The crash happened in Port Jefferson at about 11:30 a.m. on Wednesday, Nov. 2, the Suffolk County Police Department said.
SCPD said the Smithtown man was riding a 2019 Harley Davidson Trike on East Broadway near Bridle Path Lane when he struck a deer that was crossing the roadway.
The man was knocked off the motorcycle and struck his head on the pavement, authorities said.
The unoccupied three-wheel motorcycle continued traveling for about a quarter of a mile until it left the roadway, striking several bushes before stopping in a side yard on Hoyt Lane, police reported.
The deer was gone when officers arrived, SCPD said.
Detectives asked anyone with information about the crash to call police at 631-852-8652.
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