Alongside its proposals for the third Heathrow runway on 15 January, the Government announced plans to expand the motorway system by opening up the hard shoulders, normally used for emergency stopping, for through traffic at times of peak demand. Trials have indicated that this would deliver safe travel and more reliable journey times, while adding a third more capacity at peak, at a lower cost than conventional road widening schemes.   A programme of up to £6 billion is planned for some of the most congested parts of the network.

Use of the hard shoulders is accompanied by an enforced speed limit of 60mph for reasons of safety.   The detailed proposals indicate that current average speed at peak periods on sections of the motorway that would be managed is 55mph, which is expected to increase by 17% with hard shoulder running.   Curiously, such an increase would take the average to 64mph, which is higher than the proposed limit.

One virtue of enforced speed limits is to smooth traffic flows by reducing stop-start driving.  This helpfully reduces carbon emissions.   On the other hand, allowing an increase in speed will increase traffic since over time people will tend to travel further, given that travel time is stable on average.   In respect of carbon emissions and traffic generally, managed motorways with enforced speed limits are preferable to unmanaged, widened motorways.   But it would be best to set the speed limit at 55mph, to avoid any increase in carbon.