Three adults and a child were killed in a horrific fireball smash on a notorious main road in the Highlands, prompting campaigners to demand an updated dual-carriageway system.Eyewitnesses said a pick-up truck swerved across the A9 Perth-Inverness road and crashed head on with a Dutch-registered car, bursting into flames.A spokesperson for the Northern Constabulary said the victims were a man and a woman from the pick-up and a man and two-year-old boy travelling in the car.A woman and a four-year-old girl, also from the car, were taken to Raigmore Hospital in Inverness, where the woman is said to be seriously injured.One eyewitness described how he narrowly avoided being caught up in the smash.Ben Hayes, 34, was driving north with his partner and nine-week old baby when he saw the pick-up swerve towards him from the opposite side."It started to drift across the road so I pulled in as tight as I could. It hit the car behind us head on," he said."I saw it in the mirror, and it burst into flames."It didn't look like anybody would be walking away from it."He said the southbound pickup moved across the carriageway slowly, "as if the driver had fallen asleep".The A9 is one of Scotland's worst accident blackspots and campaigners have long called for upgrades.Chief Inspector Jim Neil said: "This was a horrific incident and our thoughts are with the families involved.