Around nine cars were involved in a pile-up on the A41 in the middle of the sudden hail storm that swept across the area this morning.
An air ambulance has been called to the crash, which happened by the Bourne End junction just after 11.45am.
East of England Ambulance Service spokeswoman Sophie Taylor said four people were being treated at the scene for neck and back injuries.
Their injuries do not look life-threatening at this stage.
Herts Fire and Rescue Service wrote on Twitter: “We are currently in attendance at a multi-vehicle RTC on A41 just before Hemel turn Northbound. Road closed, please find alternative route.”
Fire crew Red Watch Watford posted a photo of collision on the social networking website.
The dual carriageway has been closed in both directions between Bourne End and Hemel Hempstead.