

He checks what's called the drag factor of the road, either by dragging that concrete-filled partial tire on the road surface or putting a vehicle into a skid on the surface.

"I get the vehicle up to a known speed, say 30 mph, apply the brakes, and skid to a stop, then get out and measure my skid marks." A mathematical formula is used to determine the drag factor, which is degree of friction of that particular road surface.

Though there have been many documented tests of various types of surfaces in various weather conditions, Sullenberger likes to test the actual surface at the crash site rather than using textbook figures.

There's only so far he'll go though.

"If the vehicle tire mark is 145 feet, I'm not going to do that, I'm not going to do 75 or 105 mph," he said.

If the details of reconstruction are complex, the principles are simple, Sullenberger said.

"Anyone can understand that the faster a vehicle is going the longer the skid marks," said Sullenberger. "That's square one, where we start."

That's exactly where the people who came up with the science of accident reconstruction started. By applying the laws of physics to cars, roadways and people, they came up with formulas for determining the friction of a given roadway -- the drag factor -- and for transforming that factor and the length of a tire mark into the speed the vehicle was going when it made the mark.

Investigators measure how severely the vehicle was crushed and use figures from government crash testing that determine a vehicle's "stiffness coefficient" -- what it takes to crush it -- to determine its speed at the time of the crash.

Finally, there is the branch of reconstruction known as occupant kinetics or kinematics. If someone is sitting in the front seat of a car going 50 mph and that vehicle comes to a sudden and complete stop -- say, against the trunk of a thick tree -- the occupant will continue going forward at 50 mph. The speed of the car, as well as the angle at which the collision occurs and the nature of the roadway, will determine where the occupant ends up.

This can be crucial in a case in which there are two drunken occupants in a vehicle that crashes, and the survivor claims he was the passenger.

If the dead man has a big contusion on his forehead that matches a spiderweb pattern break in the windshield, and the man claiming he was the passenger has a semicircular bruise on his abdomen consistent with jamming into a steering wheel, the people in a courtroom are more likely to believe an accident reconstruction expert's testimony that the survivor was the driver.

Sullenberger says that on all but the most complex accidents, he usually has a pretty good idea of what happened even before he starts working with the crash data. But he runs the computations and tests and often plays devil's advocate to himself.

"If I come up with what I think is the proper solution, I'll try other possibilities, and if they won't work and mine will, that increases my confidence in mine," he said.

There are computer programs for testing different sets of data with formulas, and also models that allow him to put things in motion on the screen.

When he's done, he's usually got a crash story that he's confident is right.

"If I'm sitting on the stand in courtroom," Sullenberger said, "and someone says, 'How do you know [a formula] works?' I can say I know not just because it's in books but because I tested it and it was accurate."


Lillian Thomas can be reached at lthomas@post-gazette.com or 412-263-3566.

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