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The Fundamental Law of Road Congestion

Traffic jam
Traffic jam

I have to admit I’m late this party.

I’ve been aware that you can’t road-build your way out of congestion for some time, and I looked into quite a few sources when arguing against the so-called Harrogate Relief Road.

My favourite quote in the materials I looked at then was from Professor Bert van Wee of the Technische Universiteit Delft.

‘Building more roads is in any case senseless. It is often the first reaction from ministers who want to look as though they are doing something. Of course you do sometimes have to make roads wider, and re-work junctions, but if you think you’re going to solve the problem of congestion like that, you’re wrong.’

professor bert van wee, tu delft

Professor van Wee referred to American research that showed that building roads made driving more attractive and induced extra traffic. He may have been referring to 2011 research by Duranton and Turner on the Fundamental Law of Road Congestion.

Abstract of Duranton & Turner research
Abstract of Duranton & Turner research

The Fundamental Law of Road Congestion was first proposed by Downs in 1962. He stated that on urban commuter expressways, peak-hour traffic congestion rises to meet maximum capacity.

Headlines from the Duranton & Turner Research

The headlines from the Duranton & Turner research are:

  • for interstate highways, Vehicle Kilometres Travelled (VKT) increase in proportion to roadway lane kilometres, so widening roads does not relieve congestion
  • increasing lane kilometres for one type of road diverts little traffic from other types of road, so building a bypass or ring road does not make local roads significantly quieter
  • the provision of public transport has little effect on Vehicle Kilometres Travelled

Full Text of the Duranton & Turner Research

The full text is 57 pages.

It confirms that for interstate highways, VKT increases 1:1 with increases in interstate highway lane kilometres.

‘People drive more when the stock of roads in their city increases; commercial driving and trucking increase with a city’s stock of roads; and people migrate to cities that are relatively well provided with roads. Surprisingly, our data also suggest that a new lane kilometer of roadway diverts little traffic from other roads.’

duranton & turner

This is what they have to say about public transport.

‘The fundamental law of road congestion requires that new road capacity be met with a proportional increase in driving. A corollary is that if we were somehow to remove a subset of a city’s drivers from a city’s roads, then others would take their place. We can think of public transit in this way. Public transit serves to free up road capacity by taking drivers off the roads and putting them in buses or trains. Thus, the fundamental law implies that the provision of public transit should not affect the overall level of VKT in a city.’

duranton & turner on traffic and transit

The percentages of the total that each source of extra VKT adds are:.

  • Trucking – 19-29%
  • Changes in individual behaviour – 9-39%
  • Diversion of traffic – 10%
  • Migration – 5-21%

A 10% increase in interstate lane kilometres diverts just 0.52% of traffic from major urban roads.

For major roads in urban areas other than interstate highways, the ‘elasticity’ of VKT is not 1.0 (i.e. traffic increases in direct proportion to lane kilometres added) but 0.67 to 0.89.

Conclusion

‘These findings suggest that both road capacity expansions and extensions to public transit are not appropriate policies with which to combat traffic congestion. This leaves congestion pricing as the main candidate tool to curb traffic congestion.’

duranton & turner conclusion
The Fundamental Law of Road Congestion

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