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City road networks like flowers

Although summer in MN is very short, it is often very nice. The flowers in the yard in front of my apartment are flourishing now. When I was enjoying their beauty and fragrance, I also realized that the texture of some flowers is similar to some road networks. To illustrate, here is the picture of a flower I took this afternoon:


And here is the road map of the Big Boston Region:

road map of boston.jpg
(Source: mapquest.com)

This road network exhibits itself in a hub-and-spoke shape, with some minors roads attached to the major roads. A study about the growth of city road networks and their similarities to biological systems can be found here. The researchers from France have examined this kind of road network growth from the perspective of probability models; yet no economic mechanism was included explicitly.

It should be noted that not all road networks look like that. Some networks are very grid-like, such as the road network in the Twin Cities, MN:

(Source: http://www.aaccessmaps.com/show/map/us/mn/minneapolis_metro)