The 27-year-old passenger in the car died at the scene.
A spokesperson for Avon and Somerset police said: “We’re investigating a single-vehicle fatal collision which happened on the B3224 at Exford on Sunday.
“At about 8.45pm, a BMW 430 with two occupants was in collision with a tree. The passenger, a 27-year-old man, sadly died at the scene from his injuries. The driver, a 31-year-old man, suffered a serious head injury and was taken to hospital.
“The road was closed while an investigation was carried out at the scene, and re-opened at just after 4.30am on Monday.
“If you have any information which could help our investigation, please call us on 101 and give the call handler the reference number 5221230833.
Griffiths had been booked to ride at the season opener of Chepstow’s jumps this Friday.