We must believe Finance Secretary John Swinney when he says that the Scottish government "will continue to invest in infrastructure that tackles congestion . . . reduces journey times" and "encourages people out of their cars". It is just that it has a very funny way of showing this when one of its first acts was to cut the cost of commuting to Edinburgh from Fife by car by £5 a week at a time when the cost of making the same journey by public transport is rising.

Yesterday, a report commissioned by the previous administration spelled out the cost of scrapping tolls on the Forth and Tay bridges: extra congestion and pollution, longer journey times and significantly more car journeys.

To judge this issue, it is necessary to separate out the different arguments used to justify or oppose tolls. The first is that those who use infrastructure should help pay for it. More than £12m is still outstanding on the Tay Bridge and, after launching the consultation process yesterday, the government is now committed to replacing the Forth Road Bridge at huge public expense. The argument that drivers already pay for roads and bridges through road tax and fuel duty is a weak one. Once the indirect costs are factored in, such as treating accident victims, motoring taxes cover only around one-third. In other countries, including France, the principle of tolls on roads and bridges enjoys widespread public support and in England the M6 toll road has been judged a success. A counter-argument is that high collection costs make it an inefficient revenue-raiser, but the new technology of transponders and vehicle-tracking should tackle that. The second argument is that abolishing the Forth and Tay tolls removes an injustice suffered by the communities of Fife, Tayside and the Lothians. Why should they be treated differently from the people of Skye? But there is a difference. Without a competing ferry and with no rail link, the Skye Bridge is a lifeline service. By contrast, in the areas served by the Forth and Tay bridges travellers can opt for rail or bus services.

This, in turn, leads to the third argument, which is about the principle of the polluter paying. Here, things become complicated because the tolling arrangements at the Tay Bridge are so cumbersome that queuing to pay is the main source of pollution and congestion, though that might improve with transponders. On the Forth Bridge, abandoning tolls will simply make a bad situation worse, with more standing traffic and frustrated drivers. Also, it accentuates a trend towards making public transport relatively more expensive. Yesterday's transport figures tell the story: with the cost of motoring stagnant or falling, road traffic was up by 3% to record levels last year, while bus journeys (up in price by 42% since 1980) have plummeted.

Fulfilling one manifesto promise merely puts others at risk. What about the pledge to cut Scottish emissions by an average 3% a year or to offset any increases in greenhouse emissions? Sooner or later the Scottish government will have to make tough decisions about transport or risk the label of climate wreckers. It is irresponsible not to consider issues such as bridge tolls in the wider context of an environmentally sustainable transport policy.