How the road network determines traffic capacity

ETH researchers have shown that we can use the structure of urban road networks to predict their traffic capacity. This information enables urban and transportation planners to quantify how changes will influence traffic volumes.

People who commute by car will have an idea of what “traffic capacity” means, drawn from their own experience: as a stream of cars heads into a city early in the morning, the flow of traffic initially increases—until a critical point is reached in terms of the number of vehicles on the roads. From this point on, every additional car reduces the flow of traffic, and vehicles come to a standstill or form a traffic jam. This critical point represents a city’s traffic capacity, or its possible maximum traffic volume.

The number of vehicles a road network can support differs from city to city, and the factors that influence these differing traffic capacities were previously unquantified. ETH researchers led by Kay Axhausen, Professor of Traffic and Transport Planning, and Professor Monica Menendez (now at New York University Abu Dhabi) have now used billions of traffic measurements to reveal a set of rules that enable us to easily estimate the critical number of vehicles, and by extension, the traffic capacity of a city’s road network.

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