Next time you're stuck in traffic, wondering why an accident up ahead is taking so long to be cleared, just picture Concord police Detective Michael Cassidy on his hands and knees. He's searching for evidence, like a single shard of broken glass or a scrap of a pedestrian's clothes ground into the pavement.

Cassidy is one of four city detectives trained to investigate serious or fatal car crashes with pedestrians, bicycles or other cars. They use physics, trigonometry and algebra to figure out whether the crashes truly were unavoidable. They rarely are, Cassidy said.

"Oftentimes a lot of people think we just show up and take a report," Cassidy said. "That's not the case."

So far this month, the police said Concord has seen an unusually high number of pedestrian accidents. In separate crashes over the past three weeks, three people have been seriously injured. All of those cases are still under investigation.

A fourth person received minor injuries this month after apparently cutting into traffic while jogging, according to the police.

No one has been charged in any of the incidents.

The reason is because investigations last for three months on average, and some can go much longer.

"We try to give victims justice in the long run and hold any drivers responsible," Cassidy said. "If someone is walking down the street, minding their own business and gets punched in the face, we consider them a victim. If someone is driving along in their car and gets hit, we consider them a victim, too."

Nose to the pavement

The detectives' tools are simple: a tape measure, a scale and a contraption called "the friction sled," nothing more than the business end of a tire cut and filled with 86 pounds of lead.

With cheat sheets for commonly used physics equations, detectives can tell how fast a car was traveling based on the skid marks it left.

All it takes is understanding the force needed to pull the friction sled against a surface.

Everything gets recorded at an accident scene where someone was seriously hurt or killed. The weather, temperature and cloud cover. The width of a single lane and the entire roadway. Hundreds of photographs are also taken.

"If we miss one thing, or if we don't measure something correctly, it can throw off the whole investigation," Cassidy said.

Cassidy, who has more than 1,000 hours of practice doing traffic accident reconstruction, said small things become apparent after a while, such as finding stray shoes at a bad crash scene.

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