Margaret Foulds, 67, had been returning from a shopping trip with two friends when she left the M74 motorway at the wrong junction.
But rather than continuing along a slip road, she pulled in to the hard shoulder and stopped her car near Uddingston, Lanarkshire.
She then waited for a few seconds before turning her Vauxhall Meriva back across two lanes of northbound traffic on the M74 near junction 4.
This sudden movement caused a van driven by gas engineer David Gilchrist, 26, to crash into the side of her car at 60mph.
Another car driven by pharmaceutical representative Elaine Hughes, 46, swerved to avoid the van but was hit by Foulds’ car as it spun away from the van.
The impact left Foulds’ friend and passenger Sandra Muir, 71, with five fractured ribs, a cut in her liver, a bleed in her lung, a fractured left ankle and a fractured right knee.
Foulds, of Paisley, Renfrewshire, had gone on trial at Hamilton Sheriff Court where she denied a charge of dangerous driving in December 2014 but was convicted by a jury.
Ms Muir, who had been the front seat passenger, said she had been left with a catalogue of injuries but claimed Foulds had not carried out a U-turn.
She said: “We came off and we stopped in the hard shoulder, and it was a slip road going up. Margaret said we had to get on to the slip road and said ‘that’s us clear, we are going to go now’ and she did.
“The next thing I knew the car was getting banged and twirled round.
“I didn’t see the van coming because I couldn’t see past her but I had no reason to check because she was driving but she wasn’t trying to do a U-turn.”
Sentencing Foulds, Sheriff Vincent Smith said: “You have been found guilty of driving dangerously as a result of which your friend was seriously injured.
“This is a grave offence however I have to consider you have never come to the attention of the courts before.”
Sheriff Smith fined her £1,000, banned her from the roads for two years and ordered her to resit her test.