Detective Chief Inspector Mel Sutherland, leading the investigation, said the force was keen to speak to anyone who was travelling on the A19 between 12:20 BST and 12:30 BST.
“They may have mobile phone footage or dashcam footage which captures the moments leading up to the incident.
“Our thoughts remain with the man’s family at this time,” he added.
A spokesperson for the North East Ambulance Service said: “We received a call at 12:22 BST to reports of an incident in the South West Industrial Estate area of Peterlee.
“We dispatched 4 Hazardous Area Response Teams (HART), a clinical team leader, a specialist paramedic, two ambulances and requested support from the air ambulance.”
