The M5 in Somerset is closed in both directions after a 17-year-old girl died in a collision.
Avon and Somerset Police said the girl had been detained by officers shortly before the incident. She died at the scene and her next of kin have been informed.
The force described the collision as being between a pedestrian and a car. No-one else was injured.
It happened between junctions 24 for Bridgwater and 25 for Taunton at about 23:00 GMT on Monday.
The force said a mandatory referral had been made to the Independent Office for Police Conduct due to the recent prior police contact.
National Highways said the road was expected to remain closed throughout Tuesday morning, while inquiries were carried out and repairs are made to make the motorway safe.
A diversion is currently in place between the two junctions, along the A38, but drivers are advised to avoid the area and check traffic updates before setting off on journeys.
The site of the crash is near Creech Heathfield, close to Taunton. There are long delays on local roads, especially the A38 near North Petherton and Bridgwater.
Motorists travelling southbound on the M5 are experiencing delays of 90 minutes, with seven miles of congestion. There are delays of 30 minutes and congestion for two miles northbound.
Anyone who witnessed the collision or may have dashcam footage which could help the investigation is urged to contact police.