Skye Lochaber and Badenoch MSP Dave Thompson said the Liberal Democrat cabinet minister should abandon his campaign against the £2.5 million Dunblane-Inverness scheme.
The Scottish Government’s Transport Scotland agency, which is in charge of the trunk road, said today the installation of cameras on the Perth-Pitlochry section had reduced speeding from one in three drivers to one in ten - even though they do not go live until October.
The announcement comes days after opponents claimed some lorry drivers were slowing below the speed limit because of the cameras, causing tailbacks.
The lorry speed limit will be increased on single carriageway sections of the road between Perth and Inverness from 40mph to 50mph as an experiment when the cameras go live.
Cameras will cover only single carriageway stretches of the road north of Perth, but also the dual carriageway section south to Dunblane. Installation started in March.
Mr Thompson said: “With this new evidence, even the most ardent critics of speed cameras on the A9 will have to accept that they get results – Danny Alexander’s dangerous message on road safety has been shown to be totally out of line.
“Danny Alexander must now stop his political posturing on this issue in a desperate attempt to save his seat next year. He should accept the evidence – and back the Scottish Government’s plans to improve safety on the A9.”
Mr Alexander, MP for Inverness, Nairn, Badenoch and Strathspey, responded by signalling he would press ahead with the formation of an anti-camera coalition.
He announced that his Highland Infrastructure Forum on the cameras would include Scottish Council Development and Industry, Highlands and Islands Airports, the Scottish Tourism Alliance, independent Highlands and Islands MSPs Jean Urquhart and John Finnie, Conservative MSP Mary Scanlon, and anti-cameras campaigner Mike Burns.
It is expected to meet for the first time within weeks.
Mr Alexander said: “The Scottish Government has made its position quite clear, they plan to ignore thousands of Highlanders and plough on with the cameras.
“We should not be shy about voicing our opinion as the speed cameras creep north up the A9.
“People who want the A9 to be safer but have sincerely held concerns about the use of average speed cameras deserve to be listened to, not vilified.
“That is why I set up the forum and have asked representatives on all sides of the debate to join so we can have a genuine dialogue.”
Superintendent Iain Murray, head of roads policing for Police Scotland, said of the speeding figures: “This is an early indication drivers are already moderating their behaviour.
“This can only improve safety on the route and I would encourage all drivers to follow this example”.
Transport minister Keith Brown said: “The A9 Safety Group [which includes Transport Scotland and police] is clear that average speed cameras are effective in saving lives and this is backed up by the facts.
“Before and after studies of other average speed camera sites in the UK show a reduction in accidents.
“On average, they achieved a 61 per cent reduction in fatal and serious accidents.
“Average speed cameras encourage drivers to improve their behaviour and we are already seeing this on the A9.”