A new study by the Atkins consultancy and Aberdeen University, commissioned by the government itself, found increases in carbon emissions from the transport sector meant even the most ambitious policy options available were unlikely to achieve the target of a 42% reduction in CO2 by 2022.

It also raised questions over the government’s record investment in public transport infrastructure, finding measures such as rail improvements, including the intro­duction

of a new high-speed line,

would only produce a marginal environmental impact.

By contrast, policies intended to curb demand for car use, including increased workplace and public parking charges and a national road-pricing scheme, had far greater potential for reducing emissions and represented dramatically better value ­for money.

Introducing lower speed limits on trunk roads would also produce a significant cut in CO2, the report found.

It recommended investment in infrastructure to encourage cycling and walking, and improving the quality of bus services to persuade people out of their cars.

The findings, published ­quietly on a government website, proved awkward for the Scottish Government. It delighted environmentalists in June by introducing tough, binding targets to cut carbon emissions.

Labour’s Scottish transport spokesman, Des McNulty MSP, called on the government to lead a debate on issues such as road pricing, warning that there were “hard choices” to be made.

He said: “We actually need to have a serious debate about these things. We can’t go on with the consensus of setting challenging targets on climate change and emissions then run away from the debates about how we actually deliver significant change.”

The RAC Foundation said: “There is an increasingly strong case for road pricing. Not local congestion schemes, which levy extra charges on top of the taxes hard-pressed motorists already pay, but perhaps a national scheme where drivers pay as they go, combined with the abolition of road tax and fuel duty.”

Patrick Harvie, Green Party MSP, claimed the Atkins report exposed a “fundamental flaw” in the government’s transport policy. He said: “Ministers continue to claim that they can grow the economy and protect the planet if they just support rail travel alongside their plans for road expansion. The facts are against them.”

Transport accounts for nearly a quarter of greenhouse gas emissions in Scotland and is due to increase

to more than 17 megatonnes of CO2 per year by 2022, a figure that would need to be halved to meet government targets.

However, even the most ambitious package of policies examined in the Atkins report would shave off only 2.15 megatonnes of CO2 by 2022. At best, this would represent a fifth of the reduction in emissions required, leaving other sectors of the economy to achieve deeper cuts.

Professor David Gray, head of the Centre for Transport Policy at Robert Gordon University in Aberdeen, said:“If you’re going to cut transport-related emissions, you need to make some difficult and potentially unpopular choices. It’s difficult to see how the current government, with a minority in parliament, will be willing to do that.”

Jillian Anable, senior lecturer at the Centre for Tran-

sport Research at Aberdeen University and one of the report’s authors, said transport-related emissions would be driven by technological advances, such as renewable electricity generation and the marketing of new electric vehicles.

But she said “tough choices” would need to be made about other policy options.

“If we increase the cost of driving and parking relative to other measures, and enforce or possibly lower speed limits, these are effective measures, no doubt. But it doesn’t mean they’re politically acceptable,” she added.

A spokesman for the Scottish Government said that the Climate Change Act would “drive new thinking” about ways to build a sustainable, low carbon economy, but that it had no plans for road pricing.

“Key to this is having the essential research and data available to help inform decisions on how to preserve this world for generations to come by taking appropriate action during these challenging economic times,” he said.