Two people suffered injuries after a collision in Longlevens on Thursday night.
The road was closed for several hours to allow investigators to work at the scene.
Police want to hear from anyone who saw anything.
A force spokesperson said: "The incident happened shortly before 9.40pm and involved a motorbike and a Seat Ibiza at the crossroad with Longford Lane and Church Road.
"Two teenagers who had been on the motorbike, a 17-year-old boy and a 16-year-old girl, both sustained serious injuries. They were taken to Southmead Hospital in Bristol where they remain for treatment.
"Officers would like to hear from anyone who witnessed the collision and has not yet spoken to police, and anyone who travelled through the area and has dashcam footage which may assist the investigation.
"Information can be submitted here or call police on 101 and reference incident 452 of 16 May."