On September 3, 1967, Sweden switched from driving on the left to driving on the right at five in the morning. The day is called Dagen H, from the Swedish word for the right-hand traffic reorganisation, and it remains the largest single-day traffic changeover any country has attempted.
Sweden had stayed on the left while its neighbours went right, and the mismatch caused accidents along the Norwegian and Finnish borders. Cross-border traffic kept growing, and most Swedish cars already had left-hand drive, which put the driver on the wrong side of the road. A 1963 referendum went against the change, but parliament passed it anyway and set the date four years out.
Planning took four years. All non-essential traffic was banned from one in the morning until six, and in those hours crews moved roughly 360,000 road signs, rebuilt bus stops and repainted lane markings, while Stockholm and Malmö pulled their trams off the network and ran buses instead. One-way streets were reworked. Drivers were told to stop, move over, stop again, and continue.
The result surprised the safety authorities. Monday came with noticeably fewer reported accidents than a normal Monday, and insurance claims fell, probably because people were being careful and driving slowly. Numbers soon returned to normal. Sweden kept its network of roundabouts, and most were rebuilt rather than removed.