The single-track Old Military Road, which runs parallel to the main route to Kintyre through Glen Croe, will also be made more resilient in case of further landslips, some of which have closed both roads. The upgrade is expected to save ten minutes on the journey time when it is in use.
The diversion route’s defences, which include a huge protective bund or wall, will be bolstered with fences to catch landslide debris, further temporary bunds and drainage improvements. Part of the road will be realigned to make bends less severe, but “limited lengths of steep and narrow sections at the western end” will continue to operate under convoy.
Transport Scotland said proposals would “extend the length of two-way road, subject to obtaining necessary statutory approvals. However, part of it would remain single lane at the western end, including the hairpin bends.
The Scottish Government agency said the work would start “later next year” at an estimated cost of £24-32m. More details and the project’s timescale will be provided at the next A83 Taskforce at the end of January.
It has rejected other options to upgrade a forestry track or build a two-lane road, both on the opposite side of Glen Croe. It said upgrading the Old Military Road would be the quickest to complete and have least impact.
Ms Gilruth said: “These improvements will mean more certainty for locals and road users if the A83 is shut due to adverse weather conditions. All of this work underlines the Scottish Government’s commitment to work with key stakeholders and local communities to ensure that Argyll & Bute remains open for business.”
The minister also stressed the Scottish Government’s commitment to a “long-term solution to the landslip risks”. Consultants are due to produce a recommended option through Glen Croe in the spring from among the five shortlisted, which include viaducts and tunnels and would cost up to some £900m.
Transport Scotland mentioned one of these in its Old Military Road announcement comprising a new 2.7-mile road that would cross the glen and Croe Water over a 200m viaduct, along with landslip debris flow shelters.
John Gurr, chair of the Rest and Be Thankful Campaign group, said: “We would like a two-way road into Argyll that stays opens when it rains. We have not had a two-way road open for the last two-and-a-half years.
"If the OMR was the medium term solution, we would still not have a two-way road open when it rains for at least the next ten years.”