Some roads are fast, some are slow. Fast roads attract traffic, slow roads deter it. People prefer to travel on fast roads. People prefer to live on slow roads. Shops prefer roads which have lots of traffic, trying to induce passers-by to stop.
Differentiating roads by speed (and thus flow) is a core design principle in transportation planning. The belief is that this not only better satisfies preferences, it is cost-effective. There are economies of scale associated with building a few fast roads and more slow roads, rather than all roads to equal design. From an investment perspective, it allows concentrating resources. From a traffic perspective, it isolates a few roads for fewer access points than others. From a highway design perspective, it sets aside a few roads to be straighter and flatter and faster.
The plan of Manhattan, created by surveyors, was a grid. But the grid was differentiated. It set aside Avenues which ran North-South and designed to be 100 ft. wide (30.5 m). Since Manhattan is longer than it is wide, there were many more East-West streets. The standard width was to be 60 ft. (18.3 m) wide (and the blocks were 200 ft. (61 m) long). But selected streets (4th, 23rd, 34th, 42nd, 57th, 72nd, 79th, 86th, 96th, 106th, 116th, 125th, 135th, 145th and 155th Streets) were wider, set at the same 100 ft. width as the avenues. Some of these (155th, 145th, 125th, 42nd eventually became the roads that some of bridges and tunnels to the rest of New York would land, though this is not a perfect match). They would also tend to become the sites of stations on the subway system a century later.
But, it turns out, we would have a hierarchy of roads in the absence of a central network planner. All it takes is for there to be a feedback mechanism between demand and supply (so people pay based on their use of the road), between supply and demand (so people choose faster roads), and a rule about reinvesting excess revenues on the road they were earned to improve it, and roads that don’t earn enough money to decay (slow down).
Even without payment, people prefer trails others have walked on before, clearing the brush and compacting the soil. That attraction is a positive feedback system that locks-in existing roads.
Today the hierarchy has more than two levels of Manhattan, From Express HOV and toll lanes at the top, to freeways, limited access highways, principal arterials, minor arterials, collector and distributor streets, to local roads. Not every community has every layer, and standards vary between places. The hierarchy is not a strict tree.
Appleyard, Donald and Lintell, Mark (1972) “The environmental quality of city streets: the residents’ viewpoint.” Journal of the American Institute of Planners 38(2) 84-101.
Ballon, Hillary, ed. (2011) The Greatest Grid. Columbia University Press
Yerra, Bhanu and David Levinson (2005) The Emergence of Hierarchy in Transportation Networks. Annals of Regional Science. 39(3) pp. 541-553.