Three people on a quad bike were killed when it collided with a sports car.
Two women and a man died at the scene on the A602, between Upton and Hemsworth. West Yorkshire Police have not named the three.
Two men, aged 21 and 22, who were in the bronze-coloured Nissan sports car, have been arrested on suspicion of dangerous driving. They were still in police custody.
Police, who were called at about 00.30am, warned drivers to avoid the area as emergency services continued to work at the scene .
Detective Chief Inspector Richard Holmes said: "Clearly this has been a very serious and tragic incident with multiple casualties and inquiries are expected to be ongoing on that part of the A6201 for the majority of the day."
He said detectives are appealing for witnesses, including anyone who saw the incident or the quad bike or the Nissan being driven before the collision happened.