51
1 INTRODUCTION
The North Sea is one of the busiest shipping traffic
areas in the world. By monitoring AIS signals
transmitted byships, the NetherlandsCoastguard is
abletomonitorthecurrenttrafficsituationandassist
ships. For the Dutch Ministry of Infrastructure and
theEnvironment(I&M)itisalsoimport
anttomonitor
any changes in the safety levels of traffic at various
locations,particularlyatbusyjunctions.
Traditionally, the safety levels of the shipping
traffic and the impact of new developments and
measures,canbeassessedwithriskmodels,suchas
theSAMSONmodel(VanderTak&DeJong, 1996)
tha
t was developed at MARIN. In the SAMSON
model, risk is a combination of accident probability
and consequences. For the risk of collisions, the
probability is modeled by estimating the number of
encountersbetweenshipswitha statictrafficmodel,
andmultiplyingthisbytheprobabilityofacollision
givenanencounter.
The t
raffic model is used to predict routes and
shippingintensitiesinfuturesituations,butitcannot
be used to monitor the safety levels that occurred
withtheactualtraffic.
Ariskindexwasdeveloped(Koldenhofetal.2009)
toapplytheriskmodelfromSAMSONtotheact
ual
real‐time trafficinformation that isprovided byAIS
data. This index can be used to monitor the safety
levels. In the risk index, the encounters are
determinedfromactualshipspositionsinsteadofthe
statictrafficmodel.
Both in the SAMSON model and the risk index,
theprobabilityofanencounterresult
inginacollision,
is still estimated from accident statistics. These
statisticsare,fortunately,ratherscarce.Theamountof
detail that can be incorporated in the probability
model,istherefore very limited. Moreover,formost
Classifying Ship Encounters to Monitor Traffic Safety
on the North Sea from AIS Data
W.H.vanIperen
M
aritimeResearchInstituteNetherlands(MARIN),Wageningen,TheNetherlands
ABSTRACT:InstudiesfortheDutchMinistryofInfrastructureandtheEnvironment,MARINhasdeveloped
methodstoclassifyshipencountersontheNorthSeafromAISdata.ThemethodsusetheDistanceatClosest
PointofApproach(DCPA),TimetoClosestPointofApproa
ch(TCPA),andanestimateofshipdomains,to
determineforeachcrossing,head‐on,andovertakingencounter,whetherthesefollowabnormalpatterns.On
august 1 2013, the route structure on the North Sea, was rearranged to improve safety and efficiency. The
encounterclassificationmethodswereappliedtotwoyearsofAISdata.Heatma
psofencountersshowhowthe
junctionshaveshifted.Forthesejunctions,thenumbersofencounterswerecompared.Thispaperdiscussesthe
methodstoclassifyencounters,andtheresultsofthecomparisonoftheroutestructures.Theresultsshowa
decreaseofthenumberofexceptionalhead‐onandcrossingencountersinthenewroutestructure.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 9
Number 1
March 2015
DOI:10.12716/1001.09.01.06
52
statistics, the exact tracks of the ships are not
available. Characterizations of encounters therefore
only distinguish between head‐on, crossing, and
overtaking encounters. As a result it is still hard to
estimate theprobabilityof a collisionfromAIS data
directly.
Instead of using the probability ofa collision for
monitoring the safety level, I&M therefore asked
MARIN to develop methods to distinguish between
normal and exceptional encounters, or even near
misses, and thus get an indication of dangerous
locations. The methods use the progress of the
Distance at Closest Point of Approach (DCPA) and
Time to Closest Point of
Approach (TCPA) during
encounters, as well as an estimate of the normally
maintained ship domains, to determine whether an
encounterfollowsanormalorabnormalpattern.
Onaugust12013,theroutestructureontheDutch
part of the North Sea, was radically changed to
improve safety and efficiency regarding offshore
platforms,portapproachandanchorageareas,andto
allocatesafeareasthatcanbeusedforwindenergy.
MARIN applied the classification methods for
encounterstotwoyearsofAISdatafortheNorthSea:
oneyearundertheoldroutestructure,andoneyear
under the new structure. Resulting
heat maps of
encounterlocationsshowtheareaswheremanyship
encounters take place. By comparing the heat maps
andthetrafficflowsfortheoldandnewsituation,the
shiftingofjunctionsbetweenvarioustrafficflowswas
described.Anumberofspecificareasweredefinedin
both structures, and the
types of encounters were
countedandcomparedpermonth.
This paper describesthe developedmethods and
theresultsofapplyingthesetocomparetheoldand
new route structure. First, Section 2 describes the
available data. The underlying principles to the
methodsaredefinedinSection3.Section4gives
an
overview of the various criteria that are used to
classifytheencounters.Section5describestheresults
of the encounter comparison between the old and
newroutestructure.
2 AVAILABLEDATA
Each month, MARIN receives AIS data from the
Netherlands Coastguard forthepurpose of riskand
safety studies regarding
shipping traffic. Not all
tracksofrecreationalandfishingshipsareavailable,
sincesomeshipsarenotyetequippedwithAIS.
The AIS data has already been processed by
Coastguard software upon reception at its base
stations along the Dutch coastline and on various
platformsintheNorthSea.The
processingmeansthat
forexample all overlappingsignals at differentbase
stationshavebeenmergedandsometracksmayhave
been extrapolated after signal loss. Also, where
available, positions have been checked with radar
tracks.
The main information that is used from the AIS
signalsaretherecordedmomentsatwhich
thesignals
are parsed on board of the ship (‘parse times’), the
positions at those moments (tracks), speed over
ground,heading,courseoverground,shiptype,and
dimensionsoftheship.Thetrackpositionsrepresent
themiddleoftheship.
Inthepre‐processingofthedataforthisproject
at
MARIN, information is derived and stored only for
fixed intervals of 1 minute. For these ‘plot times’
relativepositionsandspeedsofshipsarecalculated.
Giventhespeeds,accelerationandmaneuverabilityof
mostships,thechoicefor1minuteintervalsisenough
toavoidmissinganycloseencounters.
Theaccuracy
oftracksisnotalwaysoptimal.The
merging process with radar positions, for example,
maycausesomefalsepositionsinthetracksinsome
areas.Suchpositionsarenottakenintoaccountinthe
encounter classification process. False positions are
detected based on peaks in the calculated average
speedbetween
twoconsecutivepositions.
3 DEFINITIONS
Theencounterclassificationmethodsuseinformation
about relative positions between ships. It is
summarized by distance, relative bearing, closest
point of approach and maintained ship domains.
Definitionsanddescriptionsofthesearegivenhere.
3.1 Encounters
An encounter is defined as the tracks of two ships
having a speed of at least 1 knot, that at certain
momentsduringtheirapproach,areexpectedtopass
eachotherwithin3nauticalmileswithin20minutes,
basedontheirspeedandcourse.Theencounterstarts
at the first of such moments, and ends 20 minutes
afterthelast
ofsuchmoments.The20minutesextra
areusedtoalsobeabletostudythetrajectoriesafter
theshipshavepassedeachother.
An encounter always takes place between two
shipsAandB,andisstudiedfromtheperspectiveof
both ships. If a third ship C is present,
this is
consideredinthreeencounters:anencounterbetween
A and B, between A and C, and between B and C
separately.
3.2 Distance
The distance between ships A and B at time t is,in
principle, the distance between the center points of
the two ships. However, sincethe
two ships do not
necessarily parse signalsat the same time, the exact
distanceattcannotbecalculated.Thedistanceofship
AtoshipB,denotedasd(A,B)(t),iscalculatedforthe
moments that signals are parsed at ship A by
extrapolatingthe position ofship B atthat
moment.
Extrapolation is based on the position, speed and
course over groundat the lastparse time of ship B.
Giventhehighfrequencyofsignals,thisissufficiently
accurateforthepurposesofthisresearch.
53
3.3 Absoluteandrelativebearing
ThepositionofshipBfromtheperspectiveofshipA
can be uniquely described by distance and relative
direction. In navigation, direction is measured in
relation to a reference direction. This is called the
bearing. The referencedirection can beabsolute(for
examplenorth)
orrelative(courseofship),resulting
inanabsoluteorrelative bearing.Forencounters,the
course over ground of the own ship is taken as
referencedirection.
TherelativebearingofshipBasseenfromshipA
is defined as the angle between the line connecting
the centers of
ships A andB, and the course line of
ship A. This is denoted as rb(B,A)(t). This angle is
measuredasthedifferenceinclockwisedirection,and
takes values between 0° and 360°. In general,
rb(B,A)(t)≠rb(A,B)(t),unlesstheshipssailinexactly
theoppositedirection.
3.4 Ship
directionsduringencounters
Theaveragedirectionofashipduringanencounter,
denotedasθ,isdeterminedastheabsolutebearingof
thelinebetweenthefirstandlastpositionoftheship
duringanencounter.
ThedifferenceofdirectionofshipBasseenfrom
shipAisdefinedas:
BA B A
(1)
BA
takes values between ‐360° and 360°, and
moreover
AB BA
(2)
The difference of direction of the encounter,
regardlessoftheship‐perspective,isdefinedas:
AB BA
(3)
3.5 Distanceatclosestpointofapproach(DCPA)
Theclosestpointofapproach(CPA)isthepositionof
ashipduringanencounterwherethedistancetothe
other ship is minimal. The distance at that point is
denoted as DCPA. During the encounter, the CPA
and DCPA can
be estimated from thecurrent speed
andcourseovergroundofbothships.Theprediction
of DCPA therefore progresses over time and is
denotedasDCPA(t),expressedinnauticalmiles.
There is an important difference regarding the
passing distance for a give‐way ship A between
passingastand‐onship
Batthesternoratthebow.
ThiscanbeexpressedbydefiningasignforDCPA:
DCPA>0: the give‐way ship from portside
passesatthebowofthestand‐onship;therelative
bearingincreases;
DCPA<0: the give‐way ship from
portside
passesatthesternofthestand‐onship;therelative
bearingdecreases.
The sign of DCPA is thus determined by the
changeofrelativebearing.
3.6 Timetillclosestpointofapproach(TCPA)
Foreach estimated DCPA‐value,the time it takes to
reachtheCPA,isdenotedas
TCPA(Timetillclosest
pointofapproach).TCPAalsohasasign:
TCPA>0:the distance between the ships
decreasesandtheCPAisstillahead;
TCPA=0:theCPAisreached;
TCPA<0:the CPA is passed; the distance
betweentheshipsincreases.
3.7 Shipcoordinatesandshipdomains
For safeand comfortable navigation, ships prefer to
maintain a certain minimal distance to other ships.
Theresultingfreezonearound theshipiscalledthe
shipdomain.Theminimaldistances canbeexpressed
in miles, but also in number of ship lengths (and
breadths).
Depending on this, domains will be
referred to here as absolute (distance) or relative
(distance)shipdomainsrespectively.
The absolute ship domain can be observed from
tracks of encounters by applying a coordinate
transformationthatputseachshipintheorigin,after
which all tracks of encountering ships can be
superimposed.Thistransformationusesthe absolute
distanceandrelative bearingbetweentheships.The
tracksofshipBasseenfromshipAinabsoluteship
coordinatesarecalculatedas:
B,A
B,A
( ) d(A,B)( ) sin(rb(B,A)( ))
( ) d(A,B)( ) cos(rb(B,A)( ))
x
tt t
y
tt t
(4)
The relative ship domain can be observed by
additionally scaling the absolute ship coordinates
accordingtothelengthofshipA,L
A:
B,A B,A A
B,A B,A A
() ()/L
() ()/L
ltxt
bt yt
(5)
Figure 2 shows all tracks of encounters (mainly
overtakingandcrossingencounters)inabsoluteship
coordinates that occurred during one month at the
busy junction in thetraffic separation schemeabove
VlielandIslandintheNetherlands.
The plot clearly suggests the ship domain where
few ship tracks are observed,
and an increased
densityoftracksaroundit.Alsovisibleinthedomain
are thetracks of (two)vessels being towed by tugs.
The clustered tracks above the origin represent the
tug, and the clustered tracks below the origin
representthetowedvessel.
The size of the ship domain (either absolute
in
miles,orrelativeinshiplengths)canbemeasuredby
determiningthedistributionoftracksandtakingfor
examplethe0.5%percentile.Forthis,adistinctionis
made in sectorsof11.25°, since distancesinfront of
theshiparelargerthanatthesideoftheship.
54
Figure1showsthe0.5%,1%and5%percentilesof
the absolute ship coordinates in Figure 2. The 5%
percentileshowsashapethatistobeexpectedfora
shipdomain.Itcanbeseenthatforexampleforthe0°
sector,only5%ofthetracksarewithin
1mileofthe
ship. To the side of the ship, 5% of the tra cks are
closerthan0.5nmoftheship.
Clearly,thetowingcombinationsaffecttheshape
ofthe0.5%and1%percentile.After removing these
tracks,thepercentilealsoshowtheexpectedcontours.
Figure1. 0.5%, 1% and 5% domain percentiles of
superimposedtracksinFigure2,measuredinnauticalmiles
Figure2.Superimposedtracksinabsoluteshipcoordinates
4 ENCOUNTERCLASSIFICATIONCRITERIA.
Fordifferenttypesofencounters,differentcriteriaare
applied to discriminate between normal and
exceptionalencounters.
4.1 Typesofencounters
Three obvious types of encounters, head‐on,
overtakingandcrossingencounters,aredistinguished
basedonthedifferenceofdirectionoftheships.The
crossing encounters are further
divided into two
subtypes, depending on whether the give‐way ship
fromportsidecrossesatthesternoratthebow.This
isdeterminedbytheDCPAvalueattheCPA.
Figure3showsthedistributionofthedifferences
in direction for over half a million encounters
between merchant
vessels on the Dutch part of the
NorthSeainthefirstthreemonthsof2011.Thegraph
is more or less symmetrical, given (2).The graph
shows three peaks. The center peak indicates the
overtaking encounters,having
≈0°. Theleft and
right peak indicate the head‐on encounters, having
≈180°.Betweenandaroundthepeaksarethetwo
typesofcrossingencounters.
Figure3. Distribution of observed differences in direction
betweenencounteringships
Table1summarizes how theencountertypesare
categorized.
Table1.Overviewofdistinguishedtypesofencounters
_______________________________________________
EncountertypeCriteria
_______________________________________________
Overtakingφ<25º
Head‐on165º<φ<195º
Crossing,give‐wayship (25º<φ<165ºor195º<φ<335º)
passesatsternandDCPA<0atTCPA=0
Crossing,give‐wayship(25º<φ<165ºor195º<φ<335º)
passes
atbowandDCPA>0atTCPA=0
_______________________________________________
Besides these types of encounters, there are a
numberofintentionalencountertypes.Theseinclude
towing combinations (either the one ship towing
another, or two tugs towing a third ship), navy
convoys, and encounters between ships and pilot
vessels. Distinction is based on relative positions,
average speeds, maximum distances during
encounter,
shiptypes,etc.
The intentional encounters are left out of
consideration when defining criteria for exceptional
and unintentional encounters, since the maintained
distances are often muchsmaller.An example ofan
intentionalencounterwasshowninFigure2,wherea
towing combination can be seen in the center. Its
effect
ontheshipdomaincanbeseeninFigure3.
4.2 Criteriaforexceptionalencounters
In preliminary discussions about detection of near
misses and hazardous encounters from AIS with
representatives of I&M and the Netherlands
Coastguard, it became clear that any definition of a
near miss will always be subject to
exceptions.
Thereforeabroadbottom‐upapproachwaschosenin
which normal encounters are excluded, leaving a
selectionof‘relevant’encounters.
Outof3152encountersonthebusyjunctionabove
VlielandIslandinMay2010,initially371encounters
wereselectedforwhichtheexpectedDCPAwasless
than 0.5 nm
at some point within 9 minutes of
55
reaching the CPA. The relevance of many of these
encounters was discussed with experts by studying
animations of the encounters. The expert panel
consistedofmembersoftheShippingAdvisoryBoard
of the North Sea (under whom representatives of
captains, pilots, port authorities and of the
Coastguard).Theanimationsled
todiscussionsabout
thecausesofthesituationsandpossibleothercriteria
that could be investigated. It was agreed that ship
domainsneededtobedeterminedfromAIS,inorder
to judge whether some ships were uncomfortably
closefromtheothership’sperspective.
Aresultingsetofencounters,thetop
ofaranking
that was based on both DCPA values and ship
domainentrees,wereclassifiedintothreecategories:
‘safeandirrelevant’,‘exceptionalandrelevant’,anda
categoryinbetween.Theencountertracksservedasa
trainingsetofnegative( ‐),neutral(+/‐),andpositive
(+) instances to determine
classification criteria for
relevantcases.
4.2.1 Give‐wayshipscrossingatthestern
Figure4showsthe14DCPA‐TCPAgraphsforthe
positive, neutral and negative instances of the
crossingencounters wherethegive‐wayshipcrosses
at the stern. An interesting area in the graph is the
area
where 0 < TCPA < 0.05 and‐0.2 < DCPA < 0.
None of the graphs of the irrelevant instances cross
thisarea, whereas the neutralandpositiveinstances
do have observed values in this area. Some of the
neutral instances pass very close to the origin, but
have a constant
graph in the last 3 minutes (0.05
hours)beforereachingCPA,indicatingthatalthough
theshippassesverycloseatthestern,ithappensina
controlled manner. The two positive instances
initially have positive DCPA values, but only just
beforereachingCPA(TCPA≈0.05=3minutes),the
course
ischanged.Intheendtheshipspassat0.2nm,
but the maneuver is far more abrupt than for the
neutralcases.
Figure4. DCPA‐TCPAgraphofthe training set where the
give‐wayshipfromportsidepassesatthestern
Based on these observations, the relevant
encountersweredefinedasencountersthathave‐0.2
<DCPA<0atsomemomentswhere0<TCPA<0.05.
Since 14 encounters only give a very rough
impression of attained DCPA values during the
encounters,
Figure 5 shows the accumulated DCPA‐TCPA
graphs
of 164646 crossing encounters on the Dutch
partoftheNorthSeaduringthelastthreemonthsof
2011 where the give‐way ship crosses at the stern.
Encounterswithspecialcraft(tugs,pilotvessels,etc)
have been left out in this graph. In the graph, lines
indicate percentiles for
the DCPA values for each
TCPA value. For a given TCPA value, the 50% line
indicatesthat50%ofallDCPAvaluesarebelowthat
lineatthegivenTCPAvalue.
Figure5. Accumulated DCPA‐TCPA values of 164646
crossingencounterswherethegive‐wayshipfromportside
passesatthestern
Thegraphshowsthatforeach value 0< TCPA <
0.05 the 90% DCPA percentile is clearly below‐0.2
nm,thereforelessthan10%oftheDCPAvaluesinthe
last3minutesbeforereachingtheCPAarewithin0.2
nm.Thegraphfurthershowsthataround10%ofthe
actual passing distances of the encounters was less
than 0.3 nm. A major part of the encounters takes
placebetween0.3and1.2nm(theredspotbetween
the60%and90%percentiles).60%oftheencounters
hadpassingdistanceslargerthan1.2nm.
4.2.2 Give‐wayshipscrossingat
thebow
Thetrainingsetcontained17encounterswherethe
give‐wayshipcrossesatthebow:1positive(relevant)
instancewasfound,nextto6neutralinstancesand10
negativeinstances.
For all 17 encounters the number of positions of
ships in the other ships domain were counted, but
both
fortheabsoluteandrelativedomainthesecounts
did not seem to discriminate between positive,
neutral and negative instances. The domains were
thereforenotusedtoformulateadditionalcriteriafor
thisencountertype.
Figure6showstheDCPA‐TCPAgraphfortheset
of 17 encounters. The graphs for the
negative
instances (the safe and normal encounters) have
positive DCPA values during the entire approach,
whereas the positiveinstance and allbut oneof the
neutral instances initially have negative DCPA
values.Thismeansthatfortheseencounterstheinitial
intentionofthegive‐wayshipwastopassatthe
stern,
butthatitlaterdecidedtopassatthebow(possibly
becauseofothertraffic).
For these encounters, the actual CPA is not yet
reachedwhentheshippassesexactlybeforethebow,
56
butsomewhatlater.TheactualCPAisthereforenot
thecriticaldomainpointthatthegive‐wayshipuses
to anticipate. This means that, in terms of TCPA,
anticipation is normally done earlier than for the
crossingsofgive‐wayshipsatthestern.
Figure6. DCPA‐TCPAgraphofthe training set where the
give‐wayshipfromportsidepassesatthebow
Thegraphsshowthatneitherthenegativenorthe
neutralgraphstakeonDCPA‐valueslessthan0.2nm
for 0 < TCPA < 0.1 (except one value just at the
border). For the positive encounter the graph does
cross this area. The DCPA‐TCPA criterion that is
applied for these
types of encounters, is that an
instanceisdismissediftherearenoDCPA‐valuesless
than0.2nmforallTCPA‐values0<TCPA<0.1(=6
minutes).
Asanindication,Figure7showstheaccumulated
DCPA‐TCPAgraphsof123736crossingencounterson
theentireDutch
partoftheNorthSeaduringOctober
–December2011wherethegive‐wayshipfromport
sidepassesatthebow.
Figure7. Accumulated DCPA‐TCPA values of 123736
encounterswherethegive‐wayshipfromportsidepasses
atthebow
Thisgraphshowsthatthedistanceatwhichships
passatthebow,ismuchmorefixedthanthedistance
atwhichshipspassatthestern.
4.2.3 Overtakingencounters
Becauseoftherelativelyslow and long approach
of ships in overtaking encounters, a DCPA‐TCPA
criteriondidnotreallydiscriminate
betweenrelevant
and irrelevant overtaking encounters. Two relevant
overtakingencountersinthetrainingsetcouldnotbe
distinguished based on the DCPA values. The ship
domainsweremorediscriminating.
Astrainingsetthereforeallovertakingencounters
werestudiedthathadatleastonedomainentranceof
the other ship, either
for the relative or absolute
domain.Oftheencounters,3werejudgedasrelevant,
5wereneutral,andtheremaining35wereirrelevant.
Forthetraining set the countofpositionsinsidethe
relative domain of the other ship seemed to be
reasonably discriminating factor between irrelevant
and relevant or neutral
encounters. No clear
additionalcriterionwasformulatedotherthanhaving
entered the relative domain of the other ship and
havingaminimalpassingdistancesmallerthan 0.35
nm.
For overtaking encounters, the applied criterion
fora relevantencounteristherefore that for atleast
one moment during the encounter, the
other shipis
within the relative domain. The relative domain is
definedasthe0.5%percentilecontourbasedon3204
overtaking encounters (excluding intentional
encounters)inadiversetrafficareaintheDutchpart
oftheNorthSeainMay2010.Thisdomainisshown
astheinnercontourinFigure
8.Thefigureshowsthat
thedomainisabout6or7shiplengthsinfrontofthe
ship,andaround3to4shiplengthsattheside.
Figure8. 0.5%, 1% and 5% percentiles of the relative
distancesfor3204overtakingencountersinMay 2010ina
diversetrafficareaintheDutchpartoftheNorthSea
4.2.4 Head‐onencounters
Criteria for exceptional head‐on encounters are
similar to thosefor overtaking: atleast one moment
where the other ship is inside the relative domain,
andaminimalpassingdistanceofatleast0.35nm.
The relative domain that is used in this case, is
basedon
1960head‐onencountersinMay2010inthe
same diverse traffic area in the Dutch part of the
NorthSea.ThedomainisshowninFigure9.
The figure shows that, as is to be expected, the
domainstretchesmuchmoreforward(around20ship
lengths)thaninthe
caseofovertakingencounters.
57
Figure9. 0.5%, 1% and 5% percentiles of the relative
distances for 1960 head‐on encounters in May 2010 in a
diversetrafficareaintheDutchpartoftheNorthSea
The number of lengths at the side of the ship is
comparabletothatfortheovertakingencounters.
5 COMPARISONOFENCOUNTERSINTHEOLD
ANDNEWROUTESTRUCTURE
5.1 Theoldandnewroutestructure
Animportantmeasureinthenewroutestructureto
improve safety and efficiency, is by
reducing the
variety of routes that ships take, and creating clear
andconfinedareasforencounters.
Figure10showstheshiftsoftheroutesinthisnew
routestructure,bycomparingthetrafficdensities(the
averagenumberofshipspresentpersquareunit.The
black line demarcates the area that
contains the
changesintheroutestructure.Thenumberofroutes
toandfromthenorthhasbeenreducedinparticular.
Figure11showsamapofallcrossingencounters
in the area for the first year that the new route
structure was in effect. The green dots indicate
locationsof
exceptionalencounters.
Figure 12 shows the shift of junctions by
comparing the number of crossing encounters per
gridcellof1x1km.Themapalsoshowsthejunction
areas that were defined to compare changes in the
number of encounters between specifictraffic flows.
Areas 5, 6 and 7
particularly show how the area
whereencounterstakeplace,ismorecompactinthe
newsituation.
Figure10. Shifts of the main traffic flows due to the new
routestructure,basedontwoyearsofAISdata
Figure11.Heatmapofcrossingencountersinthenewroute
structure
5.2 Resultsofclassificationandcomparison
DuringAugust2013–July2014,anaveragedecrease
of the various traffic intensities in the area was
observed of 10.6%. Due to these lower intensities, a
decreaseofthenumberofcrossingencounterscanbe
expectedof100%×(1–(1–0.106)
2
)=20%.
Table 2 contains the numbers of crossing
encountersperareafor2011(theoldroutestructure)
andthefirstyearunderthenewroutestructure.The
table shows that in the entire area where the routes
have changed, the encounters have decreased by
31.1%. On the specific junction
areas, the reduction
58
was20.0%,thereductionthatcouldbeexpecteddue
tothedecreasedintensities.Outsidethejunctions,the
reduction was 47.9%, showing that the encounters
takeplacemoreindesignatedjunctionareas.
The fraction of crossing encounters that are
classified as exceptional, has decreased in the new
routestructure,inthe
junctionareas(0.2%insteadof
0.4%),butevenmoreoutsidethejunctionareas(0.4%
insteadof0.9%).ThiscanbeseeninTable3.Thisis
attributedtothereductionofthediversityofroutes.
In the new route structure, traffic is better able to
anticipateothertraffic.
Figure12.Shiftsofthemainjunctionsoftrafficflowsdueto
thenewroute structure,basedon differenceofnumberof
crossingencounterspercellintwoyearsofAISdata
Table2.Numbersofcrossingencountersin2011(oldroute
structure)andAugust2013–July2014(newroute
structure)
_______________________________________________
Area2011 2013‐2014 Increase
_______________________________________________
18607 7202‐16.3%
214074 12047‐14.4%
317378 2059718.5%
513263 10865‐18.1%
621727 16410‐24.5%
730109 20877‐30.7%
812156 9073‐25.4%
920862 16030‐23.2%
1034716 25129‐27.6%
_______________________________________________
Totaljunctionareas 172892 138230‐20.0%
Outsidejunctionareas 114008 59451‐47.9%
Routechangearea 286900 197681‐31.1%
_______________________________________________
For overtaking encounters, the fraction of
exceptionalencountersishigher,andincreasedfrom
5.7%to6.4%inthenewsituation.Thisisduetothe
fact that ships sail on less and slightly narrower
routes.
Head‐onencountersalsohaveasomewhathigher
fraction of exceptional encounters, but it decreased
from
2.2%to1.4%inthenewsituation.Thisisowing
to the traffic separation zones, especially in area 2
becauseoftheseparationofthefairway.
Table3.Numbersofexceptionalcrossingencountersin2011
andinAugust2013–July2014.
_______________________________________________
Area2011 2013 Increase Percentage
‐2014exceptional
encounters
20112013‐2014
_______________________________________________
148 12‐75.0% 0.6% 0.2%
259 40‐32.2% 0.4% 0.3%
347 39‐17.0% 0.3% 0.2%
538 16‐57.9% 0.3% 0.1%
689 44‐50.6% 0.4% 0.3%
7150 37‐75.3% 0.5% 0.2%
841 18‐56.1% 0.3% 0.2%
995 41‐56.8% 0.5% 0.3%
10162 45
‐72.2% 0.5% 0.2%
_______________________________________________
Totaljunction 729 292 ‐59.9% 0.4% 0.2%
areas
Outsidejunction1056 248 ‐76.5% 0.9% 0.4%
areas
Routechange 1785 540 ‐69.7% 0.6% 0.3%
area
_______________________________________________
6 CONCLUSIONS
The numbers of normal and exceptional crossing,
head‐on and overtakingencounters, arein linewith
the objectives of the new route structure. There are
relatively less exceptional crossing and head‐on
encounters, and encounters take place in designated
junction areas. The overtaking encounters, however,
seem to be
tighter due to more dense traffic on the
somewhatnarrowertrafficroutes.
Overtaking encounters may still need additional
criteriatodeterminereallyexceptionalencounters,as
a the domain criteria still returns a relatively large
fraction of encountersas exceptional. Also,the used
criteria do notseem to apply for approach
areas, as
theseareascontainmanyexceptionalencounters.
ACKNOWLEDGEMENT
Themajorpartoftheworkwascommissionedbythe
Dutch Ministry of Infras tructure and the
Environment.
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Koldenhof,Y.,Tak,C.vander&Glansdorp,C.C.2009.Risk
Awareness; a model to calculate the risk of a ship
dynamically. Proceedings of “XIIIInternational
Scientific and Technical Conference on Marine Traffic
Engineering”,112‐119.
Tak,C.van der&Jong, J.H.de.1996.Safety Management
Assessment Ranking Tool (SMART).
8
th
International
SymposiumonVesselTrafficServices.
