BIKE: New roads can cause congestion

[Patrick Goetz]

What we all suspected anyway, now with some mathematical teeth.


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 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.