A while back, I mentioned ongoing research in the field of crowd dynamics that looks at dominant traffic patterns. Although it first seemed that left/right traffic patterns are emergent, I noted an ongoing project by South Korea to get people to walk on the right. It turns out that this is by no means the first time that this has happened.

[caption id="" align="alignright" width="370" caption="Source: telemachus unedited; red = right-hand driving, blue = left-hand "][/caption]

In Sweden, all traffic was changed from left to right on September 3, 1967, a day known as "H-Day," "Dagen H," or "Högertrafikomläggningen." From Wikipedia:

On Dagen H, Sunday, September 3, 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—from 10:00 on Saturday until 15:00 on Sunday—to allow work crews to reconfigure intersections. 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....

In order to avoid blinding the oncoming drivers, all Swedish vehicles had to have their original left-hand-traffic headlamps replaced with right-hand units. One of the reasons the Riksdag pushed ahead with Dagen H despite public unpopularity was that most vehicles in Sweden at the time used inexpensive, standardized round headlamps, but the trend towards more expensive model-specific headlamps had begun in Continental Europe and was expected to spread through most other parts of the world. Further delay in changing over from left- to right-hand traffic would have greatly increased the cost burden to vehicle owners.

The article has more on the politics of the change, including a 1955 referendum in which voters overwhelmingly chose to keep traffic on the left. Apparently such changes, while rare, occurred numerous times in the 20th century. The most common change was from left- to right-hand traffic. "Globalization" seems like a hand-wavy explanation, but it does seem that as time went on there has been more need for solving this coordination problem on an international level.