Which came first? In this case, the subjects are not the chicken and the egg but the carrot and the stick; more accurately, a particularly problematic version of the conundrum about how best to incentivise. It concerns road charging. Opponents had hoped this controversial option for reducing greenhouse gases, tackling gridlock and improving business efficiency had been parked for the foreseeable future. This was founded on a Scottish Executive study which concluded last year that road tolls were unlikely in the near future. However, recent developments have conspired to push congestion charges back up the Scottish political agenda.

The first was at the weekend, when it emerged that the Scottish Liberal Democrats favoured a cross-party convention to press for more powers to be ceded from Westminster to Holyrood. There is, of course, a political dimension to this. A convention could, potentially, prepare the ground for an SNP-LibDem Scottish government, depending on the outcome of the May poll, if the prospect of extra powers were sufficiently enticing for the Nationalists to kick a referendum on independence (unpalatable to the LibDems) into the long grass. Powers already identified for transfer from London to Edinburgh are vehicle excise duty and fuel duty. These could be substituted for road tolls.

This scenario is at present the stuff of speculation but the political pressure was ramped up yesterday when London's congestion charge zone was roughly doubled in size to include most of Westminster, Kensington and Chelsea. Go west, there in the open air. Friends of the Earth (Scotland) did not quite sing from the Pet Shop Boys' hymn sheet yesterday but, on the back of westward expansion of the scheme, the green lobby called on Scotland's political classes to explore congestion charges to tackle this country's transport problems and cut pollution.

Sir Rod Eddington, who was asked by Gordon Brown, the Chancellor, to report on the economic impact of transport on the UK economy, concluded that road charging could be worth up to £28bn a year by 2025, mostly from free-flowing traffic boosting output. The executive has, however, put more store by rail as a solution to existing and future problems. Minis-terial caution about road pricing is not surprising after it was rejected by a margin of three-to-one as an option to deal with traffic gridlock in Edinburgh.

Scotland is not London. Britain's capital city has a public transport system to give commuters unwilling to pay the charge viable alternatives (which, to a degree, they have used). It is not so straightforward in Scotland. Public transport is especially problematic in the hinterland, where travel is extremely difficult without a car. It would be neither viable nor fair to punish people off the road if practicable alternative means to take them from A to B were lacking. This also applies in urban areas where, away from peak hours, transport services are restricted and do not take account of changing work patterns. Global warming dictates that we must explore all options to reduce car use, including road charging. But it will be even more difficult to sell if introduced to pay for improved buses and trains that come later; pain first, in other words. But the public wants the carrot before the stick.