At September 3rd, 1967, all driving stopped while Sweden changed from left-side to right-side driving overnight.
The change was proposed several times before that, and was rejected by a wide margin at a 1955 referendum, but was decided in 1963 and given 4 years to execute the plan. In that time, massive public campaigns informed by a team of psychologists helped educate and prepare the public for the change, while building infrastructure and planning the change.
After the change, the amount of car accidents declined, reducing injuries by the small thousands and deaths by the hundreds (likely due to extra caution on the road though). This only lasted for about four years until it got back to "normal". The entire project cost the equivalent of about $250 million.
I'm not sure if this was worth the cost, and I couldn't easily find academic studies on it, but it was heartwarming to read about a successful coordinated change.
I'd like to thank Sawyer Bernath for telling me about this story, Aaron Gertler for encouraging people to write more factual posts, and Lizka Vaintrob for encouraging short posts.