Tahir Khan, 29, was pronounced dead at the scene of the accident on May 24.
Mr Khan, a cab driver, was driving a grey Peugeot Partner van, which was travelling along the A629 Aire Valley trunk road when it plunged down an embankment. ã€€
Officers want to speak to the drivers of a silver Toyota Hiace van and a white Suzuki Swift, which were travelling along the same road towards Silsden around 9.30am.ã€€
It is believed they may have vital information about the accident.
Sergeant Mat Tunney, of the Major Collision Enquiry Team, said: "It is likely that these drivers will have seen Tahir's van in the moments before it left the road and I would ask them to contact us immediately, so they can help us piece together the circumstances which led to the collision."ã€€
Witnesses are asked to contact the Major Collision Enquiry Team on 101.