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Dagen H (The H day) was the day, 3 September 1967, on which traffic in Sweden switched from driving on the left-hand side of the road to the right. The H stands for Högertrafik, the Swedish word for "right-hand traffic".

There were two major arguments for the change:

• All Sweden's immediate neighbours drove on the right (including Norway, with which Sweden has a long land border).
• Most Swedes drove left-hand drive (LHD) vehicles. This led to many head-on collisions when passing on two-lane highways, which are common in Sweden because of its low population density and traffic levels.

Nonetheless, the change was widely unpopular, and had repeatedly been voted down over the previous forty years. In a 1955 referendum, 83 percent voted to keep driving on the left. In 1963, the Riksdag (the Swedish parliament) approved the changeover and established the Statens Högertrafikkomission (HTK), or 'state right-hand traffic commission' to oversee it. It also began implementing a four–year education program, with the advice of psychologists.

As Dagen H neared, every intersection was equipped with an extra set of poles and traffic signals wrapped in black plastic. Workmen roamed the streets on Dagen H to remove the plastic. Similarly, a parallel set of lines were painted onto the roads with white paint, then covered with black tape. Before Dagen H, Swedish roads had used yellow lines.

On Dagen H, Sunday 3 September, all non-essential traffic was banned from the roads from 1:00 a.m. to 6:00 a.m.. Any vehicles on the roads during that time had to follow special rules. All vehicles had to come to a complete stop at 4:45 a.m. and, after waiting five minutes, carefully change to the right-hand side of the road and then stop again before being allowed to proceed at 5:00 a.m.. In Stockholm and Malmö, however, the ban was longer to allow work crews to reconfigure intersections: there it ran from 10:00 a.m. on Saturday until 3:00 p.m. on Sunday. Certain other towns also saw an extended ban: from 3:00 p.m. on Saturday until 3:00 p.m. 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.

Trams in Stockholm were withdrawn and replaced by buses, and over one thousand new buses were purchased with doors on the right-hand side. Some 8,000 older buses were retrofitted to provide doors on both sides, while Gothenburg exported its RHD buses to Pakistan and Kenya. However, in Gothenburg an extensive tram network is still operating.

All Swedish vehicles had to have their original left-traffic headlamps replaced with right-traffic 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 standardised 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 changeover from left to right traffic would have greatly increased the cost burden to vehicle owners.

On the Monday following Dagen H, there were 125 reported traffic accidents, compared with a range of 130 to 198 for previous Mondays. No fatal traffic accidents were attributed to the switch. However, many older people gave up driving rather than learn to cope with the new rule of the road. Experts had suggested that changing to driving on the right would reduce accidents, because drivers would have a better view of the road ahead. Indeed, fatal car-to-car and car-to-pedestrian accidents dropped sharply as a result. However, the accident rate rose back to its original level within two years.

• The Dagen H logo was used on various commemorative items, including women's underwear.

• Driving on the left or right
• Transportation in Sweden

• Friedlanders, Paul. "H-Day Is Coming In Sweden", New York Times, August 20, 1967. (in English)

• Berinato, Scott. "When Voice Becomes Data", CSO Online, September 21, 2006. Retrieved on 2006-10-20. (in English)