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.

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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Item Type: Ghanaian Topic  |  Size: 62 pages  |  Chapters: 1-5
Format: MS Word  |  Delivery: Within 30Mins.


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