On Dagen H, Sunday 3 September 1967 (the day on which traffic in Sweden switched from driving on the left-hand side of the road to the right), all non-essential traffic was banned from the roads from 01:00 to 06:00. Any vehicles on the roads during that time had to follow special rules. All vehicles had to come to a complete stop at 04:50, then carefully change to the right-hand side of the road and stop again before being allowed to proceed at 05:00.

In Stockholm and Malmö, however, the ban was longer to allow work crews to reconfigure intersections: there it ran from 10:00 on Saturday until 15:00 on Sunday. Certain other towns also saw an extended ban: from 15:00 on Saturday until 15:00 on Sunday.

One-way streets presented unique problems. Bus stops had to be constructed on the other side of the street. Intersections had to be reshaped to allow traffic to merge.
Dagen H, via @joemaller.

A perfect example of why it’s important to make early and critical design decisions on with a good helping of foresight. Damn, that must have been a crazy day!