Plants and animals grow by rules. Now a new study finds that the roads in a city might do the same, and the findings could have implications for city planners of the future.

Over 179 years, the streets in an urban area outside of Milan, Italy, grew by two simple processes, according to research published March 1 in the journal Scientific Reports. "We are not there yet," lead researcher Marc Barthelemy said, "but this opens the path to simple modeling."

Having a model—a mathematical prediction for how cities grow—would help urban planners shape emerging cities and reduce urban sprawl, Barthelemy told InnovationNewsDaily.

Barthelemy, a statistician and physicist at the French Alternative Energies and Atomic Energy Commission in Paris, analyzed seven sets of maps of Groane, Italy, dating back to 1833, when Groane was actually several separate farming towns. The towns grew over time, shedding their agricultural roots in favor of textile and mechanical industries after World War II. Then, in the 1990s, the factories declined, the towns got highways and high-speed trains, and they merged into one continuous urban area.

In spite of this complex history, Barthelemy found Groane's roads grew by the same, quantifiable processes, no matter the time period. "This process is actually very simple. This was a big surprise," he said. Groane's growth suggested urbanization can be quantified, which will be important for city planners as the world's population moves into cities.

Barthelemy's team also noticed some other patterns. Even though Groane has never undergone any large-scale planning efforts, its roads formed regular, rectangular blocks as it urbanized. And the most important streets of 1833 and 1914 remained the most important streets to this day. Ninety percent of the 100 most central roads in Groane already existed in 1833. "Very important roads where most of the traffic will go appear to be extremely stable," Barthelemy said.

That means that in any town, urban planners can think about the important streets when deciding where to build new infrastructure. Housing, for example, should be convenient to important streets but not too close to them, as they will become noisy and congested as the city grows, he said.

Though there doesn't yet exist a program to predict how cities will grow, at least planners know they can depend on the roads, which other studies have found last far longer than buildings or land uses. "If you plan over the long term, you have to think in respect to these roads," Barthelemy said.

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