Looking beyond the science of vulnerability assessments, this paper discusses some of the network attributes that influence the vulnerability of transport networks, influences that can be described as structure-related, nature-related or traffic-related attributes. The paper introduces vulnerability as a parameter for decision-support in cost-benefit analyses, by seeking to establish a link between the terms reliability and vulnerability vis-a-vis costs and benefits.
Following a new research strand
Taking up the invitation of Berdica (2002) to bring out and recognise the vulnerability in the road transport system as a meeting point for all the different strands of transport reliability research and other issues, the focal point of this paper is to look at a road network from a reliability and vulnerability perspective and to link this analysis to cost-benefit decisions.
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INTRODUCTION
Taking up the invitation in Berdica (2003) to bring out and recognise the vulnerability in the road transport system as a meeting point for all the different strands of transport reliability research and other issues (p.127), the focal point of this paper is to look at a road network from a reliability and vulnerability perspective and to link this analysis to cost-benefit decisions.
Looking beyond the science of vulnerability assessments, this paper discusses some of the network attributes that influence the vulnerability of transport networks, influences that can be described as structure-related, nature-related or traffic-related attributes. The paper introduces vulnerability as a parameter for decision-support in cost-benefit analyses, by seeking to establish a link between the terms reliability and vulnerability vis-à-vis costs and benefits.
Few will question that the sender, the recipient, the freight hauler, or society at large, experience additional costs when goods or persons cannot reach their destinations in time or space. Consequently, it should be obvious that a reliable transport network represents a benefit to society. Equally, a vulnerable network would represent a net cost to society (Husdal, 2004). Why then, is the reliability, or conversely, the vulnerability, of the transport network not a matter of evaluation in traditional cost-benefit analyses?
Transport networks like freeways and interstate highways are the main backbone of modern society. Consequently, the reliability, or conversely, the vulnerability of any transport network is thus a decisive factor not only in terms of market outreach and competition, but also in terms of continuity, to ensure a 24/7 operation of the community we live in. Any threat to the reliability of the transport network constitutes a vulnerable spot, a weakness. This is of particular concern when considering sparse, rural networks, because what by urban standards is a minor degradation (i.e. car accident, resulting in queuing, delays and diversions) may have severe consequences if occurring in a rural setting (i.e. blocking the only access road for hours). However, new transport projects are seldom explicitly evaluated in terms of increased reliability or lessened vulnerability. Nonetheless, vulnerability assessments should be acknowledged as an integral element within the cost-benefit analysis that is part of any transport planning process.
WHY MAKE VULNERABILITY AN ISSUE?
Disruptions in a road network are often the result of external influences such as landslides, avalanches or rock fall, flooding, lack of snow clearing during winter, accidents that require extensive clean-up, etc.; the list of scenarios is literally endless. Often seen in Norway, closures of sub sea tunnels and failures or severe degradation of ferry services are also among the many factors that will occur depending on the season and location. Ferry services may be of no concern in some parts of the country, whereas rock fall can be a major issue in other parts.
In the most common manner of speaking, the reliability of a transport network can be defined as the probability that one or more of its links functions, or in better said, the probability that one or more of its links does not fail to function. Vulnerability, on the other hand, represents the network’s or the links’ susceptibility to failure, where the term failure here expresses a considerable deviation from the normal functioning state of the link or network. A non-functioning, or at best, badly-functioning link will impose costs on the user in terms of loss of time, additional operation costs or other costs as a result of delays and diversions. Transporters of perishable goods will also experience a loss of value. These are socio-economic costs that should be taken into consideration in cost-benefit analyses of transport projects; yet, as a norm, reliability and vulnerability are not evaluated in today’s practice. This means that evaluating the benefits of developing new infrastructure and the costs of continuing to use the existing infrastructure may be lacking important criteria.
A perhaps myopic, but at the same time illustrative case from Norway may demonstrate why vulnerability should be an issue in the evaluation of transport networks: On the North-western coast of Norway (figure 1), there is a hospital in the cities of Molde and Kristiansund, 75 kilometres (47 miles) apart. In conjunction with a recent evaluation of whether one should maintain two separate hospitals versus establishing one hospital at a new location that would cover the service areas of both hospitals, a study was conducted to compare the travel times for employees, out-patients and in-patients for a set of locations. What is disturbing to this author, and which is the reason for his interest in the subject, is that the study exclusively used “ideal” travel time parameters, among which: Speed equal to speed limit, and a waiting time of maximum 15 minutes at the required ferry crossings. What from the point of this author is lacking is a consideration of a) seasonal variations of serviceability (snowy and icy roads versus clear roads), b) the possibility of ferry breakdowns due to engine failure, ferry delays or cancellations due to high winds and rough seas (a well-known event in coastal Norway), c) potential closures of the mountain passes that must be traversed in this example (a season-dependent and location-dependent issue), d) possible closure of a sub sea tunnel for certain travellers, e) closure of long bridge spans due to high winds, and f) avalanches and rock fall, to name but a few.
The aforementioned are all known incidents along the studied routes. However, these factors were not considered. Admittedly, incorporating these factors is difficult, and in a statistical sense perhaps of no significance. Furthermore, ideal parameters are, after all, easier to use for a ceteris paribus comparison of travel times. Nevertheless, should the vulnerability of the transport network not be part of the equation when locating critical public services such as hospitals?
Figure 1: Map relating to the vulnerability example in chapter 2. The service area of the hospitals in Molde and Kristiansund, on the northwestern coast of Norway, is marked by a number of potential vulnerabilities: F=Ferry, CW=Causeway, ST= Subsea Tunnel, T=Tunnel, M=Mountain Pass, B=Bridge
It is not the point here to advocate the use of worst-case scenarios; however, they should be part of the overall evaluation. In hindsight, it is usually better to have evaluated worst-case scenarios and discarded them if they do not have any influence, than to actually to have to face a worst-case scenario and to wonder why this had not been evaluated in the first place.
Introducing the reliability or vulnerability of the road network as a decision variable will bring new perspectives to cost-benefit analyses, and give greater weight to the question of whether travellers and goods indeed can traverse the network and thus arrive at their destination. From a freight hauler’s point of view, a vulnerable network is a network that is easily disrupted, resulting in unpredictable delays. This unpredictability is probably seen as a much larger problem than a congested and slow-moving network that is relatively reliable and stable. In the latter, there is at least some guarantee that the goods will arrive at their destination, and most important, the transport costs are calculable and lead times are predictable for the most part.
RELIABILITY VERSUS VULNERABILITY
The current literature acknowledges reliability and vulnerability as being two related, yet different concepts and both Berdica (2002) and Taylor and D’Este (2003a and 2003b) lament the lack of a formal definition of vulnerability.
Traditionally, when looking at the reliability of transport networks, it is done with a systems engineering approach. Reliability is here an expression of the probability that links within the network will function, reliability may thus be viewed as the degree of stability of the quality of service that a system offers. In Bell and Iida (1997), transport network reliability is focused on connectivity reliability (also named terminal reliability) and travel time reliability. Nicholson et al. (2002), in addition, list and discuss encountered reliability, capacity reliability and flow decrement reliability as ways of measuring reliability.
Whereas probability or predictability is a major concern in network reliability studies, the impacts or consequences of disruptions is the main focus of vulnerability studies. Taylor and D’Este (2003a) note that vulnerability and reliability are two related concepts, but emphasise that network vulnerability relates to network weaknesses and the economic and social consequences of network failure, not so much the probability of failure. Berdica (2002) relates vulnerability to serviceability, namely the possibility to use a link, route or road in a network at a given time. Vulnerability, then, is the inability to supply adequate serviceability and serviceability as such is determined by a set of performance measures. Taylor and D’Este (2003b) relate vulnerability to the degree of accessibility of a given node in the network, where accessibility is expressed as the travel cost needed to access the particular node, comparing optimal and alternative routes or detours. LLeras-Echeverri and Sanchez-Silva (2001) extend the terminal reliability as seen in Bell and Iida (1997), to include a progressive failure scenario approach to identify weak links.
What is apparent then, is that vulnerability, unlike reliability, needs to be more than a quantitative probability calculation related to the functioning or non-functioning of a network link. Reliability focuses on the possibility of maintaining the operability of a link or network; vulnerability focuses on the possibility of disrupting or degrading the operability of a given link or network. Reliability focuses on the possibility of maintaining a link; vulnerability focuses on the possibility of disrupting or degrading a link or network. The probability of a link being present or not does not translate into a probability of the same link being operable or not. To be of use in a vulnerability setting, the term operability needs to be approached differently for different types of roads, road conditions, goods or transports. Reliability describes the operability of the network under varying strenuous conditions (i.e. the ability to continue to function). Vulnerability describes the non-operability of the network under varying strenuous conditions (i.e. the susceptibility to fail).
A reliable network exhibits a high degree of operability as expressed by its serviceability, accessibility, and non-variability under any circumstance, due to the presence of redundancy, robustness, and resilience in the network.
A vulnerable network exhibits a low degree of operability as expressed by non-serviceability, non-accessibility, and variability under certain, due to the lack of redundancy, robustness, and resilience in the network.
Vulnerability = Non-Reliability (under said certain circumstances)
Transport networks are vulnerable to a wide range of possible scenarios, events and incidents, some probable, some improbable, some disastrous, some with only minor disturbances. If associating reliable with being operable, and associating vulnerable with being non-operable, similar to serviceability in Berdica (2002), then it is warranted to juxtapose reliability with vulnerability. Reliability, in this sense, means non-vulnerability or exhibiting a high degree of operability under any circumstances; vulnerability means non-reliability or exhibiting a low degree of operability under certain circumstances. This is how the terms vulnerability and reliability are used throughout this paper. Vulnerability can thus be defined as the consequential cost of the lack of reliability under these certain circumstances, and this consequential cost must comprise not only the immediate toll on the road-users, but the overall socio-economic costs on the community that this vulnerability would entail.
MEASURES OF VULNERABILITY
Transport networks are susceptible to a wide range of vulnerabilities that can lead to an operational degradation. These vulnerabilities are the result of certain attributes or qualities pertaining to the network itself (Srinivasan, 2002), and one categorization of attributes and influences, adapted from Bråthen and Lægran (2004), could be the following three: structure, nature and traffic.
Structure-related or structure-generated vulnerability pertains to the way the road is built, and attributes of the road network itself, not only in terms of topology, and connectivity, but also in terms of the physical body of the road, geometry, width, curvature, gradient, tunnels, bridges, weight restrictions, etc.
Nature-related or nature-generated vulnerability pertains to attributes of the natural environment, the topography and the terrain that the road traverses, and to nature-given incidents, such as flash floods, avalanches, rock fall, snow and ice, fog, earthquakes, tsunamis, consequences of climate changes, and so on.
Traffic-related or traffic-generated vulnerability pertains to attributes describing the generic flow of traffic and attributes resulting in flow decrements, such as daily rush hour and weekend highs, as well as maintenance operations, snow clearing, accident clear-up, and ongoing construction works.
Typically, these vulnerabilities will occur on a collective basis, rather than on a one-by-one basis. Even though a particular stretch of road may be susceptible to only one of the aforementioned vulnerabilities, the overall network will be exposed to the full set of all vulnerabilities, some acerbating the other. It is the collective sum of these vulnerabilities that needs to be addressed. Some links may exhibit structural deficiencies, some will be at the mercy of nature, and others are particularly vulnerable to traffic-generated incidents. A vulnerability assessment must consider each attribute separately, and, at the same time, as a whole.
Risk analysis and vulnerability analysis are related concepts. An often used definition of risk is risk being the product of consequence and probability. This equation can be extended to include vulnerability as follows:
R = V(ec) x P(ec)
V = Vulnerability to the occurrence of an external circumstance (ec) or threat P = Probability of an external circumstance or threat occurring In addition to the three categories listed above, a fourth dimension should be mentioned: the vulnerability towards an intentional terrorist attack. The issue at hand is that an attacker will seek to exploit the vulnerabilities that are present in the network. One important point with respect to terrorist threats is that, while other threats can be expected to occur in a somewhat random and stochastically predictable fashion, terrorist threats are less predictable and somewhat endogenous (in the sense that the threat is likely to be greater if the vulnerability is greater, whereas with natural threats the probability of threat and the vulnerability to the threat can be assumed to be independent of one another).
In essence, a vulnerability analysis must answer the three following questions: First, “Vulnerable…where?”, to assess the location. Second, “Vulnerable…to what?”, assessing the particular circumstances. Third, “Vulnerable…how?”, addressing the particular scenario and its impact.
VULNERABILITY/RELIABILITY AND COST/BENEFIT
The purpose of a cost-benefit analysis is to weigh the costs of a proposed project against the benefits of the project (McHarg, 1967; Adler, 1987; Nas, 1996). If the benefits exceed the costs, then the project increases society’s welfare. If the costs exceed the benefits, society will experience a loss of welfare. The argument of vulnerability versus reliability is analogous: Any vulnerability of the transport system causes disruptions that cause costs, which are a loss of welfare; vulnerability is thus a cost that is quantifiable. If society puts measures in place to reduce the vulnerability of the transport network, the increased reliability represents a benefit.
In terms of road user utility new projects are generally considered as improvements, the same normally applies to reliability. It is often taken for granted that road improvements that alleviate congestion and travel time variability also improve reliability. What remains an open-ended question is whether new projects also bring with them a lesser vulnerability. Exchanging one kind of vulnerability for the other, while supposedly increasing reliability, may in terms of socio-economic impact not be the best solution. A system approach that takes both sides into account is thus necessary. The reason is straightforward: to evaluate the cost of remaining vulnerable against the assumed benefit of becoming less vulnerable with the proposed project.
Vulnerability, represented by the consequential costs of an operational degradation, is a cost that should be included in a cost-benefit analysis. Likewise, reliability, representing the consequential benefits of an operational improvement, is a benefit value, which too needs to be included. Today’s project evaluation practice looks almost exclusively at the construction costs on one hand, and the road user utility as the benefit on the other hand, with little or no notion of reliability or vulnerability.
Example 1: A project is proposed to reduce the consequences of avalanches on a selected link on a given route, reducing the particular vulnerability of this link on this route. This improvement may cause motorists to transfer to this route from other possible routes. If the vulnerabilities of other links along the now partially improved route are neglected, then this may in turn affect a larger number of road users then before the particular improvement.
Example 2: Typical Norwegian issues of vulnerable links are ferry crossings, with a high probability of frequent closures and delays, often being the only access to a given community or between major regions. These crossings are frequently replaced by sub sea tunnels, with practically no probability of any closure, and are hence considered a considerable improvement in reliability. Normally, after the tunnel is built, the ferry jetties are dismantled. If the tunnel is then closed due to an accident, say, a fire, and remains closed for a long period of time, then this community would in fact be more vulnerable after the project then before.
The above examples can be expressed as in the figure below, illustrating the relationship between vulnerability and reliability. The constructions costs, and thus reliability, increase from left to right (dotted line), disruptions costs, and thus vulnerability, increase from right to left (solid line). From a strictly economical point of view, the cost of increasing the reliability should not exceed the cost of vulnerability for society to experience a benefit. What is apparent is that it may be straightforward to quantify the investment costs associated with increasing reliability, the costs of disruptions are much harder to quantify in measurable terms. The investment costs are included as a cost in cost-benefit analyses; however, saved disruption costs are not included as a benefit. Consequently, a proposed investment is not valued correctly, if saved disruption costs are not properly accounted for. The question that arises is whether saved disruption costs represent a utility similar to saved travel costs.
Example 3: To keep a mountain road open during winter one may consider improving the road by building a tunnel, more snow sheds, or aligning the road differently to alleviate weather exposure, or alternatively, invest in better snow-clearing equipment and increase clearing frequency. Today’s situation is (A), with high disruption costs. Building a new road and/or a tunnel is costly (B), but the probability of disruptions is lessened considerably and the disruption costs are almost negligible. Better snow-clearing equipment (C) reduces the susceptibility to disruptions somewhat – the road is still exposed to severe weather conditions – but the investment costs are much lower than in the former alternative. Consider which alternative is the most beneficial to society if investment costs and saved disruption cost are weighed against each other? Note point (D) where the two curves intersect. The expected cost of disruptions (or the expected benefits of avoiding disruptions) is there equal to the cost of countermeasures. D is the socio-economically optimal level of expected disruption costs (and the optimal expenditure on countermeasures). A movement towards D from the left means that it will be cost-effective to implement a countermeasure. A movement rightwards from D means that society will be better off “living with the vulnerability”. In this particular case, better snow-clearing equipment will be a good countermeasure, a new road/tunnel will not. The individual road user may disagree here, but from an overall cost-benefit perspective it is correct.
In terms of emergency management, one usually distinguishes between two distinct strategies, one aimed at reducing the probability of an incident occurring and the second aimed at reducing the consequences of an incident that occurs. Thus, intentionally oversimplified to make the distinction, a strategy to increase the reliability of a road network would be aimed at reducing the probability of a disruption, known as a pre-emptive measure, and a strategy to reduce the vulnerability of a road network would be aimed at reducing the consequences of a disruption, known as a mitigative measure.
Some of the elements that cost-benefit analyses should take explicitly into account in order to incorporate considerations of vulnerability are
(1) the probability of failure of a given project, given various external circumstances (i.e., the vulnerability)
(2) the probability of those external circumstances occurring (i.e., the probability of the threat)
(3) the robustness of the system (i.e., the probability that the system will continue to function even if a threat eventuates at a vulnerable point)
(4) how long it will take (and how expensive it will be) to repair the system if the threat occurs and the system fails at its vulnerable point
(5) what the costs are to the general economy of such a failure (i.e., goods and passengers not getting to their destinations, or getting there late, transportation carriers being forced to use expensive detours, etc.)
(6) the contribution of a given project to improving the robustness (and hence reliability) of the system (or possibly the costs of a project in reducing robustness if a project involves taking an existing piece of the transportation system out of service: e.g., replacing a bus system with a subway)
(7) what degree of risk aversion that should be applied in deciding what weight to place on the risk (i.e., level of threat times vulnerability) that has been identified.
Any investment into making the infrastructure more reliable or less vulnerable needs to balance these two attributes. An overemphasis on either side will lead to a loss of welfare for the society at large, as expressed in figure 1, and consequently, costs and benefits need to be weighed against each other.
CONCLUSION
This paper has sought to bring together three fields of research: reliability and vulnerability, costs and benefits, and decision-making, with the rationale that the reliability or vulnerability of the transport network only seldom is looked at as a decision parameter in cost-benefit analyses, especially for new projects.
All decision making has a degree of uncertainty, ranging from a predictable, deterministic situation to an uncertain situation. In some situations, decision making involves the risk or some uncertainty of making a “wrong” decision, because the information acquired is insufficient or the approach used is inappropriate. In the end, it is the decision maker, who determines the criteria, the factors, the constraints, the individual weighting and the decision rules. Decisions based on reliability and probability have a scientific aura around them, being absolute by number; decisions based on vulnerability are more relative and descriptive, not absolute, since vulnerability is associated with susceptibility, meaning the state of being very likely to be influenced, harmed or affected by something. This is an individual issue, not easily translated into absolute terms. By adding reliability and vulnerability to the traditional equations of costs and benefits, it is hoped that transport planners and professionals will not only consider economical arguments, but also dare to take on political statements that may be in opposition to the strictly factual costs and benefits of a project. This is only possible when these arguments are backed up by a full evaluation of the factors that make up the reliability and vulnerability of our infrastructure system.
This paper has advocated that reliability and vulnerability should be part of the project evaluation in road development projects, and that reliability and vulnerability should be accounted for in cost-benefit analyses of said projects. However, it must also be acknowledged that decisions cannot follow directly from cost-benefit assessments. Decisions related to vulnerability must pass a scrupulous review before they can be executed, and with that in mind, it is justified to assert that vulnerability, unlike reliability, is a descriptive, not a prescriptive science.
Finally, it must be said that the aim of this paper was not to find a justifiable measure of vulnerability, but to point at vulnerability as a variable that needs to be taken account of in cost-benefit analyses and in the evaluation of transport networks. Therefore, in its composition, this paper has likely raised more questions than provided solid answers. It remains to bee seen, then, whether vulnerability and reliability will be a driving force in cost-benefit analyses of future road development projects.
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