Influence of intergreen times on the capacity of signalised intersections [Elektronische Ressource] / Axel Wolfermann

Axel Wolfermann

Informations

• Publié par: technischen_universitat_darmstadt
• Publié le: 01 janvier 2009
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 9 Mo

Extrait

Influence of Intergreen Times
on the Capacity of Signalised
Intersections
Dipl.-Ing. Axel Wolfermann
geboren in Wiesbaden
Fachgebiet
Verkehrsplanung
und
Verkehrstechnik
Chair of Transport
Planning and
Traffic Engineering
Prof. Dr.-Ing.
Manfred Boltze
Vom Fachbereich Bauingenieurwesen und Geodäsie der Technischen Universität Darmstadt
zur Erlangung des akademischen Grades eines Doktor-Ingenieurs genehmigte Dissertation
Referent: Prof. Dr.-Ing. Manfred Boltze
Korreferent: Prof. Dr.-Eng. Hideki Nakamura
Tag der Einreichung: 10. 09. 2009 Tag der mündlichen Prüfung: 12. 10. 2009
D17 Darmstadt 2009 Herausgeber:
Technische Universität Darmstadt
Fachgebiet Verkehrsplanung und Verkehrstechnik
Petersenstraße 30
64287 Darmstadt
www.tu-darmstadt.de/verkehr
[e-mail]
Schriftenreihe des Instituts für Verkehr
Fachgebiet Verkehrsplanung und Verkehrstechnik
Heft V 24
ISSN 1613-8317
Darmstadt 2009 Abstract
The quality of road traffic in urban networks is determined by the bottlenecks of the network, which are
intersections in most cases. Traffic signals can provide high safety and sufficient capacity, particularly
if conflicting streams have high volumes and the grade-separated junctions are not feasible. The signal
program has to ensure that different movements do not use the same areas inside of the intersection at
the same time. This is achieved by assigning different signal groups to conflicting movements and giving
the right of way (green time) to these signal groups only subsequently.
The critical moments are the signal change intervals during which one signal group (or stage) loses its
right of way, while another receives it. These intervals have to be long enough to make sure that all
clearing vehicles left the conflict areas before any entering vehicles arrive there. Because these intervals
are framed by the ending of the green interval of one stage and the beginning of green of the next, they
are called intergreen time.
While the principle of intergreen times is as old as traffic signals are, the question for the determination
of the duration of the times is still debated. A review of the international literature related
to intergreen times shows the issues at hand. While it is apparent that too short intergreen times lead
particularly to more right angle collisions, too long intergreen times can deteriorate the acceptance,
which not only leads to a decreased capacity, but to safety problems, too.
Conspicuously, the parameters used to calculate intergreen times around the world vary more than
differences in the characteristics of the traffic flow can account for. Particularly crossing times and
entering times are treated quite differently. Intergreen times are still not based on a sound safety model
which accounts for the random character of traffic flow. In this light, it seems inappropriate to justify
any capacity reductions connected to intergreen times with a perceived safety increase. The future safety
related research can be guided towards areas promising not only safety improvements, but an increase in
capacity at the same time by scrutinising the capacity impacts of intergreen times.
A major gap in the research has to be seen in the insufficient knowledge of the exact influence of inter-
green times on the capacity. Intergreen times are taken generally as lost times, while they are in fact
partly used by vehicles to cross the intersection. The duration of signal change intervals and the effective
capacity impacts depend on the intersection layout, the signal program, and the stage settings. This in-
fluence is not taken into account to full extent so far. The presented research provides the methodology
to close this gap.
Because empirical research on signal change intervals faces major difficulties due to the manifold influ-
ences on the driver behaviour, a sound theoretical analysis of all processes connected with intergreen
times is crucial. Consequently the emphasis in this research has been placed on such a comprehensive
theoretical analysis, which leads to a transparent and flexible model to calculate the capacity of signalised
intersections with reference to intergreen times. Empirical data has been gathered at seven urban inter-
sections in Germany using video observations and speed measurements to validate the applicability of the
model and obtain results on the quantitative capacity impacts of intergreen times.
The empirical research shows that effective green times at the surveyed intersections are in fact greater
than signalled green times. At an example intersection analysed as part of this research the effective
capacity is about 5 % greater than the capacity calculated with the effective saturation headways and
the signalled green times. It is 7 % greater than the capacity according to the German Highway Capacity
Manual (HBS). The U.S. Highway Capacity Manual (HCM), which is calibrated for traffic conditions in
the United States, leads to an even lower capacity.
iIntergreen times based on the prevailing calculation procedures are commonly longer than theoretically
required, because
• the parameter values used in these procedures differ from the theoretically needed ones,
• certain parameters are not considered at all in the prevailing procedures (e.g. the entering time),
• the decisive conflict for a stage or signal group sequence does not always occur, and
• safety margins are added.
The capacity improvement potential due to minimised intergreen times was quantified based on the em-
pirical data. While the quantitative results of the model application are based on a number of simplifica-
tions due to survey constraints, they nevertheless give a good indication on the improvement potential
in general. The findings of the model application can be summarised as follows:
• Conflicts leading to very long intergreen times are commonly of low relevance for the traffic flow
(turning traffic, bicycles). The difference between intergreen times for these conflicts and the ef-
fectively occurring conflicts are termed conflict difference times. Most of the improvement potential
(up to 50 %) stems from these conflict difference times.
• Particularly under saturated conditions and at non-coordinated approaches, significant entering
times can be observed. Neglecting them consequently leads to notable capacity reductions. About
a third of the capacity improvement potential stems from this fact.
• While certain parameters vary significantly among different situations, their variation is small at a
specific intersection. It can be concluded that it is worthwhile to analyse the influencing factors and
in this way be able to predict these parameters more precisely than so far. This would reduce the
requirement for great safety margins. Crossing times and clearance speeds have to be highlighted
in this context.
• The variation of certain parameters can be reduced by a sensible signal program and intersection
layout. Low variation needs small safety margins, which results in capacity improvements. By
indicating the impending signal change from red to green, for instance, start-up lost times can
be reduced. Furthermore, the interrelation of yellow time and crossing time of clearing vehicles
should be further researched.
The achievements of the presented research can be summarised by
• providing a comprehensive description of the traffic flow during signal change intervals,
• a transparent and flexible capacity model to determine the effective capacity and the
improvement potential of signalised intersections with reference to intergreen times, and
• highlighting aspects of intergreen times which lead to significant capacity reductions while either
no safety improvement can be seen or it remains vague and unproven.
This research not only presents a comprehensive analysis of the reasons for capacity reductions caused
by intergreen times, it, furthermore, gives a first impression on the magnitude of the improvement
potential. It concludes with recommendations, how this potential may be realised, and what further
research is needed to achieve this aim.
ii AbstractZusammenfassung
Die Qualität des straßengebundenen Stadtverkehrs wird durch Engstellen bestimmt, welche in aller
Regel die Knotenpunkte sind. Lichtsignalanlagen können hohe Sicherheit und ausreichende Qualität
sicherstellen, insbesondere wenn die Verkehrsstärken sehr groß sind und planfreie Kreuzungen nicht
umsetzbar sind. Das Signalprogramm muss sicherstellen, dass unterschiedliche Fahrzeugströme die gle-
ichen Flächen im Knotenpunkt nicht zur gleichen Zeit benutzen. Dies wird erreicht, indem Ströme, die
miteinander im Konflikt stehen, unterschiedlichen Signalgruppen zugewiesen werden, denen nacheinan-
der die Freigabe erteilt wird.
Die kritischen Momente im Verkehrsablauf sind die Phasenübergänge zwischen dem Ende der Freigabe
einer Signalgruppe (oder Phase) und dem Beginn der Freigabe einer anderen. Diese Intervalle müssen
lang genug sein, um den räumenden Fahrzeugen das Verlassen aller Konfliktflächen zu ermöglichen,
bevor einfahrende Fahrzeuge dort ankom