The rider was on a Kawasaki bike, travelling northbound on the A5, when a black Mercedes drove into it just before 9pm on January 5.
The accident happened after the Mercedes driver turned right at the traffic lights into Cool Oak Lane.
An ambulance took the rider to the hospital where he is still fighting for his life.
The Mercedes driver, who was uninjured, went to a police station where they were interviewed under caution.
They have not been arrested but the Metropolitan Police is carrying out enquiries, Scotland Yard said.
Officers have asked any witnesses or anyone with dashcam footage of the incident to come forward.
Anyone with information is asked to call the Serious Collision Investigation Unit at Alperton on 0208 246 9820, or dial 101 and quote CAD 6776/05JAN24.
Earlier this month, a man in his sixties was killed in a collision which happened while he was walking near a residential road in Hounslow, south west London.
The tragedy happened at around 2.30pm on January 12 at the junction of the A4 with Springwell Road.
The London’s Air Ambulance service was called as part of efforts to revive the man, but they were unable to.