WHEN the motor car was introduced to Britain in the 19th century, a top speed of 4mph was imposed so a man waving a red flag could run ahead as it entered a town. The laws of the road have changed as cars have become zippier. But, though vehicles are now faster, safer and more efficient, they are travelling more slowly, particularly on motorways. Why?

Compared with other European countries, Britain's top legal limit of 70mph on motorways and some dual carriageways is comparatively low. That may be one reason why 49% of drivers broke it in 2010, a higher proportion than almost anywhere else. Yet that figure is falling—in 2003 it was 57%. And in that time the share of drivers exceeding the limit by more than 10mph dropped from 20% to 14%.

Since motorway traffic has fallen, congestion does not explain increasing compliance. Nor does greater policing: stringent enforcement campaigns in France and Spain have successfully cut speeds, but Britain has seen a 20% drop in the number of traffic cops in the past decade, reckons Edmund King of the AA, a motoring lobby. Speed cameras have helped to maintain limits and cut accidents at roadwork sites, but are still rare on motorways.

Drivers seem to be slowing of their own accord. The main explanation is fuel prices, which have risen by 34% in real terms since 2003. This may affect behaviour because as cars go faster they use more fuel to travel the same distance—25% more at 70mph than at 50mph, according to the Department for Transport. In-car technology has also helped make drivers aware of such costs: new cars often have dashboard dials showing how many miles to a gallon the vehicle is achieving at its current speed. People are more likely to respond to such nudges when money is tight and jobs are scarce.

Just as drivers are choosing to slow down, though, the government is considering letting them speed up: this summer it will launch a consultation to raise top motorway speeds to 80mph in England and Wales. In 1965, when the 70mph limit was set, few vehicles were able to maintain a faster speed. Cars can easily exceed that speed now and ministers claim raising the limit will provide “hundreds of millions of pounds of benefit for the economy”.

But higher speed limits are unlikely to drag the economy out of the slow lane. Drivers are already choosing cost savings over time. Raising the limit may not increase the number of cars that a motorway can carry, since stopping distances lengthen at high speed and cars are supposed to travel farther apart. And the change would not apply to lorries, a key part of the business network.

The environmental lobby is gearing up to oppose the idea. The Committee on Climate Change, which advises the government, says the current limit should be enforced instead to cut emissions—domestic transport, mostly by road, accounted for 22% of all planet-heating gases in 2009.

More influential are safety campaigners. Speed limits always trade time against safety, notes Stephen Glaister of the RAC Foundation, a motoring lobby. Cutting the top speed to 60mph—and enforcing that—would also save lives. Road deaths have already fallen by 75% since 1965, mainly because cars are safer. But many now assume that the “accepted” current speed in Britain is 80mph—changing the law would risk pushing that higher. Even increasing the limit on just a few straighter, safer motorways, as officials hope to try, may be cavalier: after Denmark increased limits on a some key routes in 2004, average speeds rose on all highways.