Retired civil servant Kenneth Braithwaite, 53, watched in horror as his wife and two grown-up children were struck by the car as they walked in Gairloch, Wester Ross.
His wife Shirley Braithwaite, 51, a primary school teacher, and daughter Natalie, 22, died at the scene.
His 19-year-old son Matthew was taken by air ambulance to Raigmore Hospital in Inverness with serious injuries but later died.
A local church minister said the tragedy had completely stunned the tight-knit area.
It happened on Friday evening, a day before the family from Barnsley, South Yorkshire, had been due to return home after their holiday in the west of Scotland.
Rev Derek Morrison, of the Church of Scotland, was among a number of people who tried to comfort Mr Braithwaite after the accident.
He said the tragedy had been difficult to comprehend and that Mr Braithwaite had been "deeply shocked and traumatised".
"In terms of the sudden and complete wiping out of a family, leaving one member of that family to grieve for those he has lost, the scale of the tragedy has been devastating.
"A place like Gairloch, a popular holiday destination, beautiful and totally peaceful, militates against a tragedy such as this ever happening.
"You would never expect something like that to happen in a place like this.
'Weekend of tragedy'
"Many people have been saying prayers today, and Kenneth Braithwaite was very much in all our thoughts."
The family had been enjoying a break at a cottage at nearby Aultbea.
It emerged that Natalie had been due to begin a PhD in nuclear physics and Matthew was undertaking a degree in horticulture.
"On a weekend when there has been so much tragedy with the story of the Cambridgeshire schoolgirls, this tragedy here has also affected us," said Rev Morrison.
"I think it is hard to imagine anything worse happening to a person."
A 28-year-old man is expected to appear at Dingwall Sheriff Court on Sunday in connection with the incident.
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