New government figures show the number of people using ferries through the Road Equivalent Tariff scheme has increased.
During the first six months of the year vehicle traffic increased by 23% on routes to the Western Isles.
However in Orkney and Shetland - where the pilot scheme is not in operation - there was a decline in traffic.
Politicians in the Northern Isles have been calling for the scheme to be extended there.
The increase in traffic has a led to a boost in tourism with many visitors visiting smaller outlying islands.
RET is due to run for another 18 months before politicians at Holyrood decide on whether to extend it.
