Drivers are facing a full year of hold-ups on one of the busiest routes into Preston.
Work began this week on creating a junction for the £17.5m Penwortham Bypass from the A59 dual carriageway at Howick.
And with one lane closed off in either direction, motorists are being warned to expect disruption until the new road is open early next year.
"We ask people to bear with us while this work takes place," said Jim Carter, chairman of City Deal which is funding the project.
And Coun Geoff Driver, leader of Lancashire County Council, added: "There will be disruption while the road is being built, but it will be worth it in the end."
The section of the A59 between the Anchor roundabout at Hutton and the centre of Penwortham has been a traffic bottleneck for years in the morning rush hour.
The creation of the Penwortham Bypass is expected to divert 22,000 vehicles a day around the town, reducing the traffic volume by between 40 per cent and 70 per cent at peak times.
The new road is expected to open in January 2020.