The carbon emissions cost of an 80mph speed limit on motorways

The Government is to take forward its proposal for increasing the motorway speed limit to 80 mph by trialling the new limit on selected managed motorways.

Roads Minister Mike Penning said this week that his Department "is carrying out work to assess the potential economic, safety and environmental impacts of trialling 80mph speed limits on motorways where variable limits are currently in place”.

He said it hadn't yet been decided which stretches of managed motorway would be included in any proposed trial.

“We plan to bring forward detailed proposals and start consultation during the next few months,” he said.

Ministers are also believed to be pressing the police to use a lower enforcement threshold than the current guideline of 10% +2mph that equates to 90mph, which is the reason why motorists aren't presently stopped for driving over 70 mph.

Carbon emissions

Environmentalists have criticised the proposal for upping speed limits on the grounds that it would increase carbon emissions.

But how much difference would it make?

Greenhouse gas emissions from transport currently account for 21% of total UK domestic emissions.

Motorways account for less than 1% of Britain's total road length, yet account for 19% of total annual road mileage, of which 75% is accounted for by cars and taxis.

This means motorway traffic accounts for emissions of 21.917 MtCO2e per year, given that UK total annual emissions last year were 549.3 MtCO2e (million tonnes of carbon dioxide equivalent).

Under relatively steady speed driving conditions carbon dioxide emissions increase by around 14% between 70 and 80 mph and decrease by around 10% between 70 mph and 60 mph, according to modelling done for the carbon reduction strategy, said Lord Shutt of Greetland (the Deputy Chief Whip in the House of Lords, a Liberal Democrat) in a Parliamentary answer recently.

An increase of 14% would therefore mean an increase of half of one percent of UK total current annual emissions.

When asked whether he would support an increase in the speed limit given its impact on carbon emissions, Energy Minister Ed Davey said last month that the increasing number of vehicles on the roads in the coming years with lower emissions would offset any increase caused by higher speeds.

Is this true? The Department for Transport’s Carbon Reduction Strategy is arguing that by 2022, vehicles will be “vastly more fuel-efficient”.

“This will primarily be delivered through advances in the efficiency of the internal combustion engine. Alongside this, new ultra-low emission vehicles will have made their transition onto the mass-market,” it says.

On the other hand, the International Energy Agency has previously suggested that a temporary reduction of motorway speed limits to 90kph as a way of addressing fuel shortages would be a low cost measure that can lead to ‘large oil savings’ and, in a report, Saving Oil and Reducing CO2 Emissions in Transport, has recommended a general lowering of motorway speed limits that would not be restricted in time.

 The effect of vehicle speed on carbon emissions, from the Environmental Change Institute at the University of Oxford

What difference would this make? An enforced 70mph speed limit could cut carbon emissions by 1 million tonnes of carbon (MtC) per year by 2010, and a new 60mph limit could double this to 1.94 MtC, according to research by the Environment Change Institute, at the University of Oxford.

If added to the transport measures in the UK Climate Change Programme, carbon savings would increase by 15% (70 mph) or 29% (60 mph), they calculated.

What next?

It was last September, when Philip Hammond was Transport Secretary, that the Government announced it would be consulting on an increase in the speed limit, but it has yet to happen.

Mr Penning has said that any proposals would come with an impact assessment that would include research about road safety as well as emissions.

The Government is currently reliant for this on 2010 research about the relationship between road safety and speed limits.

Yesterday, a consultation ended on a call for evidence on the impact of speed limit changes to support the development of an appraisal tool to help local authorities assess the full costs and benefits of local speeds.

There is currently no published timescale for the initiative.