Vulnerability Analysis for Transportation Networks

263 Pages·2017·11.889 MB·English

by  Michael Taylor (Auth.)

VULNERABILITY
ANALYSIS FOR
TRANSPORTATION
NETWORKS
MICHAEL A. P. TAYLOR
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To Jean (1925e2017),
who set me on the path,
and to
Charlie and Howie,
who let me know that the world will be a better place.
PREFACE
Transportation network reliability has featured in transportation research
for some decades, perhaps first as an interesting academic aside, but
thendin the aftermath of the great Kobe earthquake of 1994 when sub-
stantial parts of that region’s infrastructure were destroyeddas a topic of
vital interest to governments and transportation agencies. Research in the
1990s focused on travel-time reliability and network connectivity
reliability. The former topic has become an important consideration in
transport economics and project evaluation, though not without contro-
versy (but that is a story for another time). The latter was concerned with
theabilitytofindapathbetweenanytwopointsthroughadisaster-struck
network. Was the network still intact? Were there alternative paths that
could be used?
A series of international workshops were held in the 1990s where
researcher presented and discussed their work on network reliability,
culminating in the establishment of the triennial International Symposia on
Transportation Network Reliability (INSTR), with the first INSTR held at
Kyoto University in July 2001. In that event four researchers, a Swede, a
New Zealander, and two Australians, started to talk about something
slightly different: network vulnerability. What happened in networks that
were damaged but still operating in some degraded state? When could
they cope? How serviceable were they? How could this be measured?
What if the damaged network was still fully connected but the available
path (or detour) was so much longer than the disrupted path that its use
was not logical, let alone practical? What were the critical elements in a
network, where failure would have the largest effects, and how to protect
them? How could the risk of failure be assessed?
Thesequestionsinfacthavespurredadecadeandahalfofintenseinter-
national research activity, which is still ongoing, and with new players
emergingdas well as a growing list of influences from the modern world
thatcanaffectvulnerabilityandinfrastructureperformance,withmajorim-
plications for transportation network planning, design, and operations.
Beyond the natural disasters that have been part and parcel of concerns on
vulnerability, issues such as climate change, international terrorism, and
j
ix
x Preface
post-disaster relief have come to prominence. A recent report by the
OECD1hasstressedtheimportanceofvulnerabilityinitsreviewofthepo-
tentialeffectsofextremeweatherandclimatechangeontheperformanceof
transportation systems. As society places more reliance on its infrastructure
systems and their ability to provide safe, efficient, and equitable services
oncall;aswestressthosesystemsmoreandmorethroughgrowingdemand
that pushes system capacity; and as our expectations of system performance
growandwedemandmorefromthetransportationsystemwhilesqueezing
its spare capacity, the concepts of vulnerability become more relevant and
important.Thisbookexplorestheseissuesandconsiderstheissuesinherent
invulnerabilityanalysis,themethodsproposedandadoptedtoassessvulner-
ability, to identify critical infrastructure, locations and facilities, and the
metrics proposed and techniques employed in those methods. It seeks to
defineanintegratedapproachtovulnerabilityanalysisthatusesthecommon
features(suchasdatarequirements)behindthealternativeapproacheswhile
maintaining the rounded, multidimensional insights that are possible by
using the full set of those approaches.
The book charts the development of transportation network vulnera-
bility analysis and the widening spread of the research on that topic. It
alsodescribesapersonaljourneyfortheauthor,fromthoseearlybeginnings
inKyototothepresentday.Theauthorbeganthewritingbelievinghimself
tobeanexpertinthefieldandwhilethatbeliefisstilltheretoalargedegree
(and is reinforced by the ongoing citations of a couple of his papers in the
new published research), he has learnt much more about the topic in
compilingthisworkthanheeverknewatthestart.Thatisavaluablething.
Knowledgeiseverexpanding,andtheholdersanddevelopersofknowledge
are diverse and spread across the globe. There is no single repository of
knowledge or, indeed, wisdom. That has always been the casedfor
example, think of Newton, Leibnitz, and the calculus. At the same time
knowledge isephemeral. Transportation engineers of the1920swere using
trip distribution models, originedestination matrices, and desire line maps,
yet that knowledge disappeared (presumably due to economic depression
and a world war) until it was reinvented from scratch, by necessity, in the
1950s. To keep knowledge of a topic alive and growing needs constant
attention and development. The author hopes that this book will provide
1 ITF(2016).Adaptingtransporttoclimatechangeandextremeweather:Implicationsforinfrastructure
ownersandnetworkmanagers.InternationalTransportForum,ITFResearchReports,Paris:OECD
Publishing.Retrivedfromhttp://dx.doi.org/10.1787/9789282108079-en.
Preface xi
asignificantsteptowardthedisclosureofthestateoftheartinvulnerability
analysisandtheraison d’êtretopreserve,extend,andutilizetheaccomplish-
ments in the field. Much has been done, but there is much more still
possibledand required.
The starting point for the discussion is the need to maintain reasonable
levels of serviceability, i.e., function, in a transportation system, and how
this can be assessed. An inherent part of this assessment is the notion of
risk; what are the implications and potential impacts of a system failure
(full or partial). No system can ever be completely immune from degrada-
tion, therefore a risk analysis is required to indicate the potential damage
ofafailureeventandthelikelycostsofsafeguardstominimizethatdamage.
Risk has two dimensions: the probability of an event and the (expected)
magnitudeofitsimpact.Riskisthenrepresentedastheproductoftheprob-
ability and the magnitudes, resulting in an expected outcome for an event.
The potential combination of events is also apparent, and the risk analysis
can handle this situation toodas discussed by Nicholson and Dalzeill2 in
their consideration of the road network of the North Island of New Zea-
land, and the potential simultaneous occurrence of volcanic eruptions and
snowstormsinpartsofthatnetwork.Innormallifeweperhapstendtoover-
value extreme eventsdor, perhaps, undervalue the more minor ones. The
airplane crash is seen as severe, the road crash death toll perhaps less so,
although the cumulative outcomes may actually indicate the reverse. Man-
agersoftransportationandotherinfrastructuresystemsneedtoconsiderthe
risks in economic, social, and political terms. Whatever the socio-political
milieu, the availability of sound data and proper risk analysis are essential
for informed decision making and network vulnerability analysis is now a
core part of it. This is the theme developed early in the book, and it leads
to the twin definitions of vulnerability relating to serviceability (ability of
atransportationsystemtofunctionatanacceptablelevelunderdifferentcir-
cumstances) and accessibility (ability of a transportation system to facilitate
acceptable levels of participation in socioeconomic activities in a region).
Transportation systems belong to a wider class of infrastructure systems,
which also include water supply, drainage, wastewater and sewerage, tele-
communications, and gas and electricity supply systems. In modern times,
governments around the world are paying special attention to “critical
2 Nicholson,A.J.,&Dalziell,E.(2003).Riskevaluationandmanagement:Aroadnetworkreliability
study.InM.G.HBell,andY.Iida(Eds.),TheNetworkReliabilityofTransport(pp.45e59).Oxford:
Elsevier.
xii Preface
infrastructure,” meaning both the systems and their components subject to
degradation.Vulnerabilityanalysisisoftenconcernedwiththeidentification
of critical components, providing planners and managers with information
onthosecomponentsofthegreatestriskoffailuredmeaningofcoursethose
componentswhosefailurewouldengenderthegreatestconsequencesasthe
product of probability and impact. In Chapter 2 the book considers critical
infrastructure,firstinbroadtermsincludingdiscussionofthepotentialinter-
actions and interdependencies between different infrastructure systems, and
then in terms of transportation systems specifically. The focus is on land
transportationsystemnetworks,whichsplitintotwobroadtypes:roadnet-
works and railway networks, each of which have their own features. The
differencesbetweenthemlargelyrelatetotheoperatingrulesfortrafficusing
the network, the technologies employed in vehicle propulsion and move-
ment, and the nature of the traffic flows on them. Rail networks perhaps
havemorecomplexitythanroadnetworks,butroadnetworksareinhabited
by numerous individual decision makers (drivers, riders, pedestrians, and
others) and the decisions made by one or more individuals can strongly
influence the way that a system operates. The study of vulnerability and
resilience in infrastructure systems is now widespread, with resilience being
concernedwiththetimeandeffortrequiredforadisruptedsystemtoreturn
to its normal operating conditions after the event.
Tostudynetworkvulnerabilityanunderstandingofthestructureofnet-
worksandhowtheelementsofnodesandlinksconnecttogetherisrequired
first. It then requires understanding of the ways in which transportation
flows occur in networks. The former is the study of network topology.
This provides basic knowledge about the network and how flows through
it will use different components. Transportation analysts have by and large
ignored network topology in the past, leaving it to the mathematicians
and the telecommunications engineers. Yet topological analysis can reveal
much core information on how any network willfunction. Study of trans-
portflowsdandhencenetworkusagedrequiresknowledgeandapplication
of the theories and models used by transportation analysts to represent the
traffic flows and the resultant network operating conditions. The basics of
traffic flow theory lie in the particular description of continuity of flow
forroadtraffic;thattherearetwomainvariablestodescribeflowinasingle
trafficstream(trafficvolumeandtrafficdensity);thatdensity(thenumberof
vehiclesperunitlengthofroad)istheprimaryvariable;andthattheratioof
densitytovolumeyieldstheaveragespeedofthetrafficstreamalongaroad
atagiveninstantoftime.Trafficvolume(therateatwhichvehiclespassan
Preface xiii
observation point over time) is insufficient to properly describe the traffic
state on the road, for a low volume may imply either light traffic flow or
a major bottleneck. Density will reveal the true state. The “macroscopic
flowdiagram”illustratesthesecharacteristicsandcanalsoshowtheobserved
phenomenonofthe“capacitydrop”difatrafficstreamisdisruptedataden-
sityexceedingacriticalvaluethentheflowregimedegradesandmaximum
achievable volumes are significantly less than those experienced before the
disruption. This condition will continue until there is a substantial drop in
density, which can only come from reduced demand to use the road.
The question of travel demand is a key consideration in transportation
networkanalysis,includingvulnerabilityanalysis.Traveldemandissumma-
rizedbytheoriginedestinationpatternandcorrespondingoriginedestination
matrices(thenumbersoftravelerswishingtomovefromagivenoriginpoint
to designated destination points across the network). For vulnerability
studies, the starting point for travel demand analysis is route choice, i.e.,
the paths that travelers take in making their journeys. Route choice is
modeled using “traffic assignment” models, which route the trips along
the links in the network between each origin and destination in response
to the conditions (travel time or travel costs) experienced on the network.
This situation where the number of trips to be made and the destinations
of those trips are fixed is known as inelastic travel demand. Full or partial
network failures (e.g., a blocked or congested link) may alter the available
and preferred routes for some journeys. The extent of these alterations is
at the heart of vulnerability analysis. However, route choice is not the
only travel decision that people make: when to travel (departure time
choice), where to travel to (destination choice), how to travel (transport
mode choice), and indeed whether to travel at all (in a given time period)
are also decisions to be made. Prolonged network failure or chronic
congestionmaymaketravelersreconsidertheirearlierchoices.Suchchanges
aretermedaselastictraveldemand.Elastictraveldemandhasstilltobeprop-
erly considered in the existing methods for vulnerability analysis.
The development of traffic congestion, which certainly contributes to
lossofserviceabilityinanetwork,isakeypartofthevulnerabilityequation.
In particular, this is because congestion manifests itself in different forms.
Thereistheregular,cyclic“recurrent”congestionwhichispartofeveryday
life: the morning rush hour, the business hours, the evening peak, the off
peak hours, and the wee hours of the morning. Cyclic patterns also span
the days of the week, the weeks, and seasons of the year. We know about
thesepatternsandgenerallyworkwithinthem.Somecongestionisanatural
xiv Preface
part of network operations, particularly in urban areas. On the other hand,
there is incident-based “non-recurrent” congestion, which can strike “at
random”anddisablesmall(orsometimeslarge)partsofanetworkfordura-
tionsfromminutestodays.Incidentsrangefromvehiclebreakdownsblock-
ing a lane and road crashes to police operations and structural collapses.
Somearequicklydispelled,otherstakelongeranddemandconsiderablere-
sources to remedy. Other influences including natural phenomena (e.g.,
floods, blizzards, landslides, and earthquakes) and manmade events (e.g.,
roadwork and road and track maintenance, road closures for special events,
and construction zones) can also affect network operations and congestion
levels. Then there are the occasional malevolent events including sabotage
andterroristattackswhichcancauseimmediatedistress,orworse,andleave
longerlastingimpacts.Vulnerabilityanalysishastobecognizantofall these
potentials.
Atthesametimevulnerabilityanalysisisimportantinruralandregional
areasaswellasourcities.Ruralroadandrailnetworkstendtobesparse,with
few alternative routes available. A network disruption can cause isolation
and economic harm in a localitydthink of the dairy farm unable to ship
its milk to the dairy because of a single road closure or bridge collapse.
Trafficcongestionisrarelyamajorconcerninruralregions,butaccessibility
and connectivity are. Natural hazards from extreme weather and environ-
mentareperhapsmorepronetoaffectruraltransportationnetworks.Rural
communitiesareoftenlessprivilegedintermsoftheiraccesstoservicesand
facilitiesthantheirurbancousins.Theroadnetworkisoftentheironlytrans-
port connection to the outside world. Loss of connectivity may have pro-
found implications for health, education, and economic wellbeing in rural
areas.
The issues are explored in depth in Chapters 3e5 of this book.
Thecruxofvulnerabilityanalysisisthentoprovidetoolsthatassisttrans-
portation planners and decision makers in identifying potential points of
weakness in their networksdcritical locations and infrastructuredand
then assist in developing plans to protect those locations or provide ready
alternativesincaseofdisruptions.Theresearchonwhichthisbookisbased
identified four broad methods for vulnerability analysis:
1. Risk-based inventory assessment, which considers the infrastructure
assets across the system and rates those assets at greater risk of failure or
disruption due to traffic and environmental influences
2. Topologically basedassessment,whichconsiders networkstructureand
connectivity, and identifies the nodes and links most critical for flows
Preface xv
throughthenetwork,aswellasthelocationswherefailuremayhavethe
gravest effects on network operations
3. Serviceability-basedassessment,whichisconcernedwiththeimpactson
network operations of failures and disruptions at different locations and
maybeusedtoidentifycriticallocationsintermsofthoseimpacts,witha
focus on transport and traffic
4. Accessibility-based assessment, which is similar to serviceability
assessment but is more concerned with socioeconomic impacts outside
the network traffic operations.
Thebookdescribeseachofthesemethodsandprovidesexamplesoftheir
applications. It alsoconsiders theapplications of themethods to road-based
transportation and to (rail-based) public transport. Most of the reported
studies have dealt with road networks, but public transport vulnerability
analysis has emerged as a substantial area of transportation research since
2010. Chapter 6 of the book deals exclusively with studies of public trans-
port networks.
The various methods share some features as well as having their own,
uniquecharacteristics.Recognizingthecommonfeatures,mainlyintheto-
pological description of networks, provides the basis for a future integrated
approachtovulnerabilityanalysisthatisoutlinedinthelatterchaptersofthe
book.Thedifferentmethodsforanalysiscanbeseenintermsoflayeringof
additionaldetaileddatadassetinformationandtraveldemandpatternsdon
top of the basis connectivity information. Development and application of
this integrated approach to vulnerability analysis should provide the firm
and comprehensive footing for its full inclusion in transportation planning
and systems analysis. We have the knowledge to enable this, and the soft-
wareplatforms,suchasgeographicalinformationsystems(GIS),tofacilitate
it. The author wishes that this book will provide the necessary blueprints.
Therearemanypeople,family,friends,andcolleagues,whohavehelped
me, inspiredme,and have contributedto thedevelopment of vulnerability
analysis in concept, theory, and application. There are others who have
encouraged and helped me in the preparation of this book. My grateful
thanks are extended to them all. I must single out the following: Glen
D’Este,forhistrail-blazinginsightsanduncannyknackfordescribingcom-
plexphenomenainwaysthatallofuscanreadilyunderstand;MichaelBell,
forhisauthority,novelthinking,andexpertadvice;SekharSomenahalli,my
GISguruand“goto”person;myPhDstudent“Susi”Susilawati,forherGIS
expertise, analytical skills, and “can do” enthusiasm; my Masters student
Sally Freeman for her modeling skills and willingness to experiment in the