Today we're looking at whether increasing the speed limit from 70 to 80mph is really the best idea.

There are many arguments for and against the government’s decision to increase the national speed limit to 80 miles per hour, with all kinds of reasons why it would help or hinder your wallet, the air you breathe and your safety on the road. Here are some reasons why raising the speed limit to 80 would not be a good idea.

80 is already the unofficial speed limit

Reacting to an AA/Populus poll in March 2011, in which 63% of the 12,865 AA members surveyed felt the motorway speed limit should be increased, AA president Edmund King said “Eighty miles per hour is already accepted by most as the de facto limit so it is better to legalise this limit and enforce it. The best speed limits are limits that drivers understand and respect.”

Even the government’s proposal of the new speed limit stated that 49% of drivers currently drive above 70 mph; but it doesn’t try to guess as to whether or not drivers would pay equal notice to a new limit. Who's to say that increasing the speed limit wouldn't do anything but increase the overally speed of drivers to see them regularly breaking this new, 80mph limit?

A new limit would see a decrease in fuel efficiency – and an increase in pollution

Director of the RAC Foundation, the independent motoring research charity, Professor Stephen Glaister argues that there are good reasons for increasing the speed limit, but just as many not to.

“Drivers travelling that 10mph quicker might reach their destination sooner, but will use about 20 per cent more fuel and emit 20 per cent more CO2.”

According to the website, a car’s optimal speed is between 55 and 60 mph. While encouraging you to shave significant time from your journeys, raising the speed limit also wastes fuel – meaning that you would have to spend more on it to achieve the same results that you would by driving slower. Studies by the American department of Energy reveal that the advertised MPG of a car is based on a speed of 55 mph, and would be less efficient as the speed increases; 17% less efficient at 70mph and 28% less efficient at 80mph.

Stephen Joseph, from Campaign for Better Transport, said: 'Increasing the motorway speed limit to 80mph would not help the economy and would increase costs for drivers.

“It would also add to pollution and increase road casualties. Responsible motorists know that driving steadily at, or below 70mph is most fuel-efficient, reduces jams and is safer.”