Baroness Vere of Norbiton and Stephen Barclay MP have declared the new £17 million A47 roundabout at Guyhirn open.
The upgrade works were completed on March 31 after 14 months of disruption for motorists between Peterborough, March and Wisbech.
As part of the ceremony, which took place today (May 13), the A47 Nene crossing will be named the Tiddy Mun Bridge - a title put forward by 13-year-old Ava McCulloch in reference to a Fenland legend.
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Baroness Vere, a junior transport minister, said: "The opening of this vital new road scheme is a significant improvement to the road network in Cambridgeshire."
Stephen Barclay, MP for North East Cambridgeshire, said: "The completion of the A47 Guyhirn junction is great news for motorists, both in facilitating faster journey times and by increasing road safety for drivers and pedestrians.
"More than 20,000 vehicles use this junction and the short-term pain of roadworks during the last year will be offset by the reduced congestion and faster journeys that these improvements bring."
The finished A47 Guyhirn junction marks the beginning of six significant upgrades and almost half a billion pounds of investment along the 115-mile route from Peterborough to Great Yarmouth.
Planned upgrades include a new dual carriageway between Wansford and Sutton in Cambridgeshire, and a new free-flowing lanes at the A11 Thickthorn interchange near Norwich.
According to National Highways, there is now 18 percent more capacity at the A47 Guyhirn junction, with journey times expected to shrink by four minutes.
The Tiddy Mun Bridge was named by Ava, who won a National Highways competition.
The title refers to the legend of Tiddy Mun, a bog spirit who is said to control the waters and mists of the Fens in Cambridgeshire and Lincolnshire.
First documented in the Folklore Society journal in 1891, it is said that the spirit is about the size of a three-year-old child, but has the appearance of an old man with grey clothing.
The river crossing, formerly known as Nene Bridge, features an extra lane for traffic approaching the junction.
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