The UK's longest average-speed camera system will be installed along a 136-mile stretch of the notorious A9 in a bid to cut deaths.

The announcement by the Scottish Government was welcomed by road campaigners, although motoring organisations warned the cameras between Dunblane and Inverness would not stop all drivers from making dangerous manoeuvres, which often lead to fatal accidents.

The cameras will start appearing early next year and yesterday Transport Minister Keith Brown visited Blackford, between Dunblane and Perth, to launch the £2.5 million system.

It will be only the second such system to be permanently introduced in Scotland, although they have been used during long-term roadworks.

The minister said the first was installed on 32 miles of the A77 in Ayrshire between Bogend Toll and Ardwell Bay in 2005, which had delivered a 46% reduction in fatal accidents and 35% reduction in serious accidents.

About 100 people have died as a result of road accidents between Perth and Inverness since 2006, and 80 miles on that section remain to be upgraded to dual carriage with a target date of 2025. Mr Brown said: "We are the first administration committed to making the road dual carriageway all the way from Perth to Inverness a reality, but we also want to make the immediate improvements that will bring positive changes to driver behaviour.

"We hope to see the first of the cameras introduced early next year and expect the system to be fully operational in the summer of 2014."

Chief Superintendent David O'Connor, President of the Association of Scottish Police Superintendents (Asps), had called for greater priority to be given to upgrading the road after the latest fatal accident earlier this month, when a mother and daughter from Edinburgh were among three people who lost their lives near Newtonmore.

Mr O'Connor, who drives the A9 every week from his home in the Highlands to Tulliallan Police College, said: "This is good news and another step towards making the A9 safer."

Meanwhile, Paul Watters, the AA's head of roads policy, said: "There will be a lot of regular drivers on the A9 who will see the distance as a challenge and may well seek to do it in a certain time regardless of conditions. So for them it will be a good disciplinary measure. But average speed cameras won't necessarily stop dangerous overtaking.

Peter Rodger, head of Driving Standards at the Institute of Advanced Motorists (IAM), said that the cameras were welcome but must not be seen as the solution, and visible police patrolling of the route had to continue.

Leading churchman Professor Donald Macleod, who had called the fatality rate on the A9 a national disgrace, said the announcement reflected real concern on the part of ministers and could bring real benefits.

"However, average speed alone is not the problem. 'Average' still allows excessive speeds in overtaking-situations; and it still allows for more or less desperate attempts to overtake before dual carriageways run out."

He said the 2025 target for dualling the road needed to be brought forward.

Dave Thompson, SNP MSP for Skye, Lochaber and Badenoch, welcomed the announcement but said the cameras could cause frustrated car drivers to take risks when travelling behind slow-moving lorries.

"However, while the A9 remains a single carriageway in many sections, an unintended consequence of this move may be to increase frustration and this could even lead to risky overtaking.

"Trucks and lorries are limited to 40mph on the single carriageway sections of the A9, and an average speed camera system will enforce this limit on them. This will make it difficult for drivers of other vehicles, who are not limited to 40mph, to pass these slower moving trucks and lorries."

The Rail Freight Group has called for more investment in the parallel Perth-Inverness railway to further reduce the danger of accidents involving heavy lorries.

Its Scottish representative, David Spaven, said: "It's important that the Scottish Government goes further to reduce the volume of HGVs on the road. HGVs are disproportionately involved in fatal accidents, and moving freight by rail is vastly safer."

He said daily freight train of Tesco supermarket supplies to Inverness took 20 lorries off the A9, and there was scope for rail to carry far more.

He added: "The big worry is that full A9 dualling will lead to freight traffic switching back from rail to the A9."