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The main influence on the simulated network as it grows is the need to efficiently connect new areas to the existing road network – a process they call "local optimisation". They say the road patterns in cities evolve thanks to similar local efforts, as people try to connect houses, businesses and other infrastructures to existing roads.

Evolution has ensured that local efficiency also drives the growth of transport networks in biology – for example, in plant leaf veins and circulatory systems.

"Cities are not just the result of rational planning – in the same way that living organisms are not simply what is in their genetic code," Barthélemy told New Scientist.

The study's results might be important for understanding urban growth and "sprawl" says Barthélemy. More than half the world's population lives in cities, a proportion that continues to increase.

"The approach could even help city planners to better predict how some street networks will evolve and to plan accordingly," he adds.

Using the local efficiency of connections to drive road network growth looks to be a truer fit with reality than using the total cost of travelling across the network, says Onnela. "Especially given that the time scale of city growth (possibly thousands of years) and the time scale of urban planning (perhaps tens of years) are so clearly different."