Police are urging a taxi driver, who stopped at the scene of a serious crash in St Albans on Saturday, to come forward.
The crash took place at 12.36am in Harpenden Road near to the junction with Heathlands Drive when a Mitsubishi Lancer Evo, which was heading north, overtook a Volvo XC90.
For reasons unknown the Evo then crashed in to the kerb, veered into the opposite carriageway and crashed with another Volvo XC90.
The front seat passenger of the Evo, a 25-year-old man, was taken to St Mary’s Hospital in London with serious head injuries. He remains there at this time in a critical but stable condition.
The Evo driver, a 26-year-old man, and another passenger, who is a 24-year-old woman were also seriously injured and taken to Watford General hospital. The driver of the second Volvo, a 45-year-old woman, suffered minor injuries but was not taken to hospital.
PC Helen Carpenter said: "I am aware that a white taxi stopped at the scene immediately after the collision but had left before police arrived.
"This driver may have witnessed something vital and I would ask that the driver make contact with me via the Herts police non-emergency number 101 quoting reference number 24 of March 29."