OC
02-01-2005, 09:12 AM
http://www.newscientist.com/article.ns?id=dn6922
New roads can cause congestion
10:12 01 February 2005
Exclusive from New Scientist Print Edition
Kate Ravilious
Traffic should flow best in cities when only a limited number of roads lead to the centre. This counter-intuitive finding could allow planners to prevent gridlock by closing roads rather than building new ones.
It comes from a new way of thinking about complex networks developed by Neil Johnson, Douglas Ashton and Timothy Jarrett at the University of Oxford, UK. The researchers began by approximating a complex city network to just a ring road and a number of the arterial roads that cross at the centre.
They then worked out how the average time for journeys changes as the number of roads increases. When the model assumed that there was no congestion in the centre, average journey times got shorter as the number of roads increased.
But that changed when the researchers modified the model to delay any journey that passes through the centre. With a small number of roads, journeys initially became faster as extra roads were added to the network. But beyond a certain number, adding more roads increased average journey times rather than cutting them. The optimum number of roads depended on just how much extra delay there was to journeys passing through the centre.
Natural networks
The model mimics effects that have been observed in cities in which short cuts generate more traffic and so increase congestion, the researchers say. But this is the first time anybody has managed to model the effect (http://www.arxiv.org/abs/%20cond-mat/?0409059) mathematically.
The same process of analysing the costs associated with moving across a network could help solve a long-standing problem in biology: why some natural networks are centralised like cities, whereas others are decentralised like the internet.
"Organisms such as fungi have managed to evolve a complex network in which there are centralised and decentralised pathways to move nutrients around," Johnson says. "Now we can look at biological systems in terms of the 'costs' and 'benefits' of the connections rather than in terms of the physical structures themselves," he says.
New roads can cause congestion
10:12 01 February 2005
Exclusive from New Scientist Print Edition
Kate Ravilious
Traffic should flow best in cities when only a limited number of roads lead to the centre. This counter-intuitive finding could allow planners to prevent gridlock by closing roads rather than building new ones.
It comes from a new way of thinking about complex networks developed by Neil Johnson, Douglas Ashton and Timothy Jarrett at the University of Oxford, UK. The researchers began by approximating a complex city network to just a ring road and a number of the arterial roads that cross at the centre.
They then worked out how the average time for journeys changes as the number of roads increases. When the model assumed that there was no congestion in the centre, average journey times got shorter as the number of roads increased.
But that changed when the researchers modified the model to delay any journey that passes through the centre. With a small number of roads, journeys initially became faster as extra roads were added to the network. But beyond a certain number, adding more roads increased average journey times rather than cutting them. The optimum number of roads depended on just how much extra delay there was to journeys passing through the centre.
Natural networks
The model mimics effects that have been observed in cities in which short cuts generate more traffic and so increase congestion, the researchers say. But this is the first time anybody has managed to model the effect (http://www.arxiv.org/abs/%20cond-mat/?0409059) mathematically.
The same process of analysing the costs associated with moving across a network could help solve a long-standing problem in biology: why some natural networks are centralised like cities, whereas others are decentralised like the internet.
"Organisms such as fungi have managed to evolve a complex network in which there are centralised and decentralised pathways to move nutrients around," Johnson says. "Now we can look at biological systems in terms of the 'costs' and 'benefits' of the connections rather than in terms of the physical structures themselves," he says.