The serious collision between a Peugeot car and a pedestrain took place at midday, police said.
The man was taken by air ambulance to the Royal Sussex County Hospital in Brighton in a life threatening condition, where he remains, a spokesman confirmed.
Police are now appealing for the three drivers who may have seen it happen to come forward.
Sergeant Alan Spicer, of the roads policing unit, said: “We have dashcam photos of two dark coloured cars who were stationary southbound in front of the collision and a white car (pictured) which was travelling northbound and would have passed the collision.
“We want to hear from those drivers and also from the driver of the Mercedes Actros lorry which passed moments before the collision happened.”
The car drivers and the lorry driver and anyone else with information about the collision are asked to report it online or call 101 quoting Operation Raynham.