A driver suffered head trauma Wednesday when he suddenly swerved during rush hour in Centereach and his vehicle flipped into the path of oncoming traffic, Suffolk police said.

Nicolls Road was shut down in both directions by Route 25, but has since reopened, police said.

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Michael Negron, 35, of Bellport, was going north on Nicolls Road at about 3:50 p.m. when he slammed into the guardrail, causing his Ford Explorer to overturn into the median, said Det. Sgt. Edward Aki of the Sixth Precinct. He was ejected, police said.

The Ford landed in the southbound lanes, but was not struck by other vehicles, he said. Detectives and crime scene investigators are trying to determine what caused the crash.

"Witnesses say he just swerved and . . . struck the guardrail," Aki said. "He's lucky because there were plenty of cars on the road."

It did not appear Negron was speeding, but was keeping up with traffic, which was substantial at the time, police said.

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Police said Negron, who had been alone in the vehicle, was listed in serious condition Wednesday night at Stony Brook University Hospital.