II INTERNATIONAL CONFERENCE________________ PO L IT EC H N IK I ŚLĄ SK IEJ 2002 TRANSPORT SYSTEM S TELEMATICS TST'02 T R A N SP O R T z.45, nr kol. 1570
dense road network, capacity of intersection R enata Ż O C H O W S K A 1
COMPUTER AIDED EVALUATION OF INTERSECTION CAPACITY IN DENSE ROAD NETWORK
From increasing number of vehicles comes the need to quantify congestion in dense road network.
The paper presents useful computer aided tools for evaluating priority-type and signalized intersection capacity parameters. The applications have been designed on the basis of popular and verified methods.
Along with related database they are essential components o f system that enables optimization of road closures placing in dense road network.
KOMPUTEROW O WSPOMAGANA OCENA PRZEPUSTOWOŚCI SKRZYŻOWAŃ W GĘSTEJ SIECI DRÓG
W zrost liczby pojazdów w sieciach drogowych zmusza projektantów do opracowania odpowiednich narzędzi służących do oszacowania przepustowości poszczególnych składników sieci oraz usprawniania ruchu. Wybór metody powinien zależeć zarówno od indywidualnych potrzeb, jak i możliwości zastosowania w określonych warunkach ruchu.
Ruch na skrzyżowaniu może być regulowany odpowiednim oznakowaniem (znaki A7 i B20) lub sygnalizacją świetlną. W Zakładzie Inżynierii Ruchu Politechniki Śląskiej w oparciu o znane i sprawdzone w warunkach polskich metody opracowano aplikacje komputerowe służące do obliczania przepustowości obu typów skrzyżowań. Programy te zapewniają łatwy dostęp do bazy danych, która została zaprojektowana i uzupełniona wartościami parametrów geometrycznych, ruchowych i sygnalizacyjnych centrum Katowic. Obliczone parametry przepustowościowe są przesyłane z powrotem do bazy danych. W ten sposób jest ona stale uaktualniana w zależności od rzeczywistych wartości natężenia ruchu. Proces ten jest niezbędny przy optymalizacji zamknięć drogowych w sieci transportowej.
Aplikacje są stale rozbudowywane i weryfikowane. Jako składniki systemu optymalizacyjnego zapewniają możliwość dołączania nowych elementów.
1. IN T R O D U C T IO N
E stim ation o f an intersection capacity is one o f the m ost im portant traffic engineering problem s. It enables rational planning and traffic im provem ent in netw ork. T he capacity evaluations resu lt in a view o f transportation abilities expressed as relationship betw een traffic volum e and conditions for individual ju n c tio n o f netw ork. T raffic conditions are characterized by travel speed and tim e, level o f service, convenience and safety [1,6,8].
T h e assum ption that intersection capacity is lim itation o f road netw ork capacity has frequently been m ade. A significant num ber o f theoretical and experim ental w orks have
1 Faculty o f Transport, Silesian University o f Technology, Krasińskiego 8, 40-019 Katowice, Poland renzoc@polsl.katowice.pl
300 R enata Z O C H O W SK A focused on the issues connected w ith evaluating the intersection capacity u nder various geom etric and traffic conditions. T hey resulted in various capacity calculation m ethods. For that reason designers should analyse very critically possibilities for adaptation o f individual m ethod in concrete situation.
T he p roper calibration o f m odel is an essential com ponent influencing the results. The param eters o f capacity m odel should be verified in such a w ay that adaptation to traffic conditions, vehicles type and driver behaviour w ould be optim al.
2. R ELA TED D A TA B A SE
It is necessary to design m athem atical m odel o f road netw ork in such a w ay that it should enable com p o n en t identification, structure description and assignm ent o f certain characteristics to individual com ponents. H ence application o f g raph theory to road netw ork issues is a natural tendency [2,7], T he netw ork structure is defined as a graph, w here transportation points (intersections) are nodes, and jo in ts betw een them are links. The perm issible traffic m ovem ents are taken into consideration.
T he related database has been designed and developed in D ept, o f T raffic Engineering, T he S ilesian T echnical U niversity as a part o f road closure optim ization system [12]. The present version allow s inserting and storing data that are necessary for netw ork com ponents specification, analysis and evaluation o f required capacity param eters. It has been designed in D base IV form at, w here data are stored in table structure files. T he base has been filled w ith geom etric and traffic param eters o f K atow ice centre. T h e application consists o f three basic form s: “O dcin k i” , “W ezly” . Screens o f them are presented in F i g .l .
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Every section is represented by num ber o f 6 digits. F irst three figures correspond to initial intersection num ber and next ones specify final intersection num ber. Inform ation on sections has been displayed on the form that enables inserting, deleting, saving and m oving data to p ro p er record.
301
T he ju n c tio n s have been described as proper jo in ts o f sections. A schem e o f the sim ple intersection has been also placed on the form . A dditional data are accessible in next form including inform ation that concerns priority road specification, traffic organization at the intersection and volum e o f pedestrian on the crossing.
T he netw ork connections have been dem onstrated in schem atic way on the form
“M apa” . T he table corresponding to this form covers graphic com ponents designed for displaying the vector m ap on the screen.
D etailed pieces o f inform ation on road database have been included in the article: [11],
3. C A P A C IT Y O F PR IO R IT Y -TY PE IN TER SEC TIO N S
T he priority-type intersections w ith m inor street approaches controlled by traffic signs:
A7 (YIELD) o r B 20 (STO P) are considered to be m ajority o f road netw ork junctions. At unsignalized intersections all perm issible m ovem ents o f traffic may be divided into three separate groups: m ajor (vehicles are not m inor w ith the others), choked m inor (vehicles are m ajor w ith som e vehicles and m inor w ith the others) and m inor (vehicles do not have priority o ver the others). O nly capacity o f m ajor m ovem ent is com pletely independent on traffic volum e at the intersection. C apacity o f m inor and choked m inor m ovem ent is closely connected w ith traffic volum e o f priority m ovem ents.
T he application PR Z E PU ST has been designed to calculate capacity o f unsignalized intersections [5]. A sim plified schem e o f the m ethod used in the program has been m entioned in fig.2. T his m ethod has been developed at T he C racow T echnical U niversity and is one of the m ost popular m ethods adapted to Polish conditions [3]. T he geom etric and traffic characteristics include traffic volum e at approach, priority approach specification, lane width, volum e o f pedestrian, traffic signs at m inor street approach, num ber o f lanes at approach, perm issib le traffic m ovem ents at individual lanes, lane use description, visibility and slope o f approach. T hese param eters should be defined for every intersection in the netw ork.
Fig.2. Block diagram of PRZEPUST application
302 R enata Z O C H O W SK A
T w o m odels o f intersections w ith various priority traffic flow s have been taken into account by d esigning the application. B oth o f them are presented in fig.3. F or each o f the m odels the p rinciple o f priority traffic volum e specification is different.
Fig.3. Two models of intersections
A nalysing m odels o f priority-type intersections one should assum e that the driver o f m inor road stops, estim ates every gap betw een vehicles o f m ain road and takes decision on crossing. T h e d river accepts a gap (a headw ay o r a portion thereof) if the gap is greater than the d riv er’s critical gap that is a param eter o f considerable im portance [1]. Its value is dependent on traffic organization, type o f m anoeuvre, m ain road perm issible speed and intersection m odel.
In accordance w ith the m ethod the intersection capacity under ideal conditions is defined. It depends on priority traffic volum e and critical gap. T o evaluate the capacity under real conditions som e factors should be applied. T he values o f these factors are d ependent on traffic lane w idth, approach type, volum e o f pedestrian and choke o f traffic.
S creens o f the form s w ith input data and capacity param eters have been presented in fig.4. T he application has been described in detail in the article: [10].
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4. C A PA C IT Y O F SIG N A LIZED IN TER SEC TIO N S
T he use o f traffic lights at an intersection provides for the orderly m ovem ent o f traffic by assigning right-of-w ay to conflicting m ovem ents. Signals also m ay increase the traffic- handling capacity o f an intersection by perm itting conflicting stream s to share the sam e intersection. In database o f K atow ice centre [11] the rate o f signalized intersections am ounts to 13 percent. In this connection the necessity to design efficient tool for evaluating capacity param eters o f such intersections occurs. T he application should also be interactive w ith database w orked o u t earlier.
O ne o f the m ost frequently used and the best applied to Polish conditions m ethods is H C M -94 that is collection o f procedures and m ethodologies for calculating the highway capacity and quality o f service [4], P ossible capacity is defined as m axim um num ber o f vehicles that can be handled by a particular roadw ay com ponent u nder prevailing conditions.
T h e H ighw ay C apacity C om m ittee o f the H ighw ay R esearch B oard has proposed six levels o f service recom m ended for application in describing the conditions existing under the various speed and volum e conditions that m ay occur on any facility. T he 1994 m anual provides great flexibility in adapting procedures to actual conditions. T he H C M is updated continuously on the basis o f the latest researches [9].
T he auxiliary application “S S S ” has been designed in D ept, o f T raffic Engineering. It calculates signalized intersection capacity as w ell as it enables to insert and change data. A sim plified schem e o f m ethod used in the application has been dem onstrated in fig. 5 [1].
Fig.5. Block diagram o f HCM-94 method
304 R enata ŻO C H O W SK A
T he num ber o f vehicles that can discharge during the green p hase is related to the saturation flow . H ence in H C M -94 capacity is defined for group o f lanes and calculated according to the equation [4]:
C ,= S r { G / Ą
w here:
G / T - rate o f the effective green G,. in cycle T for group o f lanes i, S( - saturation flow rate.
T he group o f lanes may consist o f one or several lanes. Each group is assigned to specific type defined follow ing:
- group 1 - straight lanes o r shared through and left or right turn lanes, - group 2 - separate left turn lanes,
group 3 - separate right turn lanes.
T he saturation flow fo r group o f lanes is defined as m axim um n um ber o f vehicles that cross the group o f lanes d uring the efficient green hour under specific traffic conditions. The saturation flow for lane So under ideal conditions is assum ed to be 1900 E p er efficient green hour per lane. To adapt this value to real conditions, som e factors should be applied. The saturation flow for lane m ay be expressed as follow s:
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where:
So - saturation flow u nder ideal conditions fo r lane, n- num ber o f lanes in group,
fw- factor o f traffic lane w idth, fs- factor o f approach slope,
fmp- factor o f parking on neighbouring lane fa- factor o f lane blockage by buses, f0- factor o f intersection localization, fp- factor o f right turning,
fi- factor o f left turning.
In case o f conflicting phase the com plicated set o f equations has to be adopted fo r the calculation o f facto r fi (special procedure).
T he application “S S S ” consists o f follow ing basic form s: “P aram etry geom etryczno- ruchow e” , “ S chem at skrzyżow ania” , “ S ygnalizacja” and “W yniki” . T he form “W yniki”
p resents b o th final values o f capacity param eters and auxiliary factors analysed during calculation. It enables fast verification and errors search as w ell as im provem ent o f the project.
S creens o f the form s: “P aram etry geom etryczno-ruchow e” and “W yniki” have been m entioned in fig.6 and fig.7.
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5. C A P A C IT Y IN RELA TED D A TA B A SE
T h e capacity param eters evaluated by m eans o f auxiliary applications “PR Z E PU ST ” and “S S S ” have been recorded in tables that may be used in the database. T he proper configuration o f the form “M apa” enables w ork in two m odes:
- draw ing the m ap com ponents,
- displaying the param eters for junctions.
T h e choice o f specified ju n ctio n causes displaying its characteristics like traffic volum e or capacity param eters calculated on the basis o f actual traffic volum e.
S creen o f the form “M apa” in displaying param eters m ode has been presented in fig.8.
306 R enata Z O C H O W SK A
Fig.8. Screen of form “M apa” in displaying the parameters mode
6. CO N CLU SIO N S
O ne o f the m ajor problem s facing drivers in road netw orks is congestion. In order for the highw ay to provide an acceptable level o f service to the road user, it is necessary that the service volum e be low er than the capacity o f the roadw ay. T o avoid situation w hen many decision on road im provem ents w ould have been taken w ithout adequate analytical tools, m any m ethods fo r evaluating capacity param eters have been w orked out.
D espite a great num ber o f various com puter tools for evaluating the intersections capacity, the decision on designing the new applications has been m ade. T he program s:
“ P R Z E P U S T ” and “S S S ” provide easy access to traffic data and adaptation o f capacity evaluations to individual needs. T hey are considered to be auxiliary tools in road closures optim ization system that is w orked out in D ept, o f T raffic E ngineering. In this system the necessity o f perm anent capacity param eters updating requires the usage o f the p roper tools that interact w ith the database.
T h e p roper designed related database is a source o f data for the “P R Z E P U S T ” and
“S SS” applications. T he results o f evaluations are transm itted back to the datab ase enabling capacity param eters visualization. T he w ork on the system is in the process o f developm ent and verification. T he system is designed in such a w ay that any future application can easily be added to its environm ent.
C om puter aided evaluation o f intersection capacity in dense road netw ork 307
B IB LIO G R A PH Y
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Telematyka Systemów Transportowych, Ustroń 2001.
Reviewer: Prof. R om uald Szopa
