Police are appealing for witnesses after a car left the road at Birdlip Hill viewpoint yesterday.
At around midday, emergency services were called with a report of a car leaving the road at Barrow Wake in Birdlip.
The driver, a man, has been taken to Southmead Hospital with serious injuries.
No-one else was injured.
The car is described as being a Silver Suzuki Alto.
Investigating officers spoke to members of the public at the scene but are asking anyone who they have not yet spoken to and witnessed the incident to contact them on 101.