Elevated motorway - the A40, formerly A40(M) until 1990 - running west-east from the eastern end of Western Avenue in Acton. Halfway along its length it has a junction to the A3220 going south. To the east of this it crosses over Ladbroke Grove, curves up to the north around Westbourne Park Station, swinging south again, passing over [Porchester Road]? (at which point there are onramps), to the Marylebone Flyover, where it crosses over the Edgware Road before becoming the Marylebone Road.
See here for some photographs of this west London landmark: