Police are appealing for witnesses after a blue Ford Fiesta left the road and collided with a lamp-post and tree at Hophurst Lane, Crawley Down at about 3.40am today.
The driver, a 23-year-old local man, suffered serious injuries and was flown by the Kent, Surrey and Sussex air ambulance to St George's Hospital, London, where he is in a critical but stable condition.
Hophurst Lane was closed between its junctions with Fermandy Lane and Burleigh Way for more than four hours while emergency services were at the scene.
Sergeant Alan Spicer, of Sussex Police, said: "Anyone who saw what happened or who may have seen the car being driven shortly before the collision is asked to phone Sussex Police on 101 or email firstname.lastname@example.org, quoting Operation Custodian.
"In particular, we would like to hear from a taxi driver who may have dropped off the driver and some of his friends in Crawley Down shortly before this collision occurred. The taxi driver and the injured driver's friends may have valuable information that could help with our enquiries and we would urge them to come forward."
Sgt Spicer added: "I would also like to thank anyone whose journeys were affected by the road closure for their patience and understanding."