Transport networks are constantly challenged by the occurrence of hazards. In Ireland, in recent years, the transportation network has experienced important disruptions, caused by several storms, and these episodes have shown the vulnerabilities of the network and the serious impact that they can have on the users and the economy, write UCD's Beatriz Martinez-Pastor and Vikram Pakrashi.

In addition to the severity of the weather event, uncertainty plays an important role when improving the preparedness of the network. Transport networks are highly interconnected, and the consequences of local failure can rapidly expand to other parts of the same network, to other transportation modes that are dependent on the damaged one, and to other systems that a priori may not be directly connected.

For example, during Storm Emma in 2018, transport networks were severely disrupted with direct consequences such as the immobilisation of public transport, numerous flight cancellations, and the closure of roads, which meant that some remote locations were cut off for several days.

In addition, other systems experienced important disruptions, such as power outages to more than 100,000 homes, 18,000 people without water across the country; and food shortages, as problems were experienced1 getting supplies to supermarkets.

Constant transformation

The constant transformation of our networks, which are progressively more digitally connected, can also add both positive and negative changes to vulnerability.

Transport networks are intrinsically related to computer systems – which help to improve the network behaviour – but which are also becoming a critical part of the transport system.

Thus, in addition to physical threats such as weather events, cyber-attacks can also shut down a transport system. Unlike weather events that can be predicted to some extent, cyber attacks can be started at any location and are almost unpredictable and therefore tight security is typical of transport related computer systems.

Within the context discussed, it is therefore becoming more important than ever to transform our transport network to increase its resilience.

What is resilience?

The concept of resilience has been used in many areas of knowledge, from psychology to ecology. However, only in the last few years has it been widely used in considering transport networks. Resilience in the context of transport networks can be described as the ability to plan and prepare for, absorb, recover and adapt to adverse events.2

From the definition of resilience three clear stages can be identified (see Figure 1):

  1. Pre-disruption and post-disruption: these phases take place before the hazard starts and after the recovery process has finished. During these phases, adaptation strategies are encouraged to minimise the impact of future hazards.  
  2. Disruption–perturbation: this phase takes place while the hazard is affecting the network, and the performance of the network is impacted. During this phase, the performance of the network is constantly evolving and adapting to the changing situation, trying to adapt its behaviour to the new conditions that can easily change. 
  3. Recovery-mitigation: this phase takes place once the hazard has finished, but the network is still not operating under normal conditions. During this phase, mitigation strategies should be implemented to speed up the recovery process.

Figure 1: Resilience definition. (B Martinez-Pastor (2018) Resilience of Traffic Networks to Extreme Weather Events: Analysis and Assessment.)

On top of the different phases, it is important to highlight that when we talk about resilience, we need to do it in a multidimensional approach, since all the components, users and managers of the network will play an essential role, and each of them will form a different layer in the analysis of resilience.

How to enhance the resilience of a network?

Transport networks are constantly exposed to multiple hazards, and although some of them can be predicted to some extent, it is not possible to avoid them all. For that reason, when working in risk reduction, it is important to know that the risk cannot be eliminated completely.

When transport networks are threatened by a hazard, the main goal will be to achieve the maximum reduction of the risk with the available resources.

In order to achieve that, we need to become better at planning, improving our capacity to develop new strategies for each of the resilience stages and each of the components of the network.

Thus, resilient networks will set the stage for well-designed adaptation and mitigation strategies to manage risk effectively.

It is important to understand that different parts of the network will need different strategies, and combining the knowledge of each of these parts will allow us to define at what level to invest in adaptation, and also the amount of risk that is acceptable for each of the elements.

A robust resiliency strategy will provide us with tools to have a better understanding of the network, to reduce the impact and to accelerate the recovery process, which results in increased protection for users, and an efficient approach to reduce the costs of disruption.

In order to enhance the resilience of the network two essential strategies are described below, spare capacity and vulnerability identification.

Spare capacity 

A significant number of Irish roads must deal with high volumes of traffic, and it is expected that these numbers will grow even more with the expected rise in demand.

For example, in Dublin, the M50 continues to experience growth in levels of usage as measured by Annual Average Daily Traffic flows, with the section between Junction 5 (N2) and Junction 9 (N7) carrying in excess of 140,000 AADT.3

In addition, some sections of Irish national roads managed by Transport Infrastructure Ireland are also intensively used in a daily basis – in the case of national primary roads about 15% operate at capacities higher than 100%, and for the national secondary roads this number grows to about 30%.3 

The consequences of roads operating close to or above capacity can be seen in the performance of the network, particularly when they are impacted by a hazard.

When a road is totally or partially unavailable because of an incident or extreme weather, a resilient network will show the ability to keep reasonable availability or provide alternative travel patterns.

For example, when weather events, such as floods, intense rainfall, or snowfalls happen, a common consequence is to restrict the amount of traffic. This can happen because part of the road has been blocked by water or snow.

In this situation, those roads that are already operating closer to capacity will experience a higher impact than those that can make use of the spare capacity during these situations. In addition, a zero or reduced network capacity will make the impact stronger and the recovery process slower.

Therefore, it is essential to pay special attention to those parts of the system that are operating at higher capacities, because this ratio will play a key role in the adaptation and mitigation strategies, reducing risky situations for users and improving the efficiency of the network as a whole.

Identification of vulnerabilities

Transport networks are formed from many different elements and each of these elements will create a different response when affected by a hazard or other threat.

For example, sometimes due to the lack of alternative routes, a single road can be the only way of accessing a part of the country. Therefore, when this unique route is affected by a hazard, the users trying to access that area will suffer from a reduction in their traffic performance.

This can contribute to a significant increase in their travel times since alternative options may not exist or may require significant reroutings.

Other parts of the network, even if they have several options for rerouting, may need to deal with very high volumes of traffic and, therefore, when affected, the reorganisation of all the users will result in important disruption  to the affected road and, perhaps, to a significant part of the network also.

Prioritisation of investment

An improved understanding of vulnerability and resilience will provide an effective means for prioritisation of investment to achieve a resilient network. Prioritisation is key when implementing new strategies to shape more resilient networks, since resources are inevitably limited. 

In Ireland, Transport Infrastructure Ireland (TII) manages in excess of 5,300km of the national roads network of which about 43% are classified as national secondary roads and the challenge of improving the performance of transport networks within a limited budget is constantly present.

Traditionally, the limited maintenance budgets tend to be assigned to roads based on engineering and traffic volume related indicators. However, resilience should be an additional indicator when prioritising the sections of the network that will be upgraded.

Investing in those parts of the network that are more vulnerable will have positive impacts on the daily performance, but also when the network is threatened by a hazard. The benefits will be visible not only within the specific vulnerable section but throughout the whole network.

Therefore, having a clear understanding of the vulnerable locations will permit us to anticipate our maintenance needs, to design roads reflective of the role they perform and to ensure we invest most efficiently and effectively. 

During the past few years, some steps have been taken to introduce and understand the concept of resilience in the transport area. However, there is a long way ahead to ensure that transport networks become resilient.

Effectively, models to evaluate transport resilience are still relatively new and under discussion in the scientific community. At the same time, hazards are increasingly damaging transport networks worldwide, having important impacts on human life.

Consequently, steps should be taken to include the concept of resilience as an indicator when transport projects are developed, ranging from the creation of new policies to the construction of new parts of the network.

In this context, several research projects have appeared in the past few years in an attempt to better understand transportation networks’ resilience to extreme events.

Project SIRMA

One example is Project SIRMA, Strengthening Infrastructure Risk Management in the Atlantic Area, which started in April 2019. This project aims at developing a robust framework for the transportation infrastructure’s management and the mitigation of risks associated with extreme natural events in the Atlantic region.

This region, due to the proximity to the ocean, is affected by strong corrosion processes as well as other threats such as extreme waves in coastal regions.

The strategies mentioned above will be applied in this specific context to achieve the project’s objectives. SIRMA partners, which includes UCD and Irish Rail, will do that in three main tasks:

  1. Studying climate change and assessing its impact on the evolution of climate parameters that could allow us to anticipate natural hazards, such as the already mentioned extreme waves. On the other hand, some climate parameters have a direct influence on the degradation of existing transportation structures. It is thus important to understand how much faster that can occur in a progressively more aggressive environment and the extent to which transportation structures become more vulnerable.
  2. In parallel with the previous task, SIRMA partners will try to develop tools and methodologies to better monitor the performance of transportation structures. If it is clear that their behaviour changes in time, and that change is happening faster than before, it is important to provide the tools to assess that change in order to anticipate possible failures of transportation structures.
  3. The next step consists on developing risk-based models for transportation structures. These will consider the probability of natural extreme event occurrence (studied in task 1), the vulnerability of the infrastructures (studied in task 2) and the associated risks or costs. By combining these three aspects, it will be possible to understand how vulnerable existing transportation structures are and prioritise them in terms of needs of intervention, considering the associated failure consequences.

The SIRMA project will finish by validating and implementing the developed framework in terrestrial transportation networks, which consider both road and railway infrastructure.

That will be done in two different case studies to be developed in Ireland and in Portugal, two of the countries represented in the SIRMA consortium. Together with Spain, France, and United Kingdom, these constitute the European Atlantic region countries that SIRMA’s partners come from.

SIRMA seeks to make a significant contribution to the strengthening of the resilience of these European coastal regions. Transport networks will continue to be hit by weather events, cyber attacks, man-made events, among others; thus more research projects and studies will need to be undertaken to better prepare our countries.

Furthermore, in addition to those threats that we already know about, transport networks will also face new threats and challenges. As a case in point, the current situation with COVID-19 is creating a new paradigm for transport, where public transport as we know it will need to quickly transform and adapt to the new requisites and regulations under the social distancing measures so as to avoid a huge increase in personal car use with associated fossil fuel consumption and emissions.

Therefore, all the steps we take now to have a better understanding of the behaviour of our networks will provide us with a stronger capacity to deal with future and unforeseen events, becoming more resilient and change ready.

For further information on an online seminar, 'Bridge Durability and Network Resilience', please click here.

Authors: Beatriz Martinez-Pastor and Vikram Pakrashi, School of Civil Engineering, University College Dublin.


1) Storm Emma 2019 report. Climatology and Observations Division of Met Éireann.[1]  

2) The National Academy of Sciences (NAS).

3) Transport Infrastructure Ireland. National Roads Network Indicators 2018.

Further reading

  • Martinez-Pastor, B, 2018. Resilience of Traffic Networks to Extreme Weather Events: Analysis and Assessment (Doctoral dissertation, Trinity College Dublin).
  • Nogal, M, O’Connor, A, Martinez-Pastor, B and Caulfield, B, 2017. Novel probabilistic resilience assessment framework of transportation networks against extreme weather events. ASCE-ASME Journal of Risk and Uncertainty in Engineering Systems, Part A: Civil Engineering, 3(3), p.04017004.
  • Martinez-Pastor, B, Nogal, M, O'Connor, A and Caulfield, B, 2016. A sensitivity analysis of a dynamic restricted equilibrium model to evaluate the traffic network resilience (No. 16-3456).
  • Nogal, M, O'Connor, A, Caulfield, B and Martinez-Pastor, B, 2016. Resilience of traffic networks: From perturbation to recovery via a dynamic restricted equilibrium model. Reliability Engineering & System Safety, 156, pp.84-96.
  • Martinez-Pastor, B, Nogal, M, O’Connor, A and Caulfield, B, 2015. Evaluation of resilience in traffic networks: models and characteristics. Proceedings of the Irish transportation research network, ITRN.
  • Nogal, M, Martinez-Pastor, B, O’Connor, A and Caulfield, B, 2015. Dynamic restricted equilibrium model to determine statistically the resilience of a traffic network to extreme weather events. In Proceedings of the 12th International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12, Vancouver, Canada.