From 《后汉书·明德马皇后纪》: Empress Ma observes relatives arriving to greet her and writes 车如流水，马如游龙 — 'carriages like a flowing river, horses like a roaming dragon.' The four characters are the condensed version of her line.
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Two halves: 车水 (carriages-water, a flowing stream of cars) + 马龙 (horses-dragon, a long undulating line). Modern Chinese cities fit the image perfectly, even without horses.
