The coalition is taking radical action to kickstart stuttering infrastructure plans by hiving off the Highways Agency as a Network Rail-style public company and giving it a long-term funding settlement to carry out a £28bn road-building spree.

But both Labour and City analysts have questioned the rate of delivery, with much of the money backloaded until 2020 amid few shovel-ready projects.

A green paper published on Tuesday will set out plans to guarantee the funding over six years. Funding levels for motorways and major A-roads will rise to more than £3bn a year by the end of the decade as part of the chancellor George Osborne's commitment to pour cash into infrastructure projects to stimulate economic growth.

The government also drew criticism for shying away from more radical measures pushed by the Treasury and floated last year by David Cameron to bring private funding into the roads. Proposals for tolling will not form part of the legislation.

Motoring groups broadly welcomed the news, which solidifies spending plans for capital and maintenance projects outlined in June's spending review. But campaigners sounded a warning that the government's plans threaten some of the country's most important landscapes and habitats.

The government said creating a free-standing Highways Agency with a long-term, ringfenced budget will give the construction and maintenance industry confidence to recruit and train skilled workers.

The transport secretary, Patrick McLoughlin, said decades of underinvestment in roads had seen Britain fall behind economic competitors. He said: "Today's changes will bring an end to the short-term thinking that has blighted investment in England's roads so that we can deliver the infrastructure our economy needs."

Labour said the move had "all the hallmarks of a policy in search of a purpose". Shadow transport secretary, Maria Eagle, said: "Moving to a longer funding settlement for the Highways Agency doesn't require it to be turned into a company nor require legislation.

"Considering the government's failure to translate their promises into action over the past three years, ministers must explain how this reorganisation will not become yet another distraction from the task of fixing Britain's broken roads."

Prof Stephen Glaister, director of the RAC Foundation, said: "Anyone expecting to see an army of workmen wielding shovels on the streets tomorrow is likely to be disappointed. More broadly we welcome the recognition that our roads have been woefully underfunded for decades.

"By saying these latest plans are fully funded, ministers are effectively guaranteeing to spend more of the £33bn they collect each year in fuel duty and vehicle excise duty on road infrastructure – without a fundamental change in the way motorists pay tax such as through the introduction of widescale tolling."

The CBI welcomed the funding certainty for a more independent Highways Agency, but said the measures fell "well short of dealing with the fundamental problems". Rhian Kelly of the business body said: "We need a radical overhaul in how the UK pays for and manages the road network, instead of relying on ever-tighter public finances."

An extra £500m will be pledged to encourage take-up of electric vehicles on top of a similar sum promised last week by Vince Cable. The Department for Transport said it would focus on cutting congestion and minimising the environmental impact of roads, with more investment in environmental safeguards such as tunnelling and noise barriers. However, transport and green groups are alarmed by blanket plans to widen A-roads, pointing out that national parks and areas of outstanding natural beauty are likely to be damaged by road building.

Stephen Joseph, chief executive of the Campaign for Better Transport, said: "People will be shocked by the scale of the environmental vandalism that the government is planning. Smothering Tarmac across our best loved and most visited countryside will not solve traffic problems or create jobs."

Shaun Spiers of the Campaign to Protect Rural England said: "To consider road building in five of our most cherished landscapes suggests an alarming lack of concern for our beautiful countryside and that something is very wrong indeed with the government's roads vision."

Prospective roadbuilding includes 221 extra lane miles of managed motorways and 52 major projects that have been on a Highways Agency wishlist. The flagship scheme to improve the A14, the congested freight route linking ports to delivery hubs and the motorways of central England, will be brought forward again, having been scrapped by the coalition in 2010. DfT sources said tolls remain under consideration for the A14, despite the omission from the green paper.

Feasibility studies will go ahead for works on the A303 to the southwest and the A27. The projects have caused controversy in the past, as Stonehenge and the South Downs are potentially affected.