The new zone covers three-quarters of a mile at the north end of the Lang Kames. It is intended to slow traffic down to a safe speed where wind farm access tracks will be joining the public road from both sides, at Hamarigrind and Scar Quilse.
The speed limit signs have been in place for some days with the details covered up until the Traffic Order takes effect legally.
Last month a 50mph zone was implemented at Sandwater, covering less than one mile, due to the increased number of road junctions nearby.
The lower 40mph restriction is deemed necessary for the north end of the Kames where construction vehicles will be travelling slowly between the two new access tracks, particularly when travelling up the hill in a southerly direction.
The speed limits are expected to remain in place for the duration of the wind farm construction phase, which ends in late 2024.
The changes have been implemented by Shetland Islands Council as the road authority and paid for by the main wind farm contractor RJ McLeod.