Two people are believed to have died after a driver tried to help a man who jumped from a motorway bridge.
Police and ambulance services were called to the “serious incident” on the northbound carriageway of the Watford M1 near Junction 5 at about 10.45am on Thursday.
It is thought that a Good Samaritan driver went to help the man after he fell and was killed by a passing vehicle.
Both carriageways are now open after the south and northbound roads were closed earlier in the day.
Officers are investigating the circumstances surrounding the deaths.
Rupert Gadd, from the Hertfordshire, Bedfordshire and Cambridgeshire Police Road Crash Investigation Unit, told Heart Radio: “A male is known to have jumped from the bridge parapet onto the carriageway.
"Other motorists have stopped to assist to his aid - unfortunately one of those people who stopped was struck by another vehicle.
"We have started an investigation into the collision and deaths of both males.
"We've handed the carriageway back to the Highways Agency - once they have done their final inspections it will re-open."
Footage taken from a motorway CCTV camera appeared on Twitter, which seemed to show emergency services working on a bridge above the scene of the incident as well as on the road below.
A Hertfordshire Police spokesman confirmed that authorities are at the scene, and have asked drivers to avoid the area where possible.
"Drivers are thanked for their patience but asked to find alternative routes where possible," he said.
A spokesman for the ambulance service said two ambulances, an emergency response vehicle and an air ambulance had been dispatched to the scene but were stood down after midday.
For confidential support call the Samaritans in the UK on 08457 90 90 90, visit a local Samaritans branch or click here for details.