Imagine waking up to find that every traffic law regarding road placement has been inverted overnight. On Sunday, September 3, 1967—known as Dagen H or H-Day—Sweden did exactly that. Despite surrounding European nations driving on the right, Sweden had spent decades driving on the left, even though 90 percent of Swedish vehicles were left-hand drive cars configured for right-side roads.
The mismatch was a recipe for catastrophic head-on collisions during passing maneuvers. Parliament finally mandated the swap, initiating a massive four-year logistical overhaul. Millions of new road signs were installed and covered in black plastic, bus stops were moved, and hundreds of intersections were physically re-engineered.
The most delicate operation involved traffic signals. Municipalities had to recalibrate, rewire, and physically re-angle thousands of traffic lights across major cities like Stockholm to prevent complete desynchronization at 5:00 AM. At 4:50 AM on Dagen H, all non-essential traffic was ordered to come to a complete halt for ten minutes.
Crews frantically uncovered new signs and adjusted signal timers while drivers carefully steered from the left lane to the right side of the road. At precisely 5:00 AM, loudspeakers announced that right-hand traffic was officially active. Experts predicted bloodbaths and gridlock due to signal confusion and habit, but the opposite occurred.
Terrified of crashing, Swedish motorists drove with extreme caution. Traffic accidents plummeted to record lows on the day of the switch, making Dagen H an unprecedented triumph of civil engineering.