The M1 will be closed for up to an hour while debris from a multi-vehicle collision, which is covering 100 metres of the northbound carriageway, is cleared up.
Police and paramedics were called after a collision between five cars and a lorry on the northbound M1 between junction 4 and 5.
The entry slip road at junction 4 has been closed and traffic is being taken off from junction 3.
There are no serious injuries but paramedics are treating people for minor injuries.
Katy Teather, who was at the scene, said: "It just looked like the cars had been dropped onto the road."
Hertfordshire Constabulary believe that debris from the crash is covering 100 metres of the motorway.