A £2.7m scheme to improve traffic flow on the congested A350, north of Chippenham, starts later this month.
Wiltshire Council hopes to rid the route of its significant peak-time delays and pump £5.9 million into the local economy by creating jobs.
Drivers are being told to expect improved journey times when work is finished in early 2015, slightly later than the original planned finish date of December.
In the meantime, disruption through the affected area is said to be inevitable, although the council hopes the use of traffic lights will keep it to a minimum.
The road will be kept open to two-way traffic throughout the daytime, by putting a 30mph limit in place during construction to ensure the safety of site staff.
However, there will be a series of overnight closures, to allow specific work to be carried out more safely for contractors.
At the moment, dual carriageway runs from the M4 at Junction 17 for about 2.5 miles up to the Jacksom’s Lane junction.
The work will extend it by another 600 metres in both directions, taking it to the Badger roundabout next to Morrisons.
The project is also expected to improve the pinch point Malmesbury Road roundabout and add a signal controlled crossing nearby.
John Thomson, the Wiltshire Council cabinet member for highways, said: “This is a very important route, one which has typically been a bottleneck.
“Although there will be some disruption while the work is ongoing, the outcome will help improve journey times and ease the congestion around this area.”