He said “shovel-ready” proposals for a replacement section of the A83 through Glen Croe in Argyll would follow “data gathering” to choose the best route by the end of this year.
The new stretch, which could include viaducts and tunnels, is expected to take up to ten years to build on the other side of the glen at a cost of between £268 million and £860 million.
It was ordered by ministers after a series of highly-disruptive landslides engulfed the main route between Glasgow and Kintyre, some also forcing the closure of the parallel Old Military Road diversionary route because of the risk of further rockfalls.
Mr Dey told MSPs: "We are in the phase at the moment of data gathering to inform the best choice of specific route.
"That should be concluded by the end of this year.
"All being well, at that point we will have a clearer picture on the workable options.
"We would then move to the development of a final proposal.
"There are, of course, many unknowns here – ownership of the land depending on the route, engineering requirements, ground conditions.
"We are working with all haste on this.
"I don’t want to give hostages to fortune but I think it is feasible that we would, towards the end of next year, have a firm proposal to take forward a shovel-ready proposal – we certainly hope to be at that point.”
Mr Dey also said another £8.8 million was due to be spent on further measures to prevent landslip debris blocking the A83, on top of more than £15m spent so far, such as on heavy-duty fencing and giant pits to catch debris.
Dumbarton Labour MSP Jackie Baillie asked him why the road kept being closed when it rained after so much mitigation work.
The minister responded: “There are no certainties – we are in the hands of Mother Nature.”
He said the measures being taken were to “minimise the risk”.
Mr Dey said: “There is a great deal of effort that goes in to ensuring that any risk associated with this route is minimised.
"Every mitigation possible is being deployed.”