Police have welcomed the drop of collisions on the A11, running through Norfolk and Suffolk, which fell by 80% between 2010 and 2015, from 115 to 23.
The last nine miles between Barton Mills and Thetford opened on 12 December 2014 at a cost of £134m.
Insp Julian Ditcham, of Suffolk Police, attributed the drop to the upgrade.
"The majority of the accidents were on the single carriageways, often through people overtaking at a risk to themselves and others," said Insp Ditcham.
"There was the added problem of deer getting on to roads. There is now more space between the forestry and the road."
Prof David Spiegelhalter, of Cambridge University, said: "The numbers for 2015 are certainly significantly less [in the technical sense] than 2010 to 2012."
He accepted that figures were affected by speed restrictions during construction in 2013-14.
"[But] a reduction of around 100 a year to 23 in one year could not plausibly be due to chance," he continued.
"It is a rather a dramatic reduction, and so unless there is some other plausible reason provided [for example, a massive reduction in traffic], I would think it reasonable to claim it was due to the dualling."
Highways England said: "We routinely assess the impact of our major projects on congestion and safety and, while it is too early to say for sure what the long term impacts of dualling the A11 have been, the early indications are very encouraging."
Dr Giles Smith, 68, who lived and worked near the A11 and attended crash victims, campaigned in the 1980s and 1990s for the A11 to be made safer.
"I am impressed with the A11 dualling, even if it was 20 years late," he said.
West Suffolk Conservative MP Matthew Hancock said: "Thousands of commuters are having a much safer journey thanks to the dualling."
Business leaders said the new road had been for positive for companies in Norfolk and Suffolk.
John Dugmore, chief executive of Suffolk Chamber of Commerce, said: "We hear from our members and from businesses big and small across the East of England that 12 months on the dualling of the A11 is having an improved impact on our economy."
Crashes on a stretch of road upgraded after a 40-year campaign fell "significantly" in the first year after it opened, the BBC can reveal.