Traditionally, green is for "go" but, as far as the desperately needed M74 extension is concerned, it appears that Holyrood's only two Green MSPs may be about to bring it to a halt. Faced with a tight spending round and a long list of manifesto commitments, Finance Secretary John Swinney is casting around for major projects that could be cut or delayed.

The £500m five-mile M74 extension through south-east Glasgow is high on the list of options for several reasons. Apart from a major cost saving, such a decision could be construed as boosting the SNP's green credentials. And, crucially, dropping the project could win the minority administration the support of the Greens, whose backing may be vital to winning majority approval for the budget.

Any such decision would be short-sighted and against the interests of the people of the west of Scotland. The M74 was never intended to end abruptly on the outskirts of the city but form a loop to join the west-bound M8. It will allow traffic from Ayrshire and Renfrewshire to head for Edinburgh without going through the centre of Glasgow. Without it, this stretch of the M8 has become the most congested motorway in Britain after the M25. The Kingston Bridge, Europe's busiest river crossing, carries traffic far in excess of its capacity.

The M74 completion is not only integral to Glasgow mounting a successful Commonwealth Games but vital to the regeneration of the east end of the city, which contains some of the most deprived communities in Britain. The clogging of the M8 and main roads through the south and east of Glasgow has became a daily feature of life. Stationary traffic not only represents a massive cost to business but adds to the city's shocking respiratory health problems. It is also responsible for generating large quantities of greenhouse gases, so halting or delaying the M74 completion would not be the victory for the environment that some may claim.

The SNP-led administration may be anxious to deliver better transport infrastructure to its heartland in the north-east. This is understandable and there is a good safety case for dualling the A9. But it should not be at the expense of a project that is vital to the future of Scotland's economic heartland. This project, planned for more than 40 years and subject to endless inquiries and delays, passed its final hurdle last year when the appeal against it collapsed. Millions have been spent already buying out businesses along the route.

One danger of delaying the project until 2011 is that it would then come on stream at the same time as the Scottish Government is embarking on the huge financial commitment of another road crossing for the Forth. One of the criticisms of proportional representation is that it hands disproportionate power to small minority parties. It behoves the Greens not to be seen as wreckers. This project should be seen not as a new road but the completion of an existing one. Green should be for go.