THE WHITE TOUR bus struck Joseph Heard just before 10 a.m. yesterday in the crosswalk on Franklin Street near Race. It knocked him to the ground and dragged him about 50 feet.

Heard came to a rest, his headphones lying in a tangled heap near his body. The bus, however, never stopped - it rolled onto Interstate 95 and continued to Washington, D.C., its final destination, police said.

Heard, 46, died minutes later at Hahnemann University Hospital.

Yesterday's fatal hit-and-run was the city's eighth so far this year, according to statistics from the police Accident Investigation Division.

That's a staggering number, considering it surpasses 2014's year-end total of seven deadly hit-and-runs.

Overall, the number of crashes where a driver left the scene is at 9,006 through July 31, almost 1,000 crashes higher than at the same point last year.

It's a troubling trend, an indication that selfish motorists would rather sacrifice a stranger's life than speak with police, experts said.

And to hear local investigators, prosecutors and activists tell it, explaining the spike in hit-and-runs can be as difficult and frustrating as solving the crimes themselves.

Excuses, excuses

Of the eight fatalities this year, four involved pedestrians, two involved bicyclists, and two involved passengers in vehicles, including a July 29 fatal crash on Sandmeyer Lane. Investigators consider that a hit-and-run because the driver of the vehicle allegedly left three of his passengers to die.

As of this writing, only two have been "cleared" - cop-speak for an arrest made and charges filed - according to police.

"We can't arrest somebody without probable cause, and that includes proving that someone was driving at the time of the crash," Capt. John Wilczynski, head of the Accident Investigation Division, told the Daily News during a recent interview.

"Not 15 minutes before the crash, not 15 minutes after, but when it happened," he said.

Besides witness information, essential tools for investigators are cellphone records from the suspected drivers and surveillance footage that captures the crash, Wilczynski said.

In yesterday's fatal crash in Center City, for example, some "quite disturbing" footage from nearby cameras helped investigators trace the bus to a company on 11th and Arch, the owners of which cooperated with police and turned the bus over, Wilczynski said.

Police were questioning that driver last night to determine if his bus was the one that struck Heard. It's possible, Wilczynski said, that the driver was unaware that he had struck somebody.

It sounds bogus - how could you not hear or feel that impact? - but it's an excuse that investigators often hear.

In preparation for this story, Wilczynski conducted an informal poll of his officers: He asked them what explanations they often hear from hit-and-run suspects.

They range from the obvious - general fear, DUI, a suspended license - to some that are more surprising - peer pressure from passengers to flee, paranoia that nearby residents will attack them if they try to render aid.

Stiff penalties

That last one doesn't hold up too well in court, according to Assistant District Attorney Tom Lipscomb, who specializes in vehicular homicides.

"Problem with that [explanation] is that, invariably when someone says it, they cannot explain why they didn't drive to a gas station up the road and make a call from there," he said, "or pull over when they think it's safe, call 9-1-1 and say 'I hit somebody.' "

Lipscomb said some hit-and-runs are legitimate accidents, like when a pedestrian darts out in front of a passing car.

If that's the case, and the evidence supports it, the driver may not even be liable for criminal charges. But the act of fleeing and trying to hide instantly ratchets up what may be a genuine mistake to a punishable act.

And the penalties are severe: leaving a scene that causes injury or death is a second-degree felony, one that carries a mandatory minimum sentence of three years in prison, according to the state's Crime Code & Vehicle Law Handbook.

"When I talk to the victims' families in these cases, almost always what they'll tell me is that it's one thing if the person was driving and killed their loved one," he said.

"And it's an entirely different story if they left them there to die."

Grounded in science

Do some digging and you'll find that the pathology behind leaving the scene of a fatal crash isn't a well-studied subject.

It's not hard to see why - people charged with manslaughter aren't likely to openly gush about their alleged crimes.

But Paul Clements, a psychologist and coordinator for Drexel University's forensic health-care certificate program, has spent nearly two decades studying murders and other violent deaths, including car crashes.

And in his research, he's come to understand how brain chemistry influences reactions to traumatic moments.

Like when a driver runs over another human being.

"It comes down to 'Do I stop and live with this or do I hit the gas?' " Lipscomb said. "The fight-or-flight system kicks in."

He posits that most hit-and-runs are not premeditated, and that drivers flee simply out of panic - that's the brain's default response, he says, to handling traumatic situations.

"But later, when you see your sketch and the description of your car being flooded on the Philly news, if you purposefully do not go back and report what happened, what does that say about your moral fiber?" he asked.

If drugs or alcohol are thrown into the mix, the likelihood of a driver sticking around diminishes even more, he says, and not because of the added threat of a DUI charge.

Illicit substances suppress the brain's frontal lobe, which controls a person's judgment and impulse control, Clements said.

Drivers who've had too much to drink will flee simply because it "feels" like the proper thing to do, Clements said.

"Being drunk doesn't negate your moral fiber by any means," he said, "but it does come in later when you're no longer drunk and realize what you did. Do you take responsibility for your actions?"

Change your thinking

But the conversation about hit-and-runs can't solely focus on the drivers, at least as far as Sarah Stuart is concerned.

"We need a shift in thinking about it," said Stuart, the deputy director of the Bicycle Coalition of Greater Philadelphia. "We shouldn't be calling any of these crashes accidents. They're mistakes that people made that unfortunately have very deleterious consequences.

"What we need to be doing is changing the environment so that these mistakes don't have such dire consequences."

The coalition touts the buffered bike lanes they pushed for on Spruce and Pine streets throughout Center City as an example of how tiny changes can have major impacts: Since 2009, when the city eliminated one lane of traffic from both streets in favor of the bike lane and reduced the speed limit to 20 mph, there has been a 25 percent reduction in all crashes, according to data from the coalition.

"Every crash has its own set of circumstances, but nobody deserves to die because they walked out in the street, or walked across the street or rode in the street," Stuart said.

"The airline industry doesn't tolerate anyone dying in a crash. The railroad industry doesn't tolerate anyone dying in a crash. Why should we?"