Drivers across the UK are now having their speed monitored on more than 250 miles of roads.

Fifty stretches of highways are monitored by average speed cameras, with 12 systems installed last year alone.

Research for the RAC Foundation by Road Safety Analysis revealed that the sections covered a range in length from just a quarter of a mile on London's Tower Bridge to 99 miles on the A9 between Dunblane and Inverness.

In total 256 miles of roads have average speed cameras. The devices are often used on a temporary basis during roadworks, but those were not included in the study.

Steve Gooding, director of the RAC Foundation, said more research was required to find out whether the frequency of crashes had been reduced since average speed cameras were installed.

Average speed cameras are becoming a more common fixture on Britain's roads.

Steve Gooding, director of the RAC Foundation