A temporary reduction in the speed limit on motorways and other national roads will be discussed by a cabinet sub-committee this week, as the government seeks to reverse carbon-emission increases that are moving Ireland further away from meeting its Kyoto target.

Green party ministers want to bring the limits down from 120kph to 100kph (75mph to 60mph) on motorways and from 100kph to 80kph (60mph to 50 mph) on national primary routes in order to achieve short-term reductions in the country’s carbon footprint.

The idea will be discussed at the cabinet sub-committee on climate change in advance of the publication of figures by the Environmental Protection Agency (EPA) showing an increase of about 1% in Ireland’s carbon emissions in 2007.

The programme for government commits the coalition to reducing carbon emissions by an average of 3% per annum over five years. The biggest reductions are expected closer towards 2012, the end of the government’s scheduled five-year term.

The speed-cuts proposal, which has been confirmed by Dan Boyle, the Greens’ chairman, could result in a clash with Fianna Fail as it is clearly encroaching on the political territory of Noel Dempsey, the transport minister.

Any decision on speed-limit changes would have to be endorsed by the full cabinet, but a formal proposal on a transport matter would normally be expected to come from Dempsey’s department.

The transport sector, along with agriculture, is among the biggest contributors to Ireland’s carbon footprint and one of the most difficult in which to achieve reductions in emissions without having an impact on economic productivity.

“Since going into government, we have been able to introduce a number of measures that will have a medium- and long-term effect in terms of our national carbon footprint,” said Boyle.

“What we haven’t been able to do yet is introduce some short-term measures that will deal with the problem here and now, and the type of measure that can do that is critically looking at our speed limits. It won’t actually cost people anything and they will see their own fuel bills fall.”

Maximum fuel efficiency on the open road is believed to be achieved in most cars by driving at about 90kph (56 mph). There would a lot of practical problems to address in reducing limits, however, including the need to change speed signs on a huge number of roads.

Spain’s decision to cut its speed limits a month ago was prompted by the rising price of oil. It cut motorway speeds to 80kph and urban speed limits to 40kph.

Even the Germans have been considering making the cultural leap of imposing speed limits on their extensive autobahn network. A limit of 130kph has been imposed near Bremen and Angela Merkel’s junior coalition partners, the Social Democrats, threatened at a party conference last year to impose a national 130kph limit to reduce annual carbon emissions by 2.5m tonnes.

Boyle said the extent of the speed reductions imposed in Spain could be unpopular with Irish motorists, but a 20kph reduction on motorways and national primary roads might be acceptable. “Given the scale of the credits we have to buy if we don’t tackle the problem, it is something we should give serious consideration,” he said.

Provisional EPA figures indicate that the 1% reduction in carbon emissions achieved in 2006 was reversed in 2007. Ireland’s target under the Kyoto Protocol is to limit the increase in emission levels recorded in 1990 to no more than 13% by 2012. Between 2004 and 2005, Ireland’s emissions increased from 23.5% to 26.5% above the 1990 figure, but then fell by one point to 25.5% in 2006.

The target set in the programme for government is to get back down to 19% above 1990 levels over the next five years, and then pay a fine in the form of carbon credits that are bought to bridge the gap to the 13% target.

The Green party also regards the introduction of a carbon levy — an extra tax on petrol, diesel, oil and coal — as vital to reducing carbon emissions but insists that tax reductions in other areas, particularly on labour, will offset the carbon tax and render it revenue-neutral to government.

The carbon levy is not due to be considered by government until after the Commission on Taxation reports in 2009, but the Greens are expected to push for some progress towards a carbon tax in December’s budget.

Noel Brett, chief executive of the Road Safety Authority (RSA), said he would welcome a national debate on speed limits that examined the environmental and road safety benefits and looked at the economic and social impact of any change.

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