Half of £1bn work is on Black Cat roundabout alone

Senior project manager Paul Salmon said work on the Black Cat roundabout (pictured) takes up half the total cost of the 10-mile road project

Half of the work on a £1bn road improvement project is being taken up with improving one busy roundabout, National Highways has said.

Work on a 10-mile (16km) dual carriageway, between the Black Cat roundabout in Bedfordshire and Caxton Gibbet in Cambridgeshire, began in December 2023 and is expected to be completed in the spring of 2027.

Paul Salmon, senior project manager for National Highways, said that "building such a complex three tier junction" at the Black Cat roundabout "is why it costs half of the scheme cost".

He added that it was "the most significant and complex part of the project".

There are 30 other individual structures being built along the dual carriageway scheme - as well as another flyover at Caxton Gibbet.

An artist's impression of what the road will look like with the A1 passing underneath the new A421 flyover

The new dual carriageway will replace the only stretch of single carriageway between the M1 near Milton Keynes and the east coast ports of Harwich and Folkstone, National Highways said.

"With everything linking into the flyover at the Black Cat and the A421 and the underpass of the A1 it is just not feasible to open sections" at the moment, Mr Salmon said.

As the site is also on a high water table near the River Great Ouse, "we have put in around 450 piles, which build a wall to stop ingress of water and silt in preparation of the underpass," he said.

Senior project manager Paul Salmon said they are putting in tanks and pumping stations to cope with any possible flooding in the future

Following flooding on the A421 at Marston Moretaine last year, Mr Salmon said: "We have taken learnings over that unfortunate event and we have changed some of the designs of the water tanks."

The four tanks, which he described as "basically massive fish tanks", will be built into the ground to hold any excess water.

There will also be a main pumping station at ground level as well as two back up pumps.

"So that is just planning for the future for any major flooding event," he added.

"We have over engineered that and we are pretty confident it will be able to cope."

Mr Salmon said the Black Cat signage had been repainted and was being looked after at the National Highways compound in Wintringham near St Neots

Mr Salmon said the original Black Cat roundabout statue would be returned, adding: "We will more than likely make a second one which will be put in another location so everyone can see from every direction there is a black cat there."

He said one of the most asked questions at events was where the Black Cat had gone - and where it would appear after the works.