Bridges must be built on solid foundations. With some notable exceptions, Scotland has a long and impressive tradition of building bridge that stand the test of time and the elements. But nothing lasts forever and a predicted date of 2019 for wear and tear making the Forth Road Bridge unfit for vehicle traffic led to the Scottish Government announcing yesterday that a replacement structure would be built to the west of the existing crossing at a cost of up to £4.2bn. In the face of the current bridge's expected decrepitude, John Swinney, the Finance Secretary, concluded that doing nothing was not an option.

Bridges must be built on solid foundations. With some notable exceptions, Scotland has a long and impressive tradition of building bridge that stand the test of time and the elements. But nothing lasts forever and a predicted date of 2019 for wear and tear making the Forth Road Bridge unfit for vehicle traffic led to the Scottish Government announcing yesterday that a replacement structure would be built to the west of the existing crossing at a cost of up to £4.2bn. In the face of the current bridge's expected decrepitude, John Swinney, the Finance Secretary, concluded that doing nothing was not an option.

If the predictions of the bridge's demise are correct (and nothing of substance has emerged to cast them in doubt), there would seem to be a case for building a replacement. A road crossing linking vibrant economies and bustling communities in the east is vital to the wellbeing of the region and beyond. A tunnel crossing, while being more attractive on environmental grounds, would be prohibitively expensive, would take too long and would not be suitable for light trains, trams and lorries hauling flammable liquids; all of which could use the new bridge.

On constructing a case for a new bridge, these foundations appear solid. But there are other factors to take into consideration, chief of which is the environmental impact of a new bridge. The SNP administration has made a commitment to cut Scottish emissions of greenhouse gases as its contribution to tackling global warming. Can these reductions be made outwith the framework of an environmentally-sustainable transport policy? The short answer is no. The SNP is committed to just such a policy, but building a new Forth road crossing will make it all the more difficult to achieve.

Indeed, the green lobby argues that, for the price of the bridge, it would be possible to build the Edinburgh tram scheme, re-open the Glasgow-Bathgate-Edinburgh and Waverley railways, electrify the Glasgow-Edinburgh main line and give each local authority £20 per head of population for cycle and pedestrian schemes. Instead, it is argued, putting so many transport eggs in the one basket will encourage greater car use, increase pollution and worsen congestion. On green grounds, the foundations are not so firm and could cause the administration a tremor or two, given the policies of the Greens, its one friend in government which is more an enemy than an ally on this question.

The SNP needs to demonstrate convincing green credentials to the wider public and should begin by giving serious consideration to the proposed hovercraft commuter link between Kirkcaldy and Portobello as a means of mitigating the impact of pollution from traffic on a new bridge.