Police were called near Stanmore to the A41 Edgware Way, near to the junction with Edgwarebury Lane, at 16:09hrs on Sunday, March 7, following reports of a road traffic collision.
Officers attended, as did the London Ambulance Service and the London Air Ambulance.
One man, with no further details yet provided, has sustained serious injuries and is being transported to a west London hospital.
A number of roads were closed to allow emergency services to respond. Some of these closures were needed to remain in place while the investigation was carried out.
Any witnesses, especially those with potential dash cam footage, are urged to call the Met on 101, quoting reference CAD 4702/7MAR21.