The Sustainable Development Commission (SDC), which advises on green policies, is preparing a damning critique of the Scottish government’s transport priorities. To be delivered next month, it will expose a “significant policy gap” which threatens to derail Scotland’s world-leading climate legislation.

The assessment will point out that the transport measures so far promised will deliver only half the cuts in carbon emissions needed to meet climate targets.

“Greater alignment between policy objectives based on sustainable development targets and the many transport projects is sorely needed,” says a SDC statement. “There is growing evidence for, and acceptance of, this mismatch. We believe the government is in real danger of not delivering the required emission reductions without a root-and-branch rethink of priorities.”

The SDC’s UK chair, Will Day, has accused ministers of a “curious myopia” on transport policy. Building new roads should be a policy of “last resort” not first choice, he argued.

“History shows that, if you build a road, it fills up. If you build a bigger road, it fills up too. And if you build a relief road, in a few years you have to build a relief road for the relief road.”

Day questioned the analysis that led to a decision to erect a £2 billion road bridge across the Forth: “We’re going to have to make sure that these kind of major decisions are not made for reasons of short-term political expediency but in the long-term interests of society,” he told the Sunday Herald.

“Part of what government policy should be doing is discouraging people from using cars for transport.”

His critique was reinforced by a new analysis from Friends of the Earth Scotland showing that ministers have omitted millions of tonnes of carbon from their assessment of the pollution impact of the proposed new roads.

Using the government’s own figures, the group suggests the seven largest schemes, not including the Forth bridge, could generate 4.24 million tonnes of carbon pollution by 2030. The government estimate is 213,100 tonnes.

Chief executive Duncan McLaren accused the government of “systematically underestimating” the responsibility of roads for carbon emissions. “Carbon assessment is a useful and welcome tool,” he said, “but when millions of tonnes are not properly accounted for, it risks being misleading.”

For the government to ignore substantial emissions from the roads it builds “is rather like a major bank financing a coal-fired power station, but disowning all responsibility for the emissions that result,” McLaren argued.

The Scottish government said it took its responsibilities on climate change seriously. Delivering its targets would require the “wholesale adoption” of electric vehicles, said a spokesman.

“A key aspect of cross-Forth capacity will be significant improvements to the design of connecting roads to cut congestion and protect air quality from idling engines.”