Inspired by a tweet from Mikipedia, I did a little Googling for "Dagen H", the day that Sweden reversed its traffic from the left side of the road to the right side, 40 years ago:

On September 3, 1967, at 04.50 in the morning, the traffic everywhere in Sweden was directed over to the right side of the road and stopped. Everything stood absolutely still for 10 minutes, and at 05.00, when it started again, all road users in Sweden from heavy trucks to cyclists were already on the right side of the road, and they have stayed there since.

What's even more amazing is that there were actually less fatalities and accidents during that September than in previous Septembers (although a possible result of people being more alert just after the switch).

But it could be proof that, even the most rigid and essential standards can be reversed if the warning is long enough and well-organized.