A pedestrian and a double-decker bus were involved in the collision on the A226, Highfield Road North, at 11.40pm on Saturday 14 March 2020.
A man suffered serious injuries and was taken to a London hospital where he remains in a critical condition.
Officers from the Serious Collision Investigation Unit (SCIU) are investigating the incident and would like to speak to any witnesses who were in the area or on the bus. Motorists with dashcam footage are also asked to get in touch.
People with information can call the SCIU appeal line on 01622 798 538 or email email@example.com quoting HW/DH/28/20.