A motorcyclist required hospital treatment after the crash.
Police believe tourists, possibly from the Edinburgh area, may have witnessed the incident
The collision occurred about 9.30pm on Friday, August 25 near the Halfway Garage on the Lochs Road,
The incident involved a Triumph motorcycle and a car which were both were travelling south towards Cameron Terrace at the time.
The driver of the car has not been traced.
A source suggested the occupants - two women and a man - of a small silver hatchback briefly stopped at the scene - but left before leaving contact details.
Police are keen to speak to a group of tourists, possibly from the Edinburgh area, who may have witnessed the collision.
It is possible the group were staying in holiday accommodation in either Lewis or Harris that night.
Anyone with information is asked to contact Police Scotland on 101, quoting reference NH3274/17, or Crimestoppers on 0800 555111 if they wish to remain anonymous.