Police are appealing for witnesses and information following a crash at Swanley Bar, between Potters Bar and Brookmans Park, on Friday, November 13.
The collision occurred at around 4.50pm on Hawkshead Road, near to the junction of Bluebridge Road.
A silver Audi A3 was in collision with a man who had been walking along the road with another man.
The victim, who is in his 20s, was taken to a London hospital where he remains critically ill.
No-one else was injured.
PC Dave Clarke, from the collision investigation unit, said: “I would like to appeal to anyone who saw the men walking between Potters Bar Railway Station and where the collision occurred between 4pm and 4.50pm to please contact me.”
Anyone who witnessed the collision is asked to contact PC Dave Clarke via the Herts Police non-emergency number 101.