Detective Cpl. David Cohen at the site of a fatal crash in Montgomery County. (Fredrick Kunkle/The Washington Post)

This is the final installment in “The Wreckage Speaks,”  a three-part series on Tripping examining how officers with the Montgomery County Police Department’s Collision Reconstruction Unit investigate — and, in essence, re-create — major traffic accidents. (Part One: A crash reconstruction team tells stories that victims can’t) (Part Two: How police use impound lot to recreate a fatal crash)

The driver took his eyes off the road for only the amount of time needed to adjust the air conditioning in his car. He did this because his 13-year-old daughter, who was riding in the back seat, had become cold and asked him to raise the temperature.

But as the driver did so, his Honda Accord strayed from its lane toward an oncoming vehicle. A second passenger — the driver’s wife — died in the head-on crash that followed.

It’s a chain of events that seems so simple, so unjust, so ironic and so cruel. But it was also the conclusion that fit the evidence compiled by the Montgomery County Police Department’s Collision Reconstruction Unit (CRU).

Using witness statements, gouge marks in the asphalt, onboard computer modules and even the pattern of leaking fluid from the damaged cars, the crash reconstruction team concluded that the Honda’s driver, Xiaolin Fu, was at fault.

Fu, reached at his home, declined to talk about the case.

As part of a year-long look at how the team analyzes traffic fatalities, I had been called out to the scene of the two-car crash on Bowie Mill Road last summer. At first glance, it didn’t look like a fatal crash. Neither car — Fu’s blue Honda or an orange Mustang that had been traveling in the opposite direction near Fraley Farm Road — seemed to have sustained the devastating damage one might expect from a fatal crash. At first glance, it was also easy to jump to the conclusion — unwarranted, as it would later turn out — that perhaps the driver of the flashy sports car had been pushing the limits that day as the vehicle entered a bend on Bowie Mill Road.

Both Fu and William E. Gonzalez, 22, who was driving the Mustang, told police they had been going no faster than the 40 mph speed limit when the crash occurred.

The drivers also lived less than a mile from one another in Olney. Fu, 51, had been headed to church that morning with his wife, Wenshou Zhu, 45, and their daughter. Gonzalez, who was alone in the Mustang, was on his way home after attending a gathering of car aficionados in Northern Virginia. (Gonzalez did not respond to requests for comment left with his parents.)

Detective Cpl. David Cohen peers inside a Mustang involved in a fatal head-on collision on Bowie Mill Road near Fraley Farm Road on July 16, 2017. The driver of the Mustang survived, but one of the passengers in the other car was fatally injured. (Fredrick Kunkle/The Washington Post)

Now it was up to Montgomery County’s crash reconstruction team to investigate what had happened. The team is summoned about 30 to 40 times a year, whenever there’s a crash with fatalities or life-threatening injuries. Their job is to accumulate, analyze and review all the evidence, determine who’s at fault and help decide whether criminal charges should be filed.

As Officer Jacob A. Burley, one of the newest members of the team, is fond of saying, it’s using Newtonian physics to bring clarity to the chaos of a deadly crash. The team tells the story that victims cannot.

On the day of the crash, Burley and six other team members first spent hours gathering evidence from the crash site. Some took photographs, about 200 images in this case. Others, using high-tech tools like surveyor’s instruments, logged data from points around the wreckage so that a computer could re-create the site later in 3-D maps.

After the wreckage was towed away, Burley — who was acting as lead investigator for the first time — set about trying to determine the crash’s cause. First, he checked the license, insurance and registration status of the vehicles and their drivers. He found that Fu had a clean driving record; Gonzalez did not, according to police and Maryland District Court records.

Burley next obtained the weight of each vehicle, which is done using either manufacturers’ specifications or the actual weights as recorded by special scales. Then Burley reviewed witnesses’ statements, some of which were obtained at the hospital from survivors.

Fu told police that he and his family were running late for a 10 a.m. church service in Rockville that morning. They were headed west on Bowie Mill Road, where the road turns to the right, when he took his eyes off the road.

“My daughter asked me to turn the AC down because she was cold,” Fu told police. “I looked down for one second and the crash happened.”

Gonzalez also talked to police. He said that he was driving home from Cars & Coffee at the Dulles Landing Shopping Center, having just cleaned and polished his Mustang for the event. He told police he was traveling about 35 to 40 mph when he entered the turn on Bowie Mill Road from the opposite direction.

“All of a sudden, I saw a blue car coming toward me. It was completely on my side of the road,” he told police.

Police also obtained a witness account from a friend of Gonzalez’s who had attended the car show. The witness said he was following Gonzalez from a distance of about three car lengths and traveling about 35 mph when the blue car crossed the centerline and struck Gonzalez’s Mustang.

Burley found that these witness accounts squared with the pattern of gouging in the pavement. Most of the gouges, which had been caused by the force of the crash driving the cars’ undercarriages downward into the pavement, were in the eastbound lane of Bowie Mill Road. So were most of the fluids leaking from the wreckage. The markings suggested that the crash had occurred in the lane where the Mustang was traveling — and suggested that it was the Honda that had drifted over the centerline.

Burley next sought to determine the speed of the vehicles before the crash. Using the weight of the vehicles and their reported speeds as a starting point, he used physics formulas as old as Sir Isaac Newton to analyze the momentum of the two vehicles before and after impact. The calculations appeared to fit the evidence, both the speeds as reported by the drivers and the way the two vehicles — having collided off-center — had spun into their final positions on the road.

This is one of many formulas routinely used by the Collision Reconstruction Unite to analyze fatal or life-threatening crashes. The formula — Conservation of Momentum: In-line Collisions — is meant to analyze the velocity after the impact between two vehicles. (Fredrick Kunkle/The Washington Post)

This sketch shows the position of two cars involved in a fatal head-on collision on Bowie Mill Road near Fraley Farm Road in July 2017. The sketch was prepared by the Montgomery County Collision Reconstruction Unit as part of its work analyzing the crash. (Fredrick Kunkle/The Washington Post)

Then Burley took advantage of a tool that Newton didn’t have: Inside many vehicles these days are event data recorders (EDRs). More popularly known as the “black box,” these computerized modules vary in the type and amount of information they gather. In general, the modules record brief snapshots of data while a vehicle is traveling. This can include the vehicle’s speed, the number of occupants, their positions in the vehicle, and certain actions a driver takes. The devices continuously record and write over the data during a trip, unless there is a crash.

These modules also play a key role in deploying air bags and seat belts. When the sensors detect a crash — say, because of rapid deceleration — they deploy the necessary configuration of air bags needed to protect everyone inside the vehicle. The modules also trigger special devices, known as pretensioners, that fire off small explosive charges to rapidly take up the slack in seat belts to ensure that an occupant is snugly belted in at impact.

The impact also causes the event data recorders to cease recording new data and store whatever data was recorded before the crash. Burley said the module from Gonzalez’s Mustang alone spat out 36 pages of data.

By examining the data in the collision on Bowie Mill Road, Burley determined that Gonzalez’s Mustang had been traveling at 40 mph moments before the crash and at 38 mph at impact. Fu’s vehicle had been traveling at 35 mph seconds before the crash, accelerated briefly to 40 mph, and was going 38 mph when the vehicles collided.

The data confirmed the drivers’ statements and showed that speed had not been a factor in the crash, Burley said. But the computer modules and the seat belts themselves confirmed another suspicion officers had from the first: Fu’s wife, who was riding in the back seat, had not been wearing a seat belt when the crash occurred.

Officer Jacob A. Burley, the lead investigator of a fatal crash on Bowie Mill Road in Olney last summer, goes through the presentation of findings that he made to the state’s attorney in Montgomery County. (Fredrick Kunkle/The Washington Post)

After drawing up his conclusions, Burley presented the findings to the entire crash reconstruction team for peer review, a step taken in every fatal crash investigated by the unit. Then the findings went to the state’s attorney for review and possible prosecution.

Both the state’s attorney’s office and the reconstruction team agreed with Burley’s overall conclusion: Neither Fu nor Gonzalez had been speeding at the time of the crash. But Fu had taken his eyes off the road, allowing his vehicle to drift from his lane and crash into Gonzalez’s car. The impact fatally injured Fu’s wife, who was not wearing a seat belt.

Gonzalez was charged with driving on a suspended license at the time of the fatal crash — although police also made clear that the violation had no direct bearing on the crash. Maryland court records show Gonzalez pleaded guilty Feb. 6 and received a disposition of probation before judgment, which allows a person to avoid a formal conviction on a charge after satisfying a period of probation. Gonzalez also paid a $75 fine.

His mother, Nanda Ali, reached at their home this month, said Gonzalez did not want to discuss the case. But she said she felt frustrated that her son had been under suspicion at first, at least from members of the public who harassed him online and blamed him for the crash.

“If you asked me that day, I was just upset and angry,” she said. “A life was lost.”

Fu, whose wife was one of 27 people killed in traffic accidents in Montgomery County last year, was charged with failure to stay right, negligent driving and operating a vehicle while a back-seat passenger who is older than 16 was not wearing a seat belt, according to police and Maryland District Court records.

“Some people might think this is a little harsh, issuing a seat belt ticket for his wife being deceased,” Burley said as he went over the case. But Burley said the team also feels bound to follow the evidence where it leads and to bring the appropriate charges no matter who is involved or how wrenching the circumstances.

In December, Fu pleaded no contest to the seat belt charge. He pleaded guilty to the other two counts and received fines totaling $410.

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