The number of drivers speeding on the main road north has dropped from one in three to just one in 20 after new cameras were introduced, a new report has revealed.

Meanwhile, the number of motorists caught doing more than 10mph over the limit on the A9 between Perth and Inverness has fallen by about 97% over the last three months.

Average speed cameras were installed last year at a cost of £3 million in a bid to cut the accident rate on the road - with Highland MP Danny Alexander critical of the move.

Mr Alexander, the Liberal Democrat MP for Inverness, Nairn, Badenoch and Strathspey, had previously branded the introduction of the devices a ''knee-jerk decision'' and urged the Scottish Government to bring forward its £3 billion plans to dual the road between Perth and Inverness. They are currently not due to be completed till 2025.

Chief Superintendent Iain Murray, head of road policing at Police Scotland, said the initial results were an "encouraging start".

The cameras, which have been in operation since October 28, have so far detected 298 vehicles exceeding the speed limit to a level which warranted further action - less than four per day.

Mr Murray said: "In the first three months of operation we have seen a more than eight-fold decrease in the number of people caught speeding on this stretch of road compared with the same time last year when there were 2,493 offences recorded.

"It is clear that the cameras are changing driver behaviour in the way that we expected. This will undoubtedly help to make the A9 safer for all road users."

He added: "Speeding is not the sole cause of collisions on the route and we shall continue to engage positively with those who put themselves and others in danger through risky or illegal driving behaviour.

"Safety remains the responsibility of all road users and it is important that we interact appropriately with each other on the A9, and all of the other roads in Scotland."

He spoke out after a report found there had been a "significant reduction in the number of vehicles exceeding the maximum speed limits along the A9 corridor between Dunblane and Inverness" since the cameras were introduced.

"Before cameras were deployed, around one in three vehicles was speeding," it stated.

"This figure has now dropped to one in 20 vehicles and vehicles travelling at more than 10mph above the speed limit has dropped by over 96% from an average of one in every 10 vehicles to one in 250 vehicles."

At the same time as the cameras were switched on, the speed limit for HGVs on the route was increased to 50mph, with figures from hauliers suggesting a significant reduction in journey times for lorries.

There are also no indications that drivers are avoiding the A9 in favour of other routes in a bid to escape the cameras.

Transport Scotland's Stewart Leggett, who is chair of the A9 safety group, said it was "very encouraging to see the improved driver behaviour following the introduction of the average speed cameras and HGV speed limit pilot, ahead of the dualling".

He added: "Drivers are clearly paying heed and moderating their speed, and we welcome this positive contribution to road safety on the A9.

"All the early findings on speed, journey time and journey-time reliability are in line with our predictions while traffic volumes on the A9 are remaining higher than in 2013, with no evidence of drivers diverting onto other routes.

"The low number of drivers being detected by the cameras and the speed profiles from along the route indicates the early effectiveness of the cameras in improving behaviour, but the A9 would be safer still if every driver observed the limits.

"We don't want drivers facing fines and the cameras have never been an alternative to dualling."

Director of Road Safety Scotland Michael McDonnell said: "We know that around three-quarters of A9 drivers believed average speed cameras would be 'effective' or 'very effective' in improving safety, and these new findings may strengthen that viewpoint further.

"It's important to stress that the cameras aren't happening in isolation. They are just one part of a substantial package of measures to make the A9 safer such as engineering improvements, improved lighting and signing, and targeted publicity campaigns.

"As expected, there has been a slight increase in journey times, although some of this can be explained by roadworks that were in place as part of the dualling programme and some of the recent severe weather that we've seen.

"However, it is important to compare and contrast these extra minutes with the costs, both human and financial, of a serious accident, which can also close the road for hours at a time."

Martin Reid, of the Road Haulage Association, said: "Almost universally our members report that the flow of traffic is much improved and that journey times if anything can be slightly shorter."