The number of casualties on the A9 has dropped by 45% since the introduction of average speed cameras 21 months ago, according to new figures.

The statistics, released by the A9 Safety Group, showed a 43% fall in fatalities and a 63% reduction in serious casualties on the stretch of road between Dunblane and Inverness.

Campaigners said they had also witnesses an improvement in driver behaviour.

Design work for the A9 Dualling programme on a 15-mile stretch near Inverness will begin in the new year.

WWF Scotland director Lang Banks said: "Once again we continue to see the very clear safety benefits that average speed cameras have had by helping to enforce speed limits on the A9.

"However, alongside other tools, average speed cameras are also a cost effective way to reduce climate emissions as well as fuel costs for motorists.

"To reduce health-threatening levels of air pollution and our meet our climate targets, the use of average speed cameras and other approaches to help reduce excessive speeds should be seriously considered on all major roads across Scotland.

"As part of its forthcoming climate and energy plans, we very much hope that the Scottish Government will follow the advice of its own independent climate watchdog and consider greater enforcement of speed limits to help meet carbon targets."

SNP MSP for the Highlands and Islands Maree Todd added: "The A9's improving safety record is really reassuring news for the thousands of drivers that use Scotland's longest trunk road every day as well as those travelling over the busy Christmas holidays.

"There has been a sustained improvement in driver behaviour and a corresponding fall in collisions and casualties - with 43% fewer fatalities and a 63% reduction in serious casualties between Dunblane and Inverness in the first 21 months of the average speed cameras.

"Every road death is one too many and the SNP remains steadfastly committed to reducing casualty numbers further.

"Once completed, the dual carriageway will help grow the economy of the Highlands and Islands through improvements, road safety and journey times, as well as better links for public transport and the various tourist and recreational attractions along the route."