The latest accident on the route – dubbed the most dangerous road in Scotland – has led to further calls for proposed dualling to be brought forward.
The collision between the BMW sports car and HGV resulted in long tail-backs on the road, leaving motorists with a four-hour, 152-miles detour.
Business leaders and politicians said that the Highland economy was losing out as a result of such closures.
The A9 was forced to be shut in both directions after the collision south of Dalwhinnie at around 12.10am today.
Two causalities, one man in a critical condition and another described as having minor injuries, were airlifted to Ninewells Hospital in Dundee.
Another man with non serious injuries was taken by ambulance.
A further two men, who were not seriously injured, were taken to Raigmore Hospital in Inverness by ambulance.
Police said all the injured men were in the car travelling north.
The man driving the lorry, which was heading south, was not injured.
Around 25 firefighters attended the scene from stations across the Highlands, as well as a heavy recovery unit from Inverness.
The road was closed between Wade Bridge and Dalnacardoch Lodge, half a mile south of the Dalwhinnie junction, with the A889 for around nine hours, reopening just before 9am.
With no official diversion in place, motorists had the option of travelling from Perth via the A85-A82-A86 - a four-hour drive approximately 152 miles long.
A Police Scotland spokesman said: “Police received a report of a serious injury collision between a lorry and a motor car.
“Emergency Services attended and a number of casualties were removed from the locus by helicopter and ambulance.”
Fraser Grieve, Highlands and Islands director of the Scottish Council for Development and Industry (SCDI) said: “The A9 is a key economic artery for the Highland economy, which all too often is the scene of serious accidents.
“With the economy of the North showing continued signs of growth and opportunity, businesses will rely even more on our transport connectivity in future, and poor safety on our road network and unreliable journey times will hamper their confidence to invest.
“Everything should be done as quickly as possible both to improve safety and support economic growth. Connecting Scotland’s cities with a network of modern dual carriageway roads would have a substantial, long-term, widespread and potentially transformational economic impact.”
He added: “Our thoughts and prayers go out to the family and friends of those affected by this latest accident.”
the single-track Aberdeen-Inverness line.”
MSP Murdo Fraser, who represents Mid-Scotland and Fife, has long campaigned for dualling in a bid to cut deaths on the road.
He said: “Last night’s collision highlights how urgently we need a fully dualled road.
“The suggested diversion for southbound traffic, consisted of a four drive and 152 miles.
“The impact to business, commuters and tourists is hard to quantify but every time we have an accident like this the economy of the surrounding towns and Highlands’ suffer.”
There were 72 deaths on the A9 between 2005 and 2010, with a further 14 in 2011 and another 10 in 2012.
The Scottish Government proposes to dual the A9 from Inverness to Perth by 2025, and the whole length of the A96 by 2030, but campaigners want it sooner.
Last year, a major study carried out by the SCDI revealed that businesses believe the dualling of the A9 - and the A96 Aberdeen to Inverness route - would provide a vital economic boost to the region and transform the fortunes of the north.
A total of 210 businesses in the Highlands and Moray were polled in the study, with the majority expressing fears about the current state of the region’s main trunk routes, claiming they were holding the region’s economic development back.
The A9 was rated as “poor” or “very poor” for safety by 70% of those who responded, with only 1% of businesses claiming to be “very comfortable” driving on it.
The report by SCDI - an independent economic development organisation representing 1,200 companies, SMEs, public sector bodies, charities, trade unions and faith groups - states the benefits of dualling the roads include making the area more attractive to investors, reducing transport costs and improving business confidence.
Nearly two thirds of businesses say they make allowances for constraints imposed by the current condition of the roads.
SCDI’s Highlands and Islands chairman Michael Urquhart said connecting all of Scotland’s cities with modern dual carriageways would have a “substantial, long-term, widespread and potentially transformational economic impact”.
He added: “Businesses clearly regard the existing roads as substandard and there are significant direct negative impacts on a large number of businesses across key sectors.”
David Richardson, development manager for the Highlands and Islands branch of the Federation of Small Businesses, said: “The A9 and A96 are not fit for purpose and that dualling them will reduce driving times, improve road safety and help combat the Highlands’ distance from key markets, a major barrier to economic growth.
“The Scottish Government is to be commended for getting these projects off the ground.”
He added that members feared the controversial £2.5million scheme to instal average speed cameras on the A9 would have the opposite effect.
Plans are under way to introduce controversial average-speed cameras on the A9, with 100 cameras at 40 locations, with the Government claiming it would cut fatalities on the road.
Meanwhile, an electrician who died after being crushed under his pick-up in a layby on the A835 Ullapool to Tore road near Dingwall was officially named as 44-year-old Croatian Marko Srsen, who lived in Inverness with his wife.