Twelve years earlier, Swedes had voted on it in a referendum. Eighty-three percent voted to stay on the left. They didn't want the change.
The Riksdag decided to do it anyway, on 10 May 1963. And instead of setting a date and hoping for the best, it gave itself four years. Four years for something that would happen in five minutes.
Those four years ran on two tracks at once.
The first track was concrete and sheet metal. Around 350,000 road signs to remove or replace, some 20,000 in Stockholm alone. Junctions rebuilt where a left turn suddenly became a right turn. Buses with doors on the wrong side — rebuilt or replaced. Traffic lights. Bus stops. Everything physical.
The second track was human habit. An information campaign that got in everywhere: television, radio, a song contest about the switch, the logo on milk cartons and on underwear. Not because someone in Stockholm had a sense of humour. Because the reflex to turn left sits deeper in a driver than any regulation, and you don't change a reflex with a memo.
Neither track would have worked alone. Perfect signage with drivers running last week's reflexes is a massacre. Perfectly briefed drivers at junctions built the other way round — exactly the same.