And unlike violent crime, which afflicts certain areas of the city more than others, deadly car crashes tear across the broad spectrum of neighborhood, wealth and class. Many create headlines: a hit-and-run in Williamsburg, Brooklyn, that killed a man, a pregnant woman and, a day later, their prematurely delivered son; a 6-year-old boy run over on his way to school in East Harlem. In the first, detectives caught the driver, Julio Acevedo, after he fled, charging him with manslaughter; in the second, the police issued summonses but filed no criminal charges.

Until recently, the Collision Investigation Squad — whose members readily rattle off physics equations and discuss throw weights and friction coefficients like an odd hybrid of Sherlock Holmes and Bill Nye the Science Guy — was called only to fatal crashes or those in which emergency room doctors deemed a victim likely to die.

The policy created difficulties in some recent cases — eight in 2011, according to Inspector Ciorra — in which a person survived for a time but died days later. By the time detectives returned to the scene, video images had been taped over, skid marks washed away, crash debris scattered.

The squad’s policy was the subject of a City Council hearing in February 2012 and a federal lawsuit filed last June over the death of a pedestrian, Clara Heyworth, who was struck by a vehicle on a Brooklyn street in 2011. She died from her injuries a day after the crash, and the investigation into her death did not begin until days later. The suit alleges a “systematic failure” by the Police Department to investigate serious crashes.

Amid the criticism, the department took a hard look at the unit and, this year, revamped its approach and its policy. Now, whenever a paramedic lists a patient as critical, Collision Investigation Squad detectives respond.