Police and the air ambulance were scrambled to the scene after the man, 59, suffered a head injury at just before 11am on Wednesday.
The moped was in collision with a truck at White Hart Lane and the driver stopped at the scene and was arrested at the scene on suspicion of driving under the influence of drugs.
A police statment said: “He was arrested on suspicion of driving whilst under the influence of drugs. He was taken to a west London police station where he remains at this time. Enquiries continue.”
Closures and cordons were in place around the scene of the incident for much of the day. Anyone who witnessed this incident or has information should contact police on 101 or Tweet @MetCC quoting 3542/3NOV21