The campaign to get Norfolk, Suffolk and the Fens connected will take a giant step forward this week as prime minister David Cameron will visit the region today to pledge more than £300m to upgrade the A47.

In what will be a busy week of infrastructure announcements for the region – with improvements to the speed and reliability of the train line between Norwich and London expected to be given the backing of the chancellor – details of three sections of dual carriageway, amounting to 8.4 miles, will be announced.

Improvements to the Vauxhall Roundabout in Great Yarmouth, the Thickthorn Roundabout on the edge of Norwich, the A47/A141 Guyhirn improvements and safety measures on the Acle straight will also be set out.

But with just five months left of the parliament, it will be up to the next Government elected to get diggers in the ground.

While drivers will be able to travel from Dereham to Acle on a two-lane road under the plans, the pledge falls short of the full dualling of the route.

Campaigners who have long-campaigned for major improvements to the Acle Straight, and the route between Peterborough and Dereham will miss out on a slice of the Government's £15bn funding pot.

Yarmouth North and Central councillor Mick Castle said Norfolk had been 'failed yet again', and the proposed safety scheme, which could see some barriers built on the Acle Straight did not go far enough.

However Great Yarmouth MP Brandon Lewis said the restructuring of the Vauxhall Roundabout on the route into Great Yarmouth would help to ease congestion.

Adding: 'We will do the work and deliver. You can see how quickly we have done the A11.'

Mr Cameron said: 'Improving the A47 will help the local economy and build on the extra 211,000 jobs that have been created since 2010, ensuring more people in the East of England have the financial security of a regular pay packet for them and their family. And it is worth pointing out that spending hundreds of millions of pounds on roads like this is only possible because of the difficult decisions we have taken elsewhere as part of our long term economic plan.'

George Freeman, MP for Mid-Norfolk, said: 'For too long the A47 – the clogged east to west trade artery linking the Suffolk and Norfolk coast across Norfolk to the Midlands – has been a symptom of our part of the world being treated by Brussels and Whitehall as a 'rural backwater'.

'But our area is full of world class businesses and opportunities - in engineering, science, energy, food and agriculture, wildlife, tourism, media and the arts.'

Toby Coke, chairman of Norfolk County Council's environment, development and transport committee, said he had been expecting the schemes set out to cost £500m, and the details would need more explanation, but that the announcement was 'good news'.

'We have now got to really press for the rest of the dualling. The Acle straight and King's Lynn have got to be top of the list.'

But Denise Carlo, from the Norwich and Norfolk Transport Action Group, said the government's own findings – and the schemes announced – showed traffic levels did not justify end to end dualling of the road.

She said: 'Norfolk County Council has again wasted thousands of pounds of local public money on expensive highways consultants and lobbying for end to end dualling.'

Mark Pendlington, chairman of the New Anglia Local Enterprise Partnership, welcomed the news and said: 'We are now keen to see the details of when these schemes will start and will be continuing to work with local colleagues to ensure construction can begin as soon as possible.

'We will continue to press for further improvements to the road to ensure that our goal of a fully dualled A47 remains firmly on the agenda.'

Are the right sections of the road being prioritised? Write (giving your full contact details) to Letters Editor, Prospect House, Rouen Road, Norwich NR1 1RE.