A mail truck veered off the Long Island Expressway, crashing into a guardrail and shutting down eastbound traffic between Exits 51 and 52 during the cleanup, authorities said.
Investigators are looking into whether the tractor-trailer driver may have dozed off, said Michael Sharkey, a chief in the Suffolk County sheriff's office.
The crash happened at about 3:45 p.m., Sharkey said. The driver sustained minor injuries.
"He's banged up a little bit," Sharkey said.
The roadway was expected to be closed until 6 p.m.
"We have heavy-duty wreckers out there, so they need the room to work," Sharkey said.
The roadway will also have to be cleaned up of spilled fuel.
"He tore open his fuel tank," Sharkey said.