Starting at Drumahoe, the 15-mile section of the A6 scheme includes a bypass around Dungiven.
When complete, the A6 upgrade will link Derry and Belfast via a dual carriageway.
The upgrade will cost £220 million, according to the Department for Infrastructure (DfI)
Colin Hutchinson, strategic roads improvement manager for the DfI, said road users can expect a 11-12 minute cut in their journey time between Dungiven and Drumahoe when the work is complete.
“Work is expected to take up to four years to complete,” Mr Hutchinson told BBC Radio Foyle.
“Schemes of this nature do usually take time. There have been issues with the availability of finance. The scheme that was announced in 2005 goes the whole way to Caw so there’s another 7km on top of what we’re delivering today.
“There will be major economic benefits because of this work. There will be some disruption because of the work of course and there will be traffic management in place.
“Work has already started in the Foreglen direction. I expect to see sections of the road open before the end of the four year deadline. That’s all down to the progress that the contractor makes though.”
The existing road carries around 15,000 vehicles per day.
The second phase of work continue from Drumahoe to Maydown and then to Caw.