Watching the Detectives

Like a detective, Tom Bundorf of Packer Engineering (Ann Arbor, MI) is after just the facts. Insurance companies hire the engineering consulting firm to reconstruct what happened in auto accidents on a scientific basis. The Packer analysis serves as the only objective evidence in court to reconcile conflicting testimonies. "Sometimes witnesses believe they recall the event as crystal clear," Bundorf says. "They will sometimes try to estimate speeds and distances, but given the facts of the scene we know these estimates are not physically possible."

In one of Bundorf's recent cases, a Chevy Lumina slid into oncoming traffic and struck a F150 pickup head-on. One witness reported the Lumina speeding around the curve, though the driver claimed he was within the speed limit when he lost control. In question was the matter of driver negligence, which could significantly affect the amount of an insurance award.

Unlike a police detective, the director of Laboratory Services at Packer is rarely present at the aftermath of an accident. Instead, the case file lands on his desk in his Ann Arbor, MI office months, even years, later. Bundorf would like to find, as any scientist would, accurate measurements of the crash. But in most insurance investigations, the only clues are photographs.

Due to the limitations in police resources, says Bundorf, "what you'll find is that documentation tends to be proportional to the severity of the crash. Fatalities generally receive a full mapping, but for a fender bender, there might not be any documentation at all. The majority of the cases we get fall somewhere in the middle." In the accident described earlier, Packer Engineering did not receive any numeric measurements. All that was left of the Lumina / F150 collision were six color copies of JPEG images.

But from these rough images, Bundorf and team can tell what happened in the critical seconds before impact, including the initial velocities of the vehicles, and in more spectacular crashes, the unique choreography of swerves and spins. How can a scientist proceed without any numbers just photos? A key player in this detective story is PhotoModeler, developed by Eos Systems (Vancouver, BC) as an affordable method to reverse engineer real-world objects into accurate three-dimensional measurements.

Photogrammetry

PhotoModeler automates photogrammetric analysis on photographs from film, digital, or video cameras. The software can quickly construct a 3D CAD model of an object on the basis of just two scanned photographs taken from different vantage points. To find the simplified interior parameter data, PhotoModeler performs a quick automatic calibration test on the user's camera.

Bundorf, however, has one small problem: no access to the original camera. The insurance investigation not only lacks the original camera for calibration purposes, it also cannot determine what kind of camera took the accident scene photos. Consequently, the ordinarily easy photogrammetic process must undergo another step to find the basic camera calibration.

Inverse Camera Analysis

"We make pretty exhaustive surveys when we go to the site of an accident," Bundorf explains, "In this case, we surveyed guide-rail posts, road centerlines, concrete structures, and adjacent buildings." Many accurate readings must be taken to guard against error, as some seemingly stationary objects such as posts and signs glacially shift in a few months' time. A ground survey can be achieved through surveying tools, or photo-documented with a camera with known calibration in PhotoModeler.

The inverse camera (IC) analysis uses the known objects in the photographs as control points to calculate the interior camera information. Conceptually, the control points of the survey determine the unknown camera information, which in turn can then dimension the final objects: vehicles and tire markings in the scene. PhotoModeler has an IC function, which performs this three-step process at once. As Bundorf adds more and more three-dimensional data on the on-screen photographs, PhotoModeler adjusts the six shots into a probable XYZ-coordinate system. With enough control point adjustments, the 3D version of the scene emerges, complete with distance units.

Dynamic Analysis and Crush Profile

PhotoModeler reveals true shape of the tire markings: the transferred rubber on the road indicating the extent of a vehicle's swerve and its braking distance. The program also recasts the photo pixels orthogonally--with the distortion and foreshortening effects of perspective removed, so that the scene appears like an aerial image. This bird's eye view of the mark may be included as a layer on the CAD map.

In the Lumina versus F150 scenario, Bracki observed an 85-foot skid mark, starting from the inside lane and ending in the outside lane where the point of impact of vehicles occurred. "If you just assume the Lumina runs into the truck and just stops right there as it hits, and the truck was stopped at impact," explains Bracki, "that gives the Lumina a velocity of about 40 to 41 mph, just from the skid marks alone."

This velocity figure does not take into account the full kinetic behavior of the two vehicles (the PhotoModeler analysis determined that the Lumina was pushed back a few feet after impact), nor does it give a full answer to the speed the Lumina was traveling before the driver braked. The crush profile leads to these final details.

Bracki maps out the difference of lengths between the crumpled metal and its original shape. The map of vectors, showing the force and angle required to cause these displacements, also gives the engineer clues into the storyline of the collision. In some more complicated cases, an oblique force might spin the vehicles into other objects. Or, when a car back-ends a truck hauling a John Deere backhoe, for instance, multiple points of damage on the car indicate when and where the loose construction equipment was flung into the mix. The direct head-to-head clash of the Lumina and the pickup is much more straightforward.

"Since [Tom Bundorf] knew where the vehicle was at the point of rest, he could draw these two vehicles from a bird's-eye view for me and show me what the mutual crush was between the two vehicles. From that mutual crush, I could calculate the energy of the impact itself." Bracki works out the physics through the total energy method. From the energy required to make the crush, he calculates the real initial velocities of the two vehicles.

"Essentially, the Lumina locked its wheels up from braking and it was on the inside of the turn. The Lumina followed a tangential path into the outside (or oncoming) lane of traffic where it struck the F150 head on," explains Bracki. "We have the car going--at the time when the driver stepped on the brake--somewhere between 51 and 56 miles per hour. So he's not speeding; it was a 55 mph zone. Something spooked him as he was coming around that turn and he just locked up his tires and slid right into oncoming traffic. He was at fault because he crossed the centerline, but there was an accusation that he was coming around that turn at about 100 miles per hour, and that's not true. He was somewhere within the vicinity of the speed limit. He just made a mistake. He just shouldn't have locked his brakes at that time because his velocity carried him into oncoming traffic."

Due to Packer Engineering's detective work, its insurance client could proceed with the claim, confident that the evidence will stand up in court, if challenged. "PhotoModeler is a very powerful tool," says Bracki. According to Bundorf, the only alternative to the technological solutions presented here is to go out to the scene and "eyeball" the measurements in respect to the photographs--which, like some drivers' testimony, would be less than reliable.