The emergency closure of the M1 northbound between Junction 5 and Junction 6 is expected to have a knock on affect on surrounding roads.
The motorway was closed just before 11am following a serious incident, which is currently being investigated by Herts Police.
It has been reported locally that a person died after falling from a bridge and a motorist, who stopped to help, was killed when he was hit by another vehicle.
Highways Agency officials and the police are working hard to release the trapped traffic that has built up behind the incident, with vehicles being turned around to drive the wrong way up the motorway and exit at the nearest sliproad.
The northbound carriageway is likely is remain shut until the early evening, which will affect rush hour traffic.
A signed diversion is in place with drivers advised to follow the hollow diamond symbol via the A41 and A405.
The southbound carriageway was temporarily closed to allow the air ambulance to land and reopened just after midday, although the exit sliproad stayed closed for some time after this.
Motorists are advised to plan their journeys and allow extra time or avoid the area if possible.
Police say they were called out at 10.45am amid ‘concerns for he welfare of two people’ but are not releasing any further information at this stage.