The SNP pledged in 2007 to complete dualling of the A9 between Perth and Inverness.
This involved widening 80 miles of the 110-mile route.
In 2011, ministers announced the mammoth project would be finished by 2025.
But in February 2023, they admitted that was "simply no longer achievable".
The scheme is estimated to cost £3 billion at 2008 prices - or £4.5 billion in today's money.
Only £430 million of that has been spent so far and just 11 miles of dualling added.
Two sections of dual carriageway have been built since 2007 - near Aviemore and Dunkeld.
The latest six-mile stretch, between Luncarty and Birnam, opened in 2021.