Multiple landslips have again blocked the A83 Rest and Be Thankful after heavy rainfall across Argyll.
Traffic Scotland said the local Old Military Road, which has previously been used as an alternative route, had also been blocked.
The incident happened as Scotland was battered by heavy rain on Tuesday causing localised flooding.
About 65mm of rainfall is estimated to have fallen at the Rest and Be Thankful over the course of the day.
The area has been plagued by landslips with £79.2m being invested in the maintenance of the route since 2007.
In January the road was closed for two days after being covered by 1,300 tonnes of debris.
Argyll and Bute Council has called for a permanent solution to the problem.
Specialist geotechnical contractors have carried out initial visual assessments from the roadside but initial indications are that there is still movement on the hillside and it is unsafe to begin clear-up operations or carry out further assessments.
Both the A83 and Old Military Road will remain closed overnight.
Safety assessments will resume at first light on Wednesday and a diversion route has been implemented between Tarbet and Cairndow via the A83, A82, A85 and A819.
Temporary traffic lights are also in place on the A82 at Inveruglas after heavy rain also affected a retaining wall following a day of challenging conditions.
Bear Scotland said ferry provider Western Ferries could accommodate diverted traffic.
In March, Transport Scotland said £1.9m was being invested in the construction of a new catch pit on the hillside to improve the resilience of the vital route.
The mitigation measures also included improved drainage and hillside planting.
Transport Scotland previously said work carried out since 2013 was estimated to have kept the A83 open for at least 48 days when it would otherwise have closed.
This includes the October 2018 event, the largest on record, which intercepted approximately 3,200 tonnes of material reaching the A83.
A yellow warning for heavy rain was issued by the Met office for heavy and persistent rain from 06:00 and midnight on Wednesday.
The main areas affected were Mid and North Argyll, Stirlingshire, Lochaber and Southern parts of Skye.
The Scottish Environment Protection Agency (Sepa) warned people to prepare for potential flooding around rivers, and to expect surface water in areas in the west of Scotland.
Emergency Services were called to the Hillfoots in Clackmannanshire when Tillicoultry Burn burst its banks.
Fire crews helped people evacuate flooded properties in the Hareburn Road area of the town.
In Lochwinnoch, fire crews had to push a car through flood water on the A760 following torrential rain.
Flood water covering the road at Glen Fruin in Helensburgh was also posted on social media.
The heavy rain was expected to ease off overnight.