The Conservatives say they will dual both the A30 and the A358 in the South West if they win the next general election.
Upgrading 15 miles of the A303 would cost nearly £340m
A government announcement is expected soon about which one of the two roads will be upgraded to become the primary route into the region.
Business and tourism interests in Devon and Cornwall want the A30, because it brings tourist traffic down to Exeter.
In Somerset they want the A358 chosen because it would favour Taunton.
The discussion over which of the two routes should be duelled has rumbled on for almost a decade.
There have been numerous public consultations, and even a public inquiry.
The final decision by the government is imminent but on a visit to Devon on Friday the Conservative roads spokesman Christopher Chope will confirm that a future Tory government would dual both routes as a general improvement of roads in the region.
The announcement will please businesses but upset environmentalists who want the land around the A30 left as it is.
The A30/A303 dualling scheme through the Blackdown Hills Area of Outstanding Natural Beauty (AONB), which would cost £337.9m, is shorter than dualling the A358 and is favoured by Devon County Council and the South West Regional Assembly.
The county council has calculated that vehicles travelling via the A358 and M5 would cover an extra 32.8m miles a year.
But Friends of the Earth say that an increase in carbon dioxide emissions is a price worth paying to protect the ancient landscape of the Blackdown Hills.