A 21-year-old man died after being in collision with a police car, believed to be responding to an emergency call.
The incident happened on Saturday on the westbound carriageway of the A31 Hogs Back, near Puttenham in Surrey, shortly after 3.30am, the Independent Police Complaints Commission (IPCC) said.
A Surrey Police spokesman said: "The incident has been referred to the Independent Police Complaints Commission."
An IPCC investigation has begun and investigators have been sent to the scene, an IPCC spokesman said.
He added: "The incident occurred at around 3.35am on Saturday 1 December on the A31 Hogs Back road, near Guildford.
"The man was pronounced dead at the scene.
"It is understood the police car was on an emergency response to an incident."
The westbound carriageway was closed while the scene was examined to establish how the incident happened.