The UK government is refusing to release the carbon emission figures behind its transport decarbonisation plan, which campaigners say could make proposed road schemes financially unviable.
The Department for Transport (DfT) is blocking academics from seeing the figures, which include data on how much car use would have to be reduced in order to reach net zero commitments.
Campaigners say meeting these legally binding targets will be possible only with a drastic reduction in motor traffic, which could make many new road projects financially unviable.
There are 32m cars on the UK’s roads, and they are growing both in number and size. Transport is the country’s largest emitting sector, and produced 24% of the total in 2020.

As Dom Hallas leads me towards his block of flats, he glances up at the exterior, an endless expanse of dull, brown bricks interrupted by a few tiny windows and a curious-looking zigzag design picked out in lighter stone. “It’s true,” Hallas admits, opening the heavy security door with an electronic fob. “When I tell people where I live, some do say: ‘Oh, I always thought that was Brixton prison.’”
It is an easy mistake to make. Southwyck House, only nine storeys high but almost 250 metres long, looms like a gigantic wall just outside the centre of Brixton. It is one of south London’s most obvious and, it has to be said, forbidding landmarks.
Known locally as the Barrier Block, the building looks that way for a reason. It is an almost accidental relic of an alternative history for London, one that would have made the city very different and, as most people would probably agree, significantly less appealing.
This slab of roadside frontage was designed to be not just social housing but also a vast acoustic shield, screening the neighbourhood from the deafening roar of an elevated, eight-lane urban motorway.

They are grey, rectangular and if you lumped their population of up to 2bn together they would cover roughly the same area as Connecticut, about 5,500 sq miles. Car parking spaces have a monotonous ubiquity in US life, but a growing band of cities and states are now refusing to force more upon people, arguing they harm communities and inflame the climate crisis.
These measures, along with expansive highways that cut through largely minority neighborhoods and endless suburban sprawl, have cemented cars as the default option for transportation for most Americans.
From January, though, California will become the first state to enact a ban on parking minimums, halting their use in areas with public transport in a move that governor Gavin Newsom called a “win-win” for reducing planet-heating emissions from cars, as well as helping alleviate the lack of affordable housing in a state that has lagged in building new dwellings.