Anatomy of a crash, Part 2

Skid marks are the MVP of accident clues. By tracing their paths, police can often figure out where impact occurred in the road, the speeds cars were traveling, and just how hard the drivers tried to avoid the crash.

I learned this a few weeks ago while watching accident reconstruction experts work a staged crash scene as part of a State Police training exercise. Within minutes of the crash, tire marks from the two vehicles used in the accident, involving a Chevrolet Impala and a Ford Focus, were outlined with chalk, measured and photographed, as if they were evidence at a crime scene (which in some cases they would be).

But just as I was starting to appreciate the incredible value of tire marks, Lieutenant Stephen Walsh, who spent 24 years on the State Police's accident reconstruction team, threw me a curve.

"Actually, with antilock brakes, you very rarely get any preimpact evidence anymore," Walsh said, meaning, no skid marks at all. "But you don't necessarily need tire marks. You have a lot of other tools to work with."

Police might determine car speeds by measuring how many inches a vehicle's door has been crushed in, Walsh said. Or they might calculate the linear momentum of the strewn vehicles to approximate where they collided. Or they might analyze a car's electronic "event data recorder," which logs such things as precrash speeds and the point at which the brakes were applied before impact.

"You can always do something," Walsh said.

This week's column picks up where last week's left off, offering a bit more on the detective work involved in accident reconstruction analysis.

The training exercise called for a professional driver to ram the Impala into the empty Focus. It was easy to tell how fast the Impala was going, as police had set up one of those big digital signs linked to a radar gun at the test site, an old military runway in Ayer. But what if that sign board wasn't there?

Enter Sergeant Kerry Alvino of the State Police's Collision Analysis & Reconstruction Section, which investigates just about every fatal and near-fatal accident in the state. With a tape measure in hand, Alvino scanned where the Impala had crushed in the passenger-side door on the Focus, and jotted down the depth of the dent.

The National Highway Traffic Safety Administration - as well as car insurance companies - tests the crashworthiness of every vehicle on the market, she said. Based on the data from those tests, reconstruction experts can often tell at what speed a particular make and model of a vehicle was struck, by plugging the "crush depth" into a formula.

The Ford's crush depth, about 20 inches, was consistent with a 35-mile-per-hour crash, Alvino said. A "stiffer" vehicle, such as a BMW, which has specially reinforced doors, wouldn't have incurred as deep a dent, she said.

I asked Alvino how badly the Focus would have been crushed had the Impala hit at 55 miles per hour.

"I would expect that crush to go over to somewhere near the centerline of the vehicle. To the center console," she said. "I've been doing this long enough, and I'm not bragging, but I can go to a crash like this and look at the crush and say, that's about a 30-35-mile impact by sight."

The other car, the Impala, didn't look that damaged to me. But Alvino also measured it, this time recording the car's length.

Why do that?

"It doesn't look like the car got crushed at all, but the crush is just so even because it hit the other car with such a wide, flat area," she said, pointing to the entire grille of the Chevy. "That whole front is crushed in probably 4 or 5 inches that you can't see with the eye. If we peeled off the hood, I think you'd see the reinforcement bar is pushed all the way back to the radiator."

Once back at her office, Alvino said, she'd log into a software database that tells how long the Impala is really supposed to be. The discrepancy between those figures, she said, also could be used to corroborate crash speed.

"Know what Newton's cradle is?" Walsh asked me, a few minutes later. "The five balls hanging from strings? Lift one up, let go, one comes out the other side. That means momentum going in, in this case the impact, is equal to the momentum going out. So if you can determine the amount of momentum that came out of this impact, you can determine the momentum that went into it," he said.

"Now you can adjust for vehicle angle, approach vectors of the vehicle, and establish speeds."

This is how accident reconstruction experts talk. To them, a crash scene is one big physics or math problem. It's been that way since the 1950s, and has become even more so in the computer age.

Eyewitness accounts, while recorded, are not given significant weight unless crash-scene measurements back them up, police say. Because of the speed and abruptness of most car crashes, people's brains can't register everything that happens, Walsh said.

"Physical evidence doesn't lie. It can only be misinterpreted," he said. "Witnesses can see things, but they fill in the pieces. When they didn't see something, they use logic sometimes. And of course, they can sometimes lie. So you work with the aftermath, and you don't make any assumptions."

Still, as reliable as modern methods of accident reconstruction are, I asked the police whether any crashes stump them. Do they ever come across scenes where they have no idea who hit whom?

"In most cases, you can determine how. What you can't always determine is why," Walsh answered. "Car runs off a road, hits a tree, driver is dead. You can determine how that happened, and sometimes you can rule out the whys. You might be able to rule out it was a medical episode - they suffered a heart attack or a stroke. You can rule out mechanical failure with a vehicle. But you don't know whether he fell asleep, or whether he was talking on a cell phone."

What drives you crazy about local drivers? Is there a traffic rule you've always wondered about, or a pet peeve that never fails to annoy you? Send us a message about it: ciweek@globe.com. We'll check it out.