This is why building new roads can make the network less efficient – and make traffic congestion worse

Congestion in action. Image: Flickr/Wendell, CC BY-ND.

It’s not been a good year so far for major transport projects in Australia’s capital cities. 

The scale of disagreements over the merits of Melbourne’s proposed East West Link was such that the Victorian Government recently paid out $339m simply for the project not to be built. In Queensland, Annastacia Palaszczuk became the fourth successive Premier to completely throw out her predecessor’s signature infrastructure project – in this case the Bus and Train (BaT) Tunnel.

And while Sydney’s WestConnex project is still going ahead, recent reports concluded that it will either substantially increase traffic on the much-maligned Parramatta Road, or maybe decrease it. It all depends on which report you believe.

I am not going to debate the relative merits of these schemes: my background is in applied mathematics and not in transport planning. As such, my interest is less in the conclusions of some of these predicted usage studies and more in the consequences of the assumptions made in the modelling.

How can one report filed with the NSW Major Project register predict 20,000 fewer cars per day on a section of Parramatta Road, while other reports within the Roads and Maritime Services state that no significant reductions are likely to be seen?

It is very easy to chalk up some of these differences either to wildly overoptimistic developers potentially misleading themselves and others to get a project approved. Similarly, it is sometimes alleged that feasibility studies might be influenced by political biases or pre-established views on the merits of roads or public transport schemes.

While these factors may well influence some decision making, one thing that is often missed in the reporting of such studies is the sheer complexity associated with analysing such networks. Assuming all roads are connected, the behaviour of a whole network can be hyper-sensitive to how individual parts, even seemingly minor ones, function.

A poor estimate of traffic flow in one section of a network can lead to hugely different behaviour across the whole system. Furthermore, even the simplest networks can have the potential to function in some extremely surprising and often counter-intuitive ways.

It is very easy to believe that, if a traveller is offered the choice of two routes for a journey, the addition of a third choice should not worsen his/her travel time. If the new route is slower, the traveller could simply ignore the new route and make the same choice as before.

But as the German mathematician Dietrich Braess pointed out, this is not always the case. Increasing the capacity of a network can, perhaps surprisingly, decrease the efficiency of journeys around it even without increasing the number of trips made, as was pointed out in a recent article.

The Braess’s Paradox

To take a closer look at the reasoning behind this paradox, consider the case illustrated below. There are two major cities, labelled as the Start and End locations for a journey.

Travellers between the two cities have two choices of route, either via Town A or via Town B. The roads from Start to Town A and from Town B to End are both highways, which can handle any number of cars and allow them to make each leg of the journey in 105 minutes.

The roads from Start to Town B and from Town A to End are smaller roads, which are slower when busy. When there are N cars on the road, each leg of the journey takes N minutes. There is an old road linking Town A to Town B with a journey time of 100 minutes. This road is sufficiently slow that no traveller from Start to Finish would choose a route that involves it.

Network illustrating Braess’s Paradox. Travel times along routes are listed in minutes.

If we assume that 100 cars are travelling at the same time from Start to End, then there is no advantage to going via Town A vs going via Town B. The traffic will split approximately 50/50 between the two routes and each car will do the journey in 155 minutes. This is the fastest route. In reality, the split of cars might not be exactly 50/50, but unless the ratio is heavily imbalanced, the average travel time across the network will be 155 minutes.

A driver cannot help the overall network without suffering for it in the form of a slower journey

Now suppose now that the network is “improved” by upgrading the road between Town A and Town B. Rather than taking 100 minutes to travel between the towns, it now takes only 2 minutes.

The fastest route now is for all drivers to go from Start to Town B in 100 minutes, take the 2 minute trip to Town A, then travel from Town A to End in another 100 minutes. This journey now takes 202 minutes – but that’s 47 minutes longer than on the old road layout.

There is no incentive for any driver to choose an alternative route. Opting for either of the 105 minute roads will only lengthen their trip. A driver can improve travel times for all others by selflessly choosing the slowest roads, but cannot help the overall network without suffering for it in the form of a slower journey. This, of course, is not an option which many will choose.

While the old road between Town A and Town B was hugely inefficient, this inefficiency actually ensured that the network as a whole remained reasonably efficient. It served to distribute traffic evenly between the two routes from Start to End. But by improving this relatively unimportant road, it simply redistributes traffic more unevenly and worsens the overall system.

Even more counter-intuitively, Braess’s Paradox is observed in simple physical systems as well as transport networks.

The video (above) illustrates a system whereby a weight is suspended on two springs connected both in series (by an initially tense string) and in parallel (by initially slack strings). Removing the string which is in tension actually leads to the weight being lifted upwards.


This is actually a reverse of our traffic example. The distance the weight hangs represents the longer journey time of the traffic – remove the central string (the new road) and the hanging distance is reduced as is the journey time for the traffic.

Paradox in action

This paradox is not simply a mathematical quirk, or one which can be neglected by network analysts. There are a number of examples where removing roads – rather than building news ones – has improved transport networks.

Probably the most famous example of this comes from South Korea. When the motorway network around Seoul was reworked to remove some of the 1960s-built roadways, the the result was significantly reduced transit times throughout the city. This was not because of fewer journeys through the city, rather a more efficient distribution of cars across the remaining network.

Similar phenomena have been observed during road closures in New York City in the United States and in Stuttgart in Germany.

As Braess’s Paradox points out, even a slight change to a relatively unimportant part of the whole network can lead to massive changes in travel times. While planning reports might focus on headline stories – new road X will cut travel times by Y minutes – the underlying modelling must be more robust and look at the uncertainty around such estimates.

As painstaking as this modelling may be, it is unquestionably something that needs to be answered as fully and as correctly as possible, admitting its own limitations.

A multi-billion dollar infrastructure project cannot afford to fail simply because someone didn’t do their sums correctly. The financial consequences of incorrect projections can be financially catastrophic.

Both Sydney’s Cross City Tunnel and Lane Cove Tunnel drove their initial operators into receivership. The developers of Brisbane’s Clem Jones Tunnel fared no better.

The issue is not just limited to Australia, of course. Of the 15.9m journeys expected to be taken between London and Paris during the Channel Tunnel’s first year of operation, a mere 18 per cent of those actually occurred.The Conversation

Stephen Woodcock is lecturer in mathematics at University of Technology, Sydney.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

More than 830 cities have brought essential services back under public control. Others should follow

A power station near Nottingham: not one owned by Robin Hood Energy, alas, but we couldn't find anything better. Image: Getty.

The wave of cities worldwide rejecting privatization is far bigger and more successful than anyone thought, according to a new report from the Transnational Institute, Reclaiming Public Services: How cities and citizens are turning back privatisation. Some 835 cities in 45 countries have brought essential services like water, energy and health care back under public control.

The persistent myth that public services are by nature more expensive and less efficient is losing its momentum. Citizens and users do not necessarily have to resign to paying increasingly higher tariffs for lower standard services. The decline of working conditions in public services is not an inevitability.

And the ever larger role private companies have played in public service delivery may at last be waning. The remunicipalisation movement – cities or local authorities reclaiming privatised services or developing new options – demonstrates that cities and citizens are working to protect and reinvent essential services.

The failure of austerity and privatisation to deliver promised improvements and investments is part of the reason this movement has advanced. But the real driver has been a desire to meet goals such as addressing climate change or increasing democratic participation in service provision. Lower costs and tariffs, improved conditions for workers and better service quality are frequently reported following remunicipalisation.  Meanwhile transparency and accountability have also improved.

Where remunicipalisation succeeds, it also tends to inspire other local authorities to make similar moves. Examples are plentiful. Municipalities have joined forces to push for renewable, climate-friendly energy initiatives in countries like Germany. Public water operators in France and Catalonia are sharing resources and expertise, and working together to overcome the challenges they meet.

Outside Europe, experiments in public services are gaining ground too. Delhi set up 1,000 Mohalla (community) clinics across the city in 2015 as a first step to delivering affordable primary health care. Some 110 clinics were working in some of the poorest areas of Delhi as of February 2017. The Delhi government claims that more than 2.6m of its poorest residents have received free quality health care since the clinics were set up.


Local authorities and the public are benefiting from savings too. When the Nottingham City Council found out that many low-income families in the city were struggling to pay their energy bills, they set up a new supply company. The company, Robin Hood Energy, which offers the lowest prices in the UK, has the motto: “No private shareholders. No director bonuses. Just clear transparent pricing.”

Robin Hood Energy has also formed partnerships with other major cities. In 2016, the city of Leeds set up the White Rose Energy municipal company to promote simple no-profit tariffs throughout the Yorkshire and Humberside regions. In 2017, the cities of Bradford and Doncaster agreed to join the White Rose/Robin Hood partnership.

Meanwhile, campaigners with Switched on London are pushing their city to set up a not-for-profit energy company with genuine citizen participation. The motivations in these diverse cities are similar: young municipal companies can simultaneously beat energy poverty and play a key role in achieving a just and renewable energy transition.

Remunicipalised public services often involve new forms of participation for workers and citizens. Remunicipalisation is often a first step towards creating the public services of the future: sustainable and grounded in the local economy. Inspiration can be found in the European towns and villages aiming for 'zero waste' with their remunicipalised waste service, or providing 100 per cent locally-sourced organic food in their remunicipalised school restaurants.

Public services are not good simply because they are not private. Public services must also continuously renew themselves, grow, innovate and recommit to the public they serve.

The push for remunicipalisation in Catalonia, for example, has come from a movement of citizen platforms. For them, a return to public management is not just an end in itself, but a first step towards the democratic management of public services based on ongoing civil participation.

Evidence is building that people are able to reclaim public services and usher in a new generation of public ownership. The momentum is building, as diverse movements and actors join forces to bring positive change in communities around the world.

You can read the Transnational Institute report, “Reclaiming Public Services: How cities and citizens are turning back privatisation”, on its website.