The A41 southbound is now open after nine cars crashed in the middle of a sudden hailstorm, however northbound remains closed.
Firefighters from Watford, Garston and Hemel Hempstead were called to the Bourne End junction of the A41 just after 11.45am.
Hertfordshire County Council spokesman Jo Brown said no one was trapped in their vehicles and fire left the scene at 12.45pm.
The official Twitter account for the Bedfordshire, Cambridgeshire and Hertfordshire Roads Policing Unit said the pile up involved nine cars.
East of England Ambulance Service spokesman Richard Dunne said three ambulances attended along with Hertfordshire air ambulance and a paramedic officer.
Three casualties were treated at the scene for minor neck injuries and have been taken to Watford General Hospital by land ambulance.