ABSTRACT
Increasingly, many roundabouts in Ghanaian cities are
becoming problematic due to capacity challenges resulting from rapid traffic
growth. Signalisation is known to improve capacity, shorten queue length, and
reduce delay at roundabouts but this option has not been used in Ghana. In this
study, three model options for signalising roundabouts, namely
Approach-by-Approach Control, Metered Approach, and Full Signalisation, were
explored for analysis and comparison of their effects on roundabout in Kumasi
as a case study. The aim of the study was to establish the model option that
best addresses traffic problems at the Suame Roundabout in terms of improved
capacity, reduced delays and queue lengths. Field traffic volume studies for
the entire roundabout’s approaches were performed. Travel time and delay
studies together with queue lengths measurement for the subject approach (South
East Approach) were also undertaken. Geometric data as well as data from the
field study described earlier were used to calibrate a model for the existing
situation. Capacity, delay, queue length and degree of saturation were observed
for the signalised options and existing un-signalised. The results indicated
that the Full Signalisation Model produced the best parameter results and the
Approach-by-Approach Model the worst among all the models including the
calibrated existing model. Full signalisation of the Suame roundabout is
recommended to improve capacity and reduce vehicular delays and queue lengths.
However, under budgetary constraints, the Metered Approach option may provide
more consistency in operations at the roundabout as compared to the current
situation where movements at peak periods are traffic-warden controlled.
CHAPTER 1: INTRODUCTION
1.1 Background
Increasingly, many roundabouts in
Ghanaian cities are becoming problematic due to capacity challenges resulting
from rapid traffic growth. As traffic grow and congestion continue to outpace
the capacity of the road network the roundabouts will remain bottlenecks.
Therefore the need to find ways of improving the capacity and performance of
roundabouts cannot be overemphasised. Significantly, such improvement will go a
long way to enhance flow and safety of an entire road network.
A Roundabout is a type of
intersection which primarily serves as a location within the road network for
change of direction. In order to increase capacity, improve delay, safety and
other performance parameters of roundabouts, engineers in the United Kingdom
introduced the concept of signalizing roundabouts. Signalisation of a
roundabout can be described as the use of a traffic signal system to control
traffic flow at a roundabout.
The idea of combining traffic
signals and roundabout to improve capacity and performance may sound
contradictory to many and therefore researching into such a concept with
simulation could not have been any more appropriate.
Thus, this study focused on the
effect of signalisation on roundabout capacity and other measures of
effectiveness such as delay and queues.
1.2 Problem Statement
In Ghana, when the capacity of a
roundabout is exceeded or the performance parameters become poor on more than
one of the approaches, it is common to find any of the following interventions
being adopted to solve the problem especially when there are more than four
legs:
Police personnel or traffic wardens
or private individuals controlling movement of traffic, which is inefficient,
labour intensive and unreliable
Change of control by replacing with
a traffic signal and change of intersection layout.
Grade-separation, which is usually
expensive.
These three measures have differing
costs and timelines to come on stream. Police personnel can be deployed and
often redeployed even when traffic is still very high. Additionally, in bad
weather and situations where the lighting is poor it is unsafe to deploy Police
personnel or traffic wardens. Controlling roundabouts by this method is
associated with long queues and delays because it is too arbitrary. Figure 1.1
below shows two Police personnel controlling movement of traffic at the Suame
Roundabout.
Figure 1.1. Two Police Personnel
Controlling Traffic Movement at Suame Roundabout
In the case of traffic signal
control and grade separation, huge civil and infrastructure construction is
involved. When signals are used, accidents may increase, also, poor light and unreliable power sources are
some of the challenge to signalisation. The situation is compounded when the
number of legs exceed four as is the case of Suame. Interchanges are very expensive
and take very long time to come in stream. Suame has had proposed interchange
design for over a decade. Road users continue to suffer delays and frustrations
resulting from queuing traffic. The environmental condition brought about by
the pollution from vehicles is terrible. The intersection suffers from gridlock
making queues block intersection within 1km of the approaches. In the medium
term, an intervention must be initiated to ease the congestion and improve
throughput especially at the AM peak and PM peak conditions.
1.3 Research Objectives
* To model and simulate the existing
traffic situation at the roundabout and assess its capacity, delay and queue
lengths with a calibrated VISSIM software.
* To investigate various signalised
roundabout options to improve throughput
* Study the effect of signalisation
on queuing delay and queue length of traffic
1.4 Justification of the Study
It is rare to find research on
signalised roundabouts in Ghana. This study will therefore contribute
significantly to signalisation of roundabout design and implementation in the
following ways:
This study will reveal more
efficient ways of controlling traffic in saturated conditions at roundabouts as
compared to the deployment of Police personnel or Traffic wardens to undertake
same activity.
The research will also show a
relatively less expensive means of improving short term capacity and delay at
roundabout as compared to grade-separation. Conclusions and recommendation from this research
will be useful in subsequent studies and inform policy direction regarding
roundabouts and signalisation.
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