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. Last week, the DfT published the first long-term road traffic forecasts since 2018, and the first since the 2050 net zero target was signed into law in 2019. Prof Greg Marsden, of the University of Leeds, said the figures showed the sector breaching carbon targets and not meeting its own carbon reduction plans. He has submitted a complaint to the Information Commissioner’s Office (ICO) demanding the figures be released. The projection shows car use will increase or remain stable. It says: “In the high economy scenario, car trips increase over time. In contrast, in the low economy scenario, car trips remain relatively stable until the 2040s, from which point they begin to decrease.” However, scientists say a shift to electric vehicles alone is not enough and car use needs to be drastically reduced in order to meet net zero targets, with some reports showing that a reduction of at least 20% is needed by 2030. None of the projections from the DfT show this. Marsden said: “The key question is: why is it OK to release information about scenarios which are not government policy, but not to release information about scenarios which are?” The ICO agreed with Marsden, ruling: “The commissioner considers that there is a very strong public interest in publication of data that will assist the public in understanding policy decisions – especially those designed to be as far-reaching and long-lasting as the transport decarbonisation strategy. Disclosure will help the public to understand where the government’s proposals are too ambitious, not ambitious enough or about right.” However, the DfT has appealed against the ruling, further delaying the publication of the figures. The department claimed it does not have to submit its workings on the decarbonisation plan as it is a “live policy”, arguing: “It is submitted these should not be disclosed because they are, in effect, the foundation of future policies in the making and are therefore to be considered live policies. Such information should not be disclosed because this may lead to an inhibiting effect on discussing and developing those ideas.” Ralph Smyth, from Transport Action Network, has been working with Marsden to try to get the data published. “We think this is because already a lot of the big road schemes are not showing value for money,” he said. There are hundreds of millions of pounds worth of road schemes in the pipeline. “If you reduce cars, then they are no longer financially viable,” Smyth said. “Ministers and the Treasury like big road schemes so they will not like any plan that has a hypothetical large cut in traffic.” The climate change committee has argued that the government has not factored a reduction in traffic growth into its net zero plans, instead relying on changes in technology such as a switch to electric cars. It has said: “The government has acknowledged the need to limit traffic growth, shifting travel to public transport and active travel, but action is now required to ensure this. The government has made a relatively high-risk choice to rely heavily on technology to reach its targets, with much less focus on efficiency improvements and demand management across the economy. This is a narrow approach that could lead the UK down a more expensive path to net zero, with a higher risk of failure and energy insecurity.” A DfT spokesperson declined to disclose its figures to the Guardian and said: “The department has a clear plan to reach net zero, as set out in last year’s transport decarbonisation plan. This sets out an ambitious and credible pathway to reducing transport emissions, including through electric vehicles, and increasing use of public transport and active travel.” Chris Todd, the director of Transport Action Network, said: “These predictions will be used to undermine the business case for rail, buses, walking and cycling investment. We need politicians to stop pretending that we can have our cake and eat it and to accept the reality that spending on new roads ultimately makes things worse.”