Officers were called at approximately 03:10hrs on Saturday, 8 August to reports of a collision on the westbound carriageway of the M4 approaching Junction 4 in Hillingdon.
London Ambulance Service and London’s Air Ambulance attended the scene and a man aged in his late 60s was taken to a north London hospital where he remains in a critical condition.
Next of kin have been informed.
At this early stage the reason for the collision, which involved a single car, is unknown and no arrests have been made.
Anyone who witnessed the collision, or the events leading up to the collision, is asked to contact the Serious Collision Investigation Unit based at Alperton on 020 8991 9555.