What was the basis for judging Shangtang Street and Xiatang Street in ancient times?

Houses with white walls and tiles built along the river are divided into two main streets, Shangtang Street and Xiatang Street, which are the main houses and shops respectively. Compared with other ancient towns, the streets and rivers in luzhi town are narrower. Generally, the street is between 2.5 and 5 meters, and the narrowest alley can accommodate one person. Most of the streets are paved with stone slabs or pebbles, and the Jiangnan style is full.