The motorist had one thought as he drove upward of 60 mph on local Bay Shore streets -- to catch up to the driver who had struck a teenager.
"I knew he hurt somebody bad, and I didn't want him to get away," said Joseph Hennessey, 47, a property manager from Bay Shore.
Minutes later, his target ran a red light and, just when Hennessey needed one, there was a Suffolk police cruiser.
The driver of a Ford pickup, Alfred Buckman, 51, of Central Islip, was charged in the Thursday incident with leaving the scene of a motor vehicle crash involving serious physical injury without reporting, second-degree aggravated unlicensed operation of a motor vehicle and traffic violations.
James Vaughn, 18, of Bay Shore, suffered head trauma and broken bones but was in stable condition Friday at Southside Hospital, police said.
Hennessey and his girlfriend, Louise Wild, 46, following him in her own car, were going to dinner when a bang and a scream interrupted their plans.
They saw a teenager tossed high in the air as they were waiting to turn onto Howells Road from northbound Fifth Avenue. "He just flipped in the air like a cartwheel," Hennessey said.
A pickup leaving the scene passed so close to Hennessey that he could see the whites of the driver's eyes.
"I thought he was going to pull over, but he hit the gas," the witness said. So Hennessey made a U-turn to go after him, his Jeep going over the curb.
A high-speed chase ensued as the pair turned onto a side street, he said, then north onto Clinton Avenue, almost back to the scene of the crime, then west on Howells Road. Hennessey said he was unable to dial 911 on his cellphone and also tail the pickup.
Blocks later, the Ford ran a red light, and Hennessey said he was mulling if he should do so too when a nearby police cruiser made the point moot.
The officer pulled over the pickup and, when Hennessey caught up, he said he pointed out to police the "smashed in" windshield.
Buckman, of 14 Lorigan St., told police that the teenager ran in front of his car, but it was too late to avoid him.
Hennessey shuns the hero label: "I just did what I thought anybody would do."