The report by transport consultancy Jacobs says installing £10 million of further mitigation measures alongside the A83 in Argyll would significantly reduce the risk of debris being washed from the hillside on to the trunk road. But it has rejected more expensive options, including creating a tunnel and an elevated road that would virtually eliminate the risk of further closures.
The route has been closed six times in the past five years amid heavy rainfall, leaving motorists facing a 50-mile detour.
Work on constructing an emergency relief route along the old military road near the A83 is expected to be completed in January but Transport Scotland has commissioned Jacobs to explore longer term solutions to the landslide problem.
The most costly of the six options considered by Jacobs would have seen a tunnel built at a cost of up to £520m further down the valley.
Other options that have also been rejected include building a "debris flow shelter" over the road or a viaduct that would allow debris to flow under the road. The preferred option would see 440m of debris flow barriers erected at high risk locations, improved hillside drainage next to and under the road and planting more hills and bushes.
Its findings were presented to the A83 Taskforce, which includes representatives from Argyll and Bute Council, tourism bodies and the Loch Lomond and Trossachs National Park, on Friday.
John Semple, the council's development and infrastructure spokesman, said: "We will be studying the options and weighing up which provides the best all round solution."
Transport Minister Keith Brown added: "The last few weeks have been a challenge for local communities who have had to endure another landslip at the Rest and Be Thankful only last month.
"This has brought our work sharply into focus, and all the more timely is the draft feasibility study which has been presented to the taskforce."