On September 3, 1967, Sweden executed Dagen H (Högertrafikomläggningen), switching all vehicular traffic from driving on the left side of the road to the right. While the immediate transition occurred with surprising order and few accidents, the lingering logistical nightmare extended into 1968 through traffic signal desynchronization. Municipal engineers in Stockholm, Gothenburg, and Malmö faced an unprecedented challenge: thousands of traffic lights, pedestrian signals, and sensor loops buried under asphalt had been calibrated over decades for left-hand flow dynamics.
When the directional flow flipped, the physical sequence of signal cycles produced massive gridlock during peak hours throughout early 1968. Mechanical timers and relay boxes had to be manually rewired and physically repositioned across thousands of intersections. Intersections that previously allowed smooth left turns across oncoming traffic were suddenly causing head-on gridlocks because green-wave timing protocols were completely inverted.
Drivers who adapted quickly to sitting on the right side of the road found themselves trapped in recursive traffic loops caused by misaligned magnetic sensors that failed to detect vehicles approaching from the new lane alignments. City workers labored for months in sub-zero Nordic temperatures to manually rewrite timing patterns and replace hardware, turning a seemingly smooth overnight transition into a year-long urban engineering saga.