Areas of the UK road network covered by permanent average speed cameras have more than doubled since 2013.
New figures released by the RAC Foundation showed that by the end of 2015 the UK had 51 stretches of road covered by the technology, a total of 261 miles. In 2013, that total was 127 miles.
Much of the current coverage is focused on Scotland, in particular a 99-mile stretch of the A9 between Dunblane and Inverness that was installed in 2014. Since the cameras were installed, road deaths on the A9 have dropped by 30 percent. The research does not include roads with temporary average speed cameras, such as those installed to monitor roadworks.
Experts have cautiously welcomed improved compliance with speed limits in average speed camera zones, but warned that the technology is not a catchall solution to road safety.
Average speed cameras work out the time it takes vehicles to between several set points, unlike traditional speed cameras, which take a snapshot of vehicle speeds at one location.
Richard Owen of Road Safety Analysis, which carried out the research, said a reduction in the cost of average speed camera technology was behind the rise in usage. It costs around £100,000 per mile, compared to £1.5m per mile in the early 2000s.