Westbound Sunrise Highway is closed at Smithtown Avenue in Bohemia so investigators can conduct an accident reconstruction, the Suffolk County Sheriff's Department said.
The road was still closed as of 2 p.m. Friday.
Traffic is snarled east of the closure, with traffic being diverted onto the service road for one exit at Smithtown Avenue. Eastbound traffic is unaffected by the closure.
Sheriffs did not say what accident was being investigated.