An 18-year-old is in a critical condition in hospital following a serious collision on the A23 at Hickstead, police said.
The collision, which involved a car and lorry, happened at 8.40pm last night (August 18).
Police also said an 18-year-old man had been arrested on suspicioun of dangerous driving.
A spokesman said: “Police are appealing for witnesses after a car hit the rear of a parked lorry on the A23 at Hickstead on Sunday evening (August 18).
“The Renault Scenic was travelling south when the collision occurred as the HGV was parked in a layby at 8.40pm.
“The front seat passenger, an 18-year-old man, suffered serious injuries and was taken to the Royal Sussex County Hospital, Brighton. His condition is critical.
“The driver of the car, an 18-year-old man from London, suffered minor injuries and was later arrested on suspicion of dangerous driving.
“He is currently in custody. The lorry driver was unhurt.”
The road was closed in both directions for several hours last night while the police carried out an investigation.
Sergeant Dan Eaten thanked drivers for their patience. He added: “We would like to hear from anyone who saw this serious injury collision or has any information about it.
“If you can help with the investigation report online or ring 101 quoting Operation Beaufort.”
A spokesman for South East Coast Ambulance Service said two response cars and two ambulances were sent to the incident, along with an air ambulance.
They added: “Two patients were in the car and both taken by land ambulance to Royal Sussex County Hospital, one with minor injuries and other with a serious head injury.”
Crews from West Sussex Fire and Rescue Service were also called to the incident.
A spokesman said fire engines from Henfield, Haywards Heath, Crawley and Burgess Hill were mobilised.
One person was freed from the car with hydraulic rescue equipment and electrical saws, said the spokesman.