A car rolled backwards off a dealership forecourt before smashing through a fence and landing on the M1.
One lane of the motorway remains closed southbound between junctions 2 and 1 after the vehicle, believed to be from the West London Motor Group (WLMG) Nissan dealership in Mill Hill, crashed on to the hard shoulder of the motorway.
Emergency services earlier closed off two lanes to deal with the incident, in which it is not believed anyone was hurt.
Traffic was backed up for up to a mile and remains slow in on the M1 approaching the accident, and on the neighbouring A1 Watford Way.
More to follow.