Motorway speed limits

Update (May 2012)

The No to 80 campaign was launched on 18 May 2012. It is a a coalition of road safety and environmental organisations calling on the government to scrap plans for 80mph limits on motorways.

Original article (December 2011)

In September 2011, the Department for Transport announced that it is to launch a consultation on increasing the speed limit on England and Wales' motorways from 70mph to 80mph. The consultation would begin in 2011 with a view to raising the limit in 2013.

What we think

Increasing the limit by 10mph means that given an empty stretch of motorway, you could shave very nearly eleven minutes off a 100 mile journey. The Government thinks this will bring 'huge economic benefits' and 'hundreds of millions' of pounds to the economy. But, contrary to what might be expected a higher speed limit is more likely to increase congestion and journey times than reduce them.

As speed increases so too do the typical (safe) stopping distances between vehicles3. This reduces the number of cars you can slot into any stretch of road. Move up from 70mph to 80mph and it drops from 16 cars per mile (of motorway lane) to 13 (and just 6.5 when wet and stopping distances double). That may not sound much but that's 300 fewer cars per 100 miles of motorway lane on a dry day which adds about another mile of tailback leading to more not less congestion.

Then there is safety. The energy of a moving vehicle increases in proportion to the square of its speed. This means that a one tonne car travelling at 80mph has 35% more kinetic energy than one travelling at 70mph. That's a lot of extra energy to get rid of in the event of a crash. Given that people's reaction times don't speed-up as you go faster, it will be interesting to hear the views of the NHS and emergency services on a higher speed limit, and the cost of any reduction in safety.

So what's behind this proposal? The real reason may be to do with fuel consumption. The power required by a car on a motorway is roughly proportional to the cube of its speed. What this means in practice is that at 70mph a typical car requires about 80kW of power to push it along. At 80mph this increases one and half times to 120kW of power. More power means more energy consumption per hour and more fuel consumption.

So raise the speed limit for the fleet of cars and motorcycles in England and Wales and at a stroke you dramatically increase the amount of fuel consumed. So it's difficult to escape the conclusion that this is a poorly disguised ploy to put more money in the Treasury's coffers by getting us to use more fuel. Aside from the fact that it is likely to achieve very little of positive benefit and takes the electorate for fools, it also drives a Chelsea Tractor through the Government's claim to want to be the greenest ever and to be serious responding to climate change.

Find out more

Department for Transport press release: Government proposes 80MPH motorway speed limit

BBC News: Ministers to consult on 80mph motorway speed limit