You’ve heard of D-Day, but have you ever heard of Dagen H (Swedish for H Day)? H stands for Högertrafikomläggningen, or the Right-Hand Traffic Diversion. On Sunday, September 3, 1967, Sweden changed from driving on the left-hand side of the road to driving on the right. As you might imagine, this switch was anything but easy.

The decision to move to the other side of the road was not taken lightly. In fact, the idea had repeatedly been voted down during the preceding decades. In 1955, a popular referendum showed that 83 percent of the Swedish population was opposed to the change. However, in May 1963 the Swedish Parliament voted overwhelmingly in favor of implementing the switch to right-side driving. With all of Sweden’s neighboring countries driving on the right, it made sense for Sweden to do the same. Also, despite the left-hand driving rule, cars in Sweden typically had the steering wheel on the left, leading to many accidents, especially on narrow roads.

Preparing the country for the change was a costly and complicated endeavor. Traffic lights had to be reversed, road signs changed, intersections redesigned, lines on the road repainted, buses modified, and bus stops moved. A massive PR campaign was conducted to reconcile the public to the change and educate them about how it would be implemented. Dagen H even got its own logo, which appeared on everything from milk cartons to underwear, and a song contest (the winning tune was “Håll dig till höger, Svensson” — “Keep to the right, Svensson” — by The Telestars).

Finally, everything was ready. At 4:50 a.m. on September 3, 1967, as crowds of people gathered to watch, all vehicles on the road were instructed to come to a halt. They were then directed to move carefully from the left side of the road to the right, and wait. At the stroke of 5:00, following a radio countdown, an announcement was made — “Sweden now has right-hand driving” — and traffic was allowed to resume. Time Magazine called the event “a brief but monumental traffic jam.”

The Swedish Minister of Communication (and later Prime Minister), Olof Palme, said on the radio on the morning of Dagen H:

This is a very large change in our daily existence, our everyday life. The doubts have naturally been great. But our innate hesitancy towards a fundamental transformation of our daily traffic environment has given way before a rational internationalism, before a reform that we are confident will benefit traffic safety. I dare say that never before has a country invested so much personal labor, and money, to achieve uniform international traffic rules.

Overall, the change went smoothly. For about two years after Dagen H the number of traffic accidents dropped, perhaps partly as a result of increased caution on the part of drivers still getting used to the new rules.

So was it all worth it? From a safety standpoint, it’s hard to say. A couple of years after the switch, accident levels returned to their earlier levels, despite the hopes that bringing vehicle design and road rules into harmony would improve matters. But given the ever-increasing number of cars on the road, it’s possible today’s accident levels would be even higher if the change had not been made. And given the numbers of travelers driving across borders these days, not having to switch to the other side of the road when entering and leaving Sweden must surely be a good thing.

Swedish speakers may be interested in this video about the change to right-side driving.