Richard Chitehwe R142829W HSVG Dissertation topic Assessment of the relationship between traffic congestion and road designs on main entrance and exit points of cities
Richard Chitehwe
R142829W
HSVG
Dissertation topic
Assessment of the relationship between traffic congestion and road designs on main entrance and exit points of cities: Case study of Harare (Mbudzi Round-About).
Chapter 1
1.1Introduction
Traffic congestion has become a menace in the roads of Harare especially on the main entrance and exit points of the Central Business District (CBD). Traffic congestion means there are more vehicles trying to use a given road facility than it can handle ( Kumarage,2004). Some hours of the day are characterized by long queues of vehicles which are either entering the city or exiting the city. There is a high increment in the number of road users that is more motorists over the past years due to the population growth in Harare. The emerging of traffic congestion means that vehicles are spending more time on a particular place rather than moving.
Time is being wasted when one is caught up in traffic congestion, hence route planning and road modifications can drastically reduce the time it takes to plan transportation schedules for every motorists. Not only do route planning and road modifications lower mileage but they also help cut fuel usage, decrease carbon emissions, customer service and accidents. Road needs have been strongly influenced by its demand and also by the mass movement of people and vehicles to cities. This can be shown by the volume of traffic at the Mbudzi Roundabout which has become an area known for traffic congestion. Roundabouts have been widely accepted and used in countries for decades.
A roundabout is a form of intersection design and control that accommodates traffic flow in one direction around a central island. Compared to uncontrolled intersection, roundabout can reduce speed as well as number of conflict points because of the geometrical design ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1016/j.sbspro.2012.04.088″,”ISSN”:”1877-0428″,”author”:{“dropping-particle”:””,”family”:”Wang”,”given”:”Weiqi”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Yang”,”given”:”Xiaokuan”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”8610″,”issued”:{“date-parts”:”2012″},”page”:”157-168″,”title”:”Research on Capacity of Roundabouts in Beijing”,”type”:”article-journal”,”volume”:”43″},”uris”:”http://www.mendeley.com/documents/?uuid=0d687393-26cd-4a99-8b99-f933d5020cb2″},”mendeley”:{“formattedCitation”:”(Wang and Yang, 2012)”,”plainTextFormattedCitation”:”(Wang and Yang, 2012)”,”previouslyFormattedCitation”:”(Wang and Yang, 2012)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Wang and Yang, 2012). A roundabout can also be defined as a form of intersection design and control which accommodates traffic flow in one direction around a central island and gives priority to vehicles within the roundabout (circulation flow) ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1103/PhysRevE.70.046132″,”author”:{“dropping-particle”:””,”family”:”Fouladvand”,”given”:”M Ebrahim”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Sadjadi”,”given”:”Zeinab”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Shaebani”,”given”:”M Reza”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”January”,”issued”:{“date-parts”:”2004″},”page”:”1-8″,”title”:”Characteristics of vehicular traffic flow at a roundabout”,”type”:”article-journal”,”volume”:”046132″},”uris”:”http://www.mendeley.com/documents/?uuid=cc617274-7bb1-4510-8c8f-ba7f4b0cd8c3″},”mendeley”:{“formattedCitation”:”(Fouladvand, Sadjadi and Shaebani, 2004)”,”plainTextFormattedCitation”:”(Fouladvand, Sadjadi and Shaebani, 2004)”,”previouslyFormattedCitation”:”(Fouladvand, Sadjadi and Shaebani, 2004)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Fouladvand, Sadjadi and Shaebani, 2004).
There is high volume of moving haulage trucks and busses using the A4 route also known as the Harare-Beitbridge road, also there is high volume of small vehicles entering and existing the City Centre and other public transport. It is this trend that has led to increasing travel needs and traffic congestion. The construction of new modern roads to curb such problems will result in a more efficient transportation system and more road travel. Ideally, the development of a major road system is an orderly, continuous process. The process follows several steps: assessing road needs and transport options; planning a system to meet those needs; designing an economically, socially, and environmentally acceptable set of roads; obtaining the required approval and financing; building, operating, and maintaining the system; and providing for future extensions and reconstruction ( Alberto Bull, 2003).
Traffic congestion is mainly observed around intersections and roundabouts in urban areas. Particularly, it becomes severe during peak hours ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Chimdessa”,”given”:”Yadeta”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Kassa”,”given”:”Semu M”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Lemecha”,”given”:”Legesse”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”2″,”issued”:{“date-parts”:”0″},”page”:”81-100″,”title”:”Efficiency of Roundabouts as Compared to Traffic Light Controlled Intersections in Urban Road Networks”,”type”:”article-journal”,”volume”:”5″},”uris”:”http://www.mendeley.com/documents/?uuid=27ec36f6-db4a-4e49-9240-1c711aaf7576″},”mendeley”:{“formattedCitation”:”(Chimdessa, Kassa and Lemecha, no date)”,”plainTextFormattedCitation”:”(Chimdessa, Kassa and Lemecha, no date)”,”previouslyFormattedCitation”:”(Chimdessa, Kassa and Lemecha, no date)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Chimdessa, Kassa and Lemecha, no date). The present study tries to provide an insight into ways of coming up with more comprehensive and innovative measures to mitigate traffic congestion and its impact of new developments in the city of Harare. This research is divided into various sections where there is a review of the current traffic impact mitigation process and its results. An analysis of other mitigation measures such as modern road designs and its expected results together with an examination of their effectiveness. Again an analysis of how others have done it and their results is to be examined.
The performance of single and double lane un-signalized, signalized and roundabout intersections have been investigated in Kakooza et al. (2005) in terms of expected number of vehicle and waiting time using simple mathematical modeling. Signalized roundabout means a roundabout which is controlled by a traffic light at the entrance of each arm, whereas un-signalized roundabout is governed by the give-way rule. In the same paper the authors concluded that the performance of signalized intersections is superior under heavy entering traffic volumes while the roundabout performs better under light entering traffic volume in terms of delays. However, no clear-cut was given to qualify what “heavy” and “light” entering traffic volume means. On the other hand, the model does not compare the operational performance of signalized roundabouts, unsignalized roundabouts and signalized intersections under similar traffic density ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Chimdessa”,”given”:”Yadeta”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Kassa”,”given”:”Semu M”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Lemecha”,”given”:”Legesse”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”2″,”issued”:{“date-parts”:”0″},”page”:”81-100″,”title”:”Efficiency of Roundabouts as Compared to Traffic Light Controlled Intersections in Urban Road Networks”,”type”:”article-journal”,”volume”:”5″},”uris”:”http://www.mendeley.com/documents/?uuid=27ec36f6-db4a-4e49-9240-1c711aaf7576″},”mendeley”:{“formattedCitation”:”(Chimdessa, Kassa and Lemecha, no date)”,”plainTextFormattedCitation”:”(Chimdessa, Kassa and Lemecha, no date)”,”previouslyFormattedCitation”:”(Chimdessa, Kassa and Lemecha, no date)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Chimdessa, Kassa and Lemecha, no date).
1.2Problem statement
Faulty design or maintenance of road systems causes unnecessary congestionADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“ISBN”:”9211214327″,”author”:{“dropping-particle”:””,”family”:”Libros”,”given”:”Cepal”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”0″},”title”:”Gracias por su interés en esta publicación de la CEPAL”,”type”:”book”},”uris”:”http://www.mendeley.com/documents/?uuid=3387d730-212b-481a-b500-84964a71da03″},”mendeley”:{“formattedCitation”:”(Libros, no date)”,”manualFormatting”:”(Libros, 2003.)”,”plainTextFormattedCitation”:”(Libros, no date)”,”previouslyFormattedCitation”:”(Libros, no date)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Libros, 2003.).The city of Harare has grown in its population size over the past years due to the continuous economic activities and population migration which has resulted in an increase in the use of automobiles. The high use of automobiles has resulted in traffic congestion on the commonly used Mbudzi Roundabout where one can spend hours at this particular place. The mere thought that one has to pass through this roundabout is stressful as some destinations have no other alternative routes but to pass through Mbudzi roundabout.
Mbudzi roundabout is a two- lane roundabout and ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Macioszek”,”given”:”El?bieta”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2013″},”page”:”97-98″,”title”:”Analysis of the Effect of Congestion in the Lanes at the Inlet to the Two-Lane Roundabout on Traffic The Use of Traffic Lanes at the Inlets to Multi-lane”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=02386b95-c557-40e9-9eeb-4ec285f78a95″},”mendeley”:{“formattedCitation”:”(Macioszek, 2013)”,”plainTextFormattedCitation”:”(Macioszek, 2013)”,”previouslyFormattedCitation”:”(Macioszek, 2013)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Macioszek, 2013) suggests that two-lane roundabouts are not entirely perfect solutions to ease traffic congestion since substantial number of drivers use chiefly the outer lane and in our case the left lane, both at the inlets to the roundabouts and on the roadway, even with high values of traffic intensity in the area of the intersection. This however has resulted in traffic congestion on times known as peak hours as there is the most traffic, but not only during these times where traffic congestion starts but there is also a time where traffic congestion arises due to poor road designs hence there is a need to analyze if the existing road design is satisfying the current needs of road users. Roundabouts which are congested due to poor capacity will instead increase the emissions and lead to congestion instead. Occurrence of unwarranted delays, queue formation and speed change cycles for approaching traffic have a significant impact on congestion and air quality in the adjacent areasADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Deshmukh”,”given”:”Anirban”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”8″,”issued”:{“date-parts”:”2017″},”page”:”452-462″,”title”:”The effects of vehicle traffic flow characteristics on capacity at Rotary in Bhubaneswar city ( India )”,”type”:”article-journal”,”volume”:”3″},”uris”:”http://www.mendeley.com/documents/?uuid=8e3c1868-7611-4395-9db5-9ef9b9e0a19a”},”mendeley”:{“formattedCitation”:”(Deshmukh, 2017)”,”plainTextFormattedCitation”:”(Deshmukh, 2017)”,”previouslyFormattedCitation”:”(Deshmukh, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Deshmukh, 2017).
Mudzengere and Madiro (2013) uses mathematical models to predict traffic flows and also A.Virukai (2016) uses GIS to analyse traffic flow and maps areas with traffic congestion but fails to give solutions on ways on which how to decongest traffic on intersections such as roundabouts. There are methods of interest used such as the the volume to capacity ratio discussed by A.Virukai (2016) which the research is going to use so as to prove that there is traffic congestion on the Mbudzi Roundabout.
1.2Justification
The purpose of this research is to review whether the current roundabout known as Mbudzi Roundabout is meeting the current needs of road users and to confirm if it is built to the expected standards, if not a proposal of an alternative road design is to be proposed which will ease the congestion occurring. The research tries to evaluate whether the existing road designs still meet the demands of the road users and ensuring a smooth traffic flow since traffic flow has increased in the city of Harare. There is a need to understand whether road designs influence the formation of traffic congestion. This will however give the student a more understanding on the engineering part of the program Surveying and Geomatics under road construction and road designing.
Traffic congestion along the highway inconveniences commuters in accessing the CBD, leading to wastage of time and money. A good road with smooth traffic flow reduces travel time and encourages investments. At the same time an effective transportation system helps to maximize the economic efficiency of the city, while an inferior system retards economic progress ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kibunja”,”given”:”Njoroge Joseph”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2009″},”title”:”TRAFFIC CONGESTION PROBLEMS IN NAIROBI : AN EXAMINATION OF”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=dba3161d-c5c1-45b4-8734-63b4b47974e6″},”mendeley”:{“formattedCitation”:”(Kibunja, 2009)”,”plainTextFormattedCitation”:”(Kibunja, 2009)”,”previouslyFormattedCitation”:”(Kibunja, 2009)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kibunja, 2009). The study tries to identify the traffic problems experienced at the Mbudzi roundabout with a view of coming up with possible solutions to the problems using modern road designs which can ease the problem of traffic congestion.
1.3Research problem
What is the relationship between traffic congestion and existing road designs especially on the Harare-Beitbridge entrance point into Harare (Mbudzi Roundabout)?
1.4Research objective
To evaluate the relationship between traffic congestion and existing road designs on Mbudzi Roundabout in Harare.
1.5Specific Research Questions
Is there any relationship between traffic congestion and existing road designs on entrance points into the city of Harare?
Will modern road designs curb the problem of traffic congestion on entry points of the city?
1.6Specific objectives
To evaluate existing road designs against modern road designs in decongesting entry points into the city.
Decongestion of roads on main entrance and exit points of cities (Harare)
To improve traffic flow in and out of the city.
1.7Research methodology
Data collection
Primary data collection
Secondary data collection
Review current road design (Mbudzi Roundabout)
measure the dimensions of the existing roundabout and compare with the dimensions that are on the paper
Assess the roundabout’s effectiveness
Check the relationship between traffic congestion and the road design (Mbudzi Roundabout)
Measure traffic congestion on the existing road design (Mbudzi Roundabout)
Review modern road designs
Propose alternative road designs to ease traffic congestion
Assess its effectiveness
Deliverables
Smoothen the traffic flow
Decongestion of Mbudzi Roundabout
Primary data is to be collected through observations at different times, the times to be used will be based on those justified by A.Virukai (2016) which where termed as pick hours and also on some other times. Secondary data such as the dimensions of the existing round about and also the volume of traffic expected to pass through the intersection will be collected at the ministry of transport under the department of roads. Observations are to be done on the road users, the current road network, the land use at Mbudzi and also the traffic management system. To come up with the relationship between traffic congestion and the current road design there is need to analyse traffic volume and road capacity, road conditions, design and road network, road user behavior and traffic flow/speed
Regression analysis has been made to calculate the relationship between the circulating flow and the entry flow ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1016/j.sbspro.2012.04.088″,”ISSN”:”1877-0428″,”author”:{“dropping-particle”:””,”family”:”Wang”,”given”:”Weiqi”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Yang”,”given”:”Xiaokuan”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”8610″,”issued”:{“date-parts”:”2012″},”page”:”157-168″,”title”:”Research on Capacity of Roundabouts in Beijing”,”type”:”article-journal”,”volume”:”43″},”uris”:”http://www.mendeley.com/documents/?uuid=0d687393-26cd-4a99-8b99-f933d5020cb2″},”mendeley”:{“formattedCitation”:”(Wang and Yang, 2012)”,”plainTextFormattedCitation”:”(Wang and Yang, 2012)”,”previouslyFormattedCitation”:”(Wang and Yang, 2012)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Wang and Yang, 2012). The regression model develops regression relationships between circulating flow rate and the capacity at each entrance approach, using capacity as the dependent variable and circulating flow rate as the independent variable ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1016/j.jtte.2016.03.005″,”ISSN”:”2095-7564″,”author”:{“dropping-particle”:””,”family”:”Chen”,”given”:”Xuanwu”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Lee”,”given”:”Ming S”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”container-title”:”Journal of Traffic and Transportation Engineering (English Edition)”,”id”:”ITEM-1″,”issue”:”2″,”issued”:{“date-parts”:”2016″},”page”:”154-165″,”publisher”:”Elsevier Ltd”,”title”:”ScienceDirect A case study on multi-lane roundabouts under congestion : Comparing software capacity and delay estimates with field data”,”type”:”article-journal”,”volume”:”3″},”uris”:”http://www.mendeley.com/documents/?uuid=d937ff1a-351c-4ff5-b18a-f5f80da35419″},”mendeley”:{“formattedCitation”:”(Chen and Lee, 2016)”,”plainTextFormattedCitation”:”(Chen and Lee, 2016)”,”previouslyFormattedCitation”:”(Chen and Lee, 2016)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Chen and Lee, 2016).
Study Area
Fig. 1 Aerial view of Mbudzi Roundabout. Source: Google Map 20.09.2018
Mbudzi roundabout is an intersection with four roads off to Masvingo. Chitungwiza, Glen Norah and City Center (Harare). The roundabout is located at 17°55’13″South latitude and 30°59’6″East longitude. This area is mainly used by busses, haulage trucks, cars, commuter omnibuses and other vehicles which want to enter the CBD or travel to the nearest neighbourhood.
1.8Summary
Traffic congestion is a state where there are more vehicles using a particular road than the road can handle. Traffic congestion can be attributed by poor road designs or road designs which are built upon poor standards. The research aims to review whether the Mbudzi Round about was built according the specified standards by comparing the standards or measurements at the ministry of transport, department of roads and the measurements which I am going to take and compare if they are the same. There is also a need to review if the size of the roundabout is compatible with the volume of traffic passing through it. If the roundabout comes out to be of the correct size and compatible with the volume of traffic then there is a need to investigate further on the reason of traffic congestion, but if the results are negative then there is a need to propose another way of regulating traffic using modern road designs which is expected to reduce traffic congestion.
This is important to research because there is need to maintain a smooth traffic flow, cut fuel usages and also improving customer services. Traffic congestions are also a source of accidents hence they all have to be reduced and eradicated if possible. A properly designed transport network system boosts economic activities.
Chapter 2
Literature Review
2.1Introduction
Roundabouts have been widely accepted and used in countries for decades. A roundabout is a form of intersection design and control that accommodates traffic flow in one direction around a central island ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1016/j.sbspro.2012.04.088″,”ISSN”:”1877-0428″,”author”:{“dropping-particle”:””,”family”:”Wang”,”given”:”Weiqi”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Yang”,”given”:”Xiaokuan”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”8610″,”issued”:{“date-parts”:”2012″},”page”:”157-168″,”title”:”Research on Capacity of Roundabouts in Beijing”,”type”:”article-journal”,”volume”:”43″},”uris”:”http://www.mendeley.com/documents/?uuid=0d687393-26cd-4a99-8b99-f933d5020cb2″},”mendeley”:{“formattedCitation”:”(Wang and Yang, 2012)”,”plainTextFormattedCitation”:”(Wang and Yang, 2012)”,”previouslyFormattedCitation”:”(Wang and Yang, 2012)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Wang and Yang, 2012). Roundabouts have the ability to reduce speeds of traffic and the number of conflicting points because of the geometrical design of it. Currently in Harare Traffic lights, roundabouts, give way signs and other intersections are used to ease traffic congestion. Time has evolved and modern ways such as grade separated roads are being used as a measure to reduce congestion as well as modifying the existing road designs such as roundabouts so as to curb traffic congestion.
2.2What is traffic congestion and how it is formed?
Traffic congestion is a temporal condition on networks that occurs as utility increases, and is characterized by slower speeds, longer trip times, and increased queuing ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Lund”,”given”:”Jay R”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Craig”,”given”:”Paul P”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2009″},”page”:”7-19″,”title”:”Chapter-2″,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=d1fb7bcf-16c3-4a36-ae08-2add91bb77b7″},”mendeley”:{“formattedCitation”:”(Lund and Craig, 2009)”,”plainTextFormattedCitation”:”(Lund and Craig, 2009)”,”previouslyFormattedCitation”:”(Lund and Craig, 2009)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Lund and Craig, 2009). When volume of traffic increases and so heterogeneous that the interaction between vehicles slows down the speed of traffic, traffic congestion is the result. As demand approaches the capacity of a roundabout traffic congestion sets in.
A common feature across road networks in many urban regions in the developing world is the presence of critical congestion areas; we refer to a critical congestion area as one where a network of roads converge and a large amount of traffic needs to traverse the common congestion area ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“ISBN”:”9781450312622″,”author”:{“dropping-particle”:””,”family”:”Jain”,”given”:”Vipin”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Sharma”,”given”:”Ashlesh”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”0″},”title”:”Road Traffic Congestion in the Developing World”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=cd1150df-895e-4d12-a8d0-8e71c8a6339c”},”mendeley”:{“formattedCitation”:”(Jain and Sharma, no date)”,”plainTextFormattedCitation”:”(Jain and Sharma, no date)”,”previouslyFormattedCitation”:”(Jain and Sharma, no date)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Jain and Sharma, no date). Currently in Harare traffic congestion is being regulated by traffic lights, roundabouts, give way signs and sometimes the traffic police tries to regulate traffic congestion but it is not enough when curbing traffic congestion.
The fundamental cause of congestion is the friction or mutual interference between vehicles in the traffic flow. Up to a certain level of traffic, vehicles can circulate at a relatively freely determined speed which depends on the legal speed limit, the frequency of intersections, and other conditioning factors. At higher levels of traffic, however, every additional vehicle interferes with the circulation of the others: in other words, the phenomenon of congestion appear ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“ISBN”:”9211214327″,”author”:{“dropping-particle”:””,”family”:”Libros”,”given”:”Cepal”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”0″},”title”:”Gracias por su interés en esta publicación de la CEPAL”,”type”:”book”},”uris”:”http://www.mendeley.com/documents/?uuid=3387d730-212b-481a-b500-84964a71da03″},”mendeley”:{“formattedCitation”:”(Libros, no date)”,”manualFormatting”:”(Libros, 2003)”,”plainTextFormattedCitation”:”(Libros, no date)”,”previouslyFormattedCitation”:”(Libros, no date)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Libros, 2003).
Vehicles of different size and mass have different operating characteristics that must be taken into consideration in the design. Besides being heavier, lorries are generally slower and occupy more road space and consequently impose greater effect than passenger vehicles ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kibunja”,”given”:”Njoroge Joseph”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2009″},”title”:”TRAFFIC CONGESTION PROBLEMS IN NAIROBI : AN EXAMINATION OF”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=dba3161d-c5c1-45b4-8734-63b4b47974e6″},”mendeley”:{“formattedCitation”:”(Kibunja, 2009)”,”plainTextFormattedCitation”:”(Kibunja, 2009)”,”previouslyFormattedCitation”:”(Kibunja, 2009)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kibunja, 2009). Also Kibunja (2009) also suggests that variations in truck movements at intersections may be substantial and will influence the geometry of the layout hence they need to be considered when coming up with road designs such as roundabouts. The consequence of factors that cause regular demand surges on the transportation system, such as commuting, shopping or weekend trips.
However, even recurrent congestion can have unforeseen impacts in terms of its duration and severity. Mandatory trips are mainly responsible for the peaks in circulation flows, implying that about half the congestion in urban areas are recurring at specific times of the day and on specific segments of the transport system ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kibunja”,”given”:”Njoroge Joseph”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2009″},”title”:”TRAFFIC CONGESTION PROBLEMS IN NAIROBI : AN EXAMINATION OF”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=dba3161d-c5c1-45b4-8734-63b4b47974e6″},”mendeley”:{“formattedCitation”:”(Kibunja, 2009)”,”plainTextFormattedCitation”:”(Kibunja, 2009)”,”previouslyFormattedCitation”:”(Kibunja, 2009)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kibunja, 2009). Among others congestion can be caused by insufficient capacity of an intersection such as roundabouts and faulty designs which cannot accommodate the required number of vehicles over a certain interval.
2.2Roundabouts and their characteristics
A roundabout is a channelized at grade intersection at which all traffic moves clockwise around a central traffic island ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Manual”,”given”:”Road Planning and Design”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”January”,”issued”:{“date-parts”:”2006″},”title”:”Chapter 14″,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=fa21bf01-87c5-4125-a469-a1145ba18d82″},”mendeley”:{“formattedCitation”:”(Manual, 2006)”,”plainTextFormattedCitation”:”(Manual, 2006)”,”previouslyFormattedCitation”:”(Manual, 2006)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Manual, 2006). The same Manual (2006) suggests that roundabouts perform better at the intersection of roads with roughly similar traffic flows and a high proportion of left turning traffic. More recently, modern roundabouts have been embraced as a standard alternative design to conventional signalized intersections ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Buasali”,”given”:”Ahmed”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2017″},”title”:”Broadening Understanding of Roundabout Operation Analysis : Planning-Level Tools and Signal Application”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=c439de2a-d72d-49eb-98e9-33d9802cbc6f”},”mendeley”:{“formattedCitation”:”(Buasali, 2017)”,”plainTextFormattedCitation”:”(Buasali, 2017)”,”previouslyFormattedCitation”:”(Buasali, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Buasali, 2017). Roundabouts have a primary function of regulating traffic and Buasali (2017) states that the geometric design of the roundabout uses the first come first serve approach and forces drivers to slow down while navigating the facility. The roundabout size, lane width and number of lanes, position, and alignment and arrangement of approach legs are some of the principal elements of a roundabout.
Fig 2. General Roundabout Features. Source ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Island”,”given”:”Central”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Roadway”,”given”:”Circulatory”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Line”,”given”:”Entrance”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”0″},”page”:”1-19″,”title”:”7.10.1 Definitions”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=1a26091c-1d8d-457d-8081-eb847c96b850″},”mendeley”:{“formattedCitation”:”(Island, Roadway and Line, no date)”,”plainTextFormattedCitation”:”(Island, Roadway and Line, no date)”,”previouslyFormattedCitation”:”(Island, Roadway and Line, no date)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Island, Roadway and Line, no date) and modified.
2.3Types of roundabouts used to ease congestion
2.3.1Multilane Roundabouts
Multilane roundabouts have at least one entry or exit with two or more lanes and more than one circulating lane. The operational practice for trucks negotiating roundabouts is to straddle adjacent lanes ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Design”,”given”:”Wsdot”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”July”,”issued”:{“date-parts”:”2018″},”page”:”1-26″,”title”:”Chapter 1320 – Roundabouts – Design Manual M 22-01″,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=e9333ed0-e052-4f53-a274-f1f4959983b8″},”mendeley”:{“formattedCitation”:”(Design, 2018)”,”plainTextFormattedCitation”:”(Design, 2018)”,”previouslyFormattedCitation”:”(Design, 2018)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Design, 2018). At multi-lane roundabouts, the width of lanes within the circulatory roadway is usually governed by light trucks and not the design vehicle’s overhang while turning, as the design vehicle is assumed to utilize the mountable curb and adjacent area to perform their maneuver ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Buasali”,”given”:”Ahmed”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2017″},”title”:”Broadening Understanding of Roundabout Operation Analysis : Planning-Level Tools and Signal Application”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=c439de2a-d72d-49eb-98e9-33d9802cbc6f”},”mendeley”:{“formattedCitation”:”(Buasali, 2017)”,”plainTextFormattedCitation”:”(Buasali, 2017)”,”previouslyFormattedCitation”:”(Buasali, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Buasali, 2017). According to Buasali (2017), the width which is required for the necessary number of adjacent lanes for vehicles to travel simultaneously through the roundabout should be used to establish the circulatory roadway width so as to minimize conflicting points.
Fig: Example of a two-lane roundabout, Source ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Buasali”,”given”:”Ahmed”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2017″},”title”:”Broadening Understanding of Roundabout Operation Analysis : Planning-Level Tools and Signal Application”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=c439de2a-d72d-49eb-98e9-33d9802cbc6f”},”mendeley”:{“formattedCitation”:”(Buasali, 2017)”,”plainTextFormattedCitation”:”(Buasali, 2017)”,”previouslyFormattedCitation”:”(Buasali, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Buasali, 2017)
Fig 3: Example of a three-lane roundabout, Source ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Buasali”,”given”:”Ahmed”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2017″},”title”:”Broadening Understanding of Roundabout Operation Analysis : Planning-Level Tools and Signal Application”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=c439de2a-d72d-49eb-98e9-33d9802cbc6f”},”mendeley”:{“formattedCitation”:”(Buasali, 2017)”,”plainTextFormattedCitation”:”(Buasali, 2017)”,”previouslyFormattedCitation”:”(Buasali, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Buasali, 2017)
2.3.2Teardrop Roundabout
Teardrops are usually associated with ramp terminals at interchanges: typically, at diamond interchanges. Teardrop roundabouts allow the “wide node, narrow link” concept ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Design”,”given”:”Wsdot”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”July”,”issued”:{“date-parts”:”2018″},”page”:”1-26″,”title”:”Chapter 1320 – Roundabouts – Design Manual M 22-01″,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=e9333ed0-e052-4f53-a274-f1f4959983b8″},”mendeley”:{“formattedCitation”:”(Design, 2018)”,”plainTextFormattedCitation”:”(Design, 2018)”,”previouslyFormattedCitation”:”(Design, 2018)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Design, 2018) . Unlike circular roundabouts, Design (2018) states that teardrops do not allow for continuous 360° travel resulting in less vehicle conflicts as traffic traveling on the crossroad (link) between ramp terminal intersections (nodes) does not encounter a yield as it enters the teardrop intersections. They provide free-flowing turns to the onramps and eliminate the need for turn signals and lanes. They also prevent queuing on narrow two-lane bridges ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.4038/engineer.v49i4.7237″,”author”:{“dropping-particle”:””,”family”:”Mampearachchi”,”given”:”Wasantha”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”November”,”issued”:{“date-parts”:”2017″},”title”:”Review of Roundabout Design Standards and the Development of a Roundabout Design Guideline for Sri Lanka”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=330a7789-d8cc-4b61-b59b-2a260642f66e”},”mendeley”:{“formattedCitation”:”(Mampearachchi, 2017)”,”plainTextFormattedCitation”:”(Mampearachchi, 2017)”,”previouslyFormattedCitation”:”(Mampearachchi, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Mampearachchi, 2017).
2.3.3Turbo Roundabouts
Turbo roundabouts provide a forced spiraling flow of traffic, requiring motorists to choose their direction of travel before entering the roundabout ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.4038/engineer.v49i4.7237″,”author”:{“dropping-particle”:””,”family”:”Mampearachchi”,”given”:”Wasantha”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”November”,”issued”:{“date-parts”:”2017″},”title”:”Review of Roundabout Design Standards and the Development of a Roundabout Design Guideline for Sri Lanka”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=330a7789-d8cc-4b61-b59b-2a260642f66e”},”mendeley”:{“formattedCitation”:”(Mampearachchi, 2017)”,”plainTextFormattedCitation”:”(Mampearachchi, 2017)”,”previouslyFormattedCitation”:”(Mampearachchi, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Mampearachchi, 2017). This type of roundabout eliminates many conflicting paths and choices at the roundabout itself, Mampearachchi (2017) points out that traffic safety as well as speed and capacity is increased and also a turbo roundabout will allow U-turns only from two directions.
2.3.4Roundabout Signalization
When an unbalanced flow occurs with a heavy flow it increases delay time at an intersection and in such situations, signals can be used to initiate gaps in the traffic flow and hence balance the capacity ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Mahmood”,”given”:”Kashif”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”August”,”issued”:{“date-parts”:”2016″},”title”:”IMPROVING ROUNDABOUT PERFORMANCE USING ADAPTIVE METERING TECHNIQUE”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=b71b310a-8983-4042-acd6-951c5e84ff7d”},”mendeley”:{“formattedCitation”:”(Mahmood, 2016)”,”plainTextFormattedCitation”:”(Mahmood, 2016)”,”previouslyFormattedCitation”:”(Mahmood, 2016)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Mahmood, 2016). Signalization alters the functionality of a roundabout, they may provide solutions in situations such as an unbalanced flow of traffic (Stevens, 2005). (Stevens, 2005), argues that the benefits brought by signalization of roundabouts can provide shorter delays, reduced traffic queue lengths, an increase in capacity and safety. The roundabouts still have many benefits over the traffic signals; roundabouts have fewer conflict points thus reducing collisions ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Mahmood”,”given”:”Kashif”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”August”,”issued”:{“date-parts”:”2016″},”title”:”IMPROVING ROUNDABOUT PERFORMANCE USING ADAPTIVE METERING TECHNIQUE”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=b71b310a-8983-4042-acd6-951c5e84ff7d”},”mendeley”:{“formattedCitation”:”(Mahmood, 2016)”,”plainTextFormattedCitation”:”(Mahmood, 2016)”,”previouslyFormattedCitation”:”(Mahmood, 2016)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Mahmood, 2016). A case study of a roundabout in Sheffield, U.K., found that the introduction of traffic signals turned out to be the most effective and economical solution for sharing out or balancing the entry flows between the approachesADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Yang”,”given”:”Xiaoguang”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Li”,”given”:”Xiugang”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Xue”,”given”:”Kun”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”4″,”issued”:{“date-parts”:”2004″},”page”:”282-287″,”title”:”Technical Correspondences __________________________________________________ A New Traffic-Signal Control for Modern Roundabouts : Method and Application”,”type”:”article-journal”,”volume”:”5″},”uris”:”http://www.mendeley.com/documents/?uuid=38ca5a42-bd84-484b-adba-82bc8530820f”},”mendeley”:{“formattedCitation”:”(Yang, Li and Xue, 2004)”,”plainTextFormattedCitation”:”(Yang, Li and Xue, 2004)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Yang, Li and Xue, 2004)2.4How roundabouts contribute to congestion
According to ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Mahmood”,”given”:”Kashif”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”August”,”issued”:{“date-parts”:”2016″},”title”:”IMPROVING ROUNDABOUT PERFORMANCE USING ADAPTIVE METERING TECHNIQUE”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=b71b310a-8983-4042-acd6-951c5e84ff7d”},”mendeley”:{“formattedCitation”:”(Mahmood, 2016)”,”manualFormatting”:”Mahmood (2016)”,”plainTextFormattedCitation”:”(Mahmood, 2016)”,”previouslyFormattedCitation”:”(Mahmood, 2016)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}Mahmood (2016), roundabouts have shown significant improvement over the traffic circles in many aspects especially in alleviating traffic congestion but they may prove to be ineffective in certain situations. A roundabout may experience longer delays as well as long queues when the traffic is high and unbalanced. So during an unbalanced traffic flow, the vehicles from one approach may prevent the vehicles in another approach entering the circulating roadway resulting in longer delays and queues ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Mahmood”,”given”:”Kashif”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”August”,”issued”:{“date-parts”:”2016″},”title”:”IMPROVING ROUNDABOUT PERFORMANCE USING ADAPTIVE METERING TECHNIQUE”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=b71b310a-8983-4042-acd6-951c5e84ff7d”},”mendeley”:{“formattedCitation”:”(Mahmood, 2016)”,”plainTextFormattedCitation”:”(Mahmood, 2016)”,”previouslyFormattedCitation”:”(Mahmood, 2016)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Mahmood, 2016).
A four-legged roundabout with roadways intersecting perpendicularly should provide that the traffic volumes are reasonably balanced and the geometry does not deviate substantially ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Deshmukh”,”given”:”Anirban”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”8″,”issued”:{“date-parts”:”2017″},”page”:”452-462″,”title”:”The effects of vehicle traffic flow characteristics on capacity at Rotary in Bhubaneswar city ( India )”,”type”:”article-journal”,”volume”:”3″},”uris”:”http://www.mendeley.com/documents/?uuid=8e3c1868-7611-4395-9db5-9ef9b9e0a19a”},”mendeley”:{“formattedCitation”:”(Deshmukh, 2017)”,”plainTextFormattedCitation”:”(Deshmukh, 2017)”,”previouslyFormattedCitation”:”(Deshmukh, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Deshmukh, 2017), when these conditions are not met it means that traffic congestion is highly likely to occur. According to Deshmukh (2017), it is important on multilane roundabouts to balance the use of each lane, because otherwise some lanes may be overloaded while others are underused. Poorly designed exits may influence driver behavior and lane imbalance and congestion at the opposite leg can be caused.
Fig 3. Conflict points for a roundabout with two circulatory lanes. Source ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Yang”,”given”:”Xiaoguang”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Li”,”given”:”Xiugang”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Xue”,”given”:”Kun”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”4″,”issued”:{“date-parts”:”2004″},”page”:”282-287″,”title”:”Technical Correspondences __________________________________________________ A New Traffic-Signal Control for Modern Roundabouts : Method and Application”,”type”:”article-journal”,”volume”:”5″},”uris”:”http://www.mendeley.com/documents/?uuid=38ca5a42-bd84-484b-adba-82bc8530820f”},”mendeley”:{“formattedCitation”:”(Yang, Li and Xue, 2004)”,”plainTextFormattedCitation”:”(Yang, Li and Xue, 2004)”,”previouslyFormattedCitation”:”(Yang, Li and Xue, 2004)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Yang, Li and Xue, 2004)
The above roundabout has two circulatory lanes. The eight white points are vehicle–vehicle conflict points, while the eight gray points are vehicle-merging points. If we need to increase the capacity, more than two circulatory lanes should be set ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Yang”,”given”:”Xiaoguang”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Li”,”given”:”Xiugang”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Xue”,”given”:”Kun”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”4″,”issued”:{“date-parts”:”2004″},”page”:”282-287″,”title”:”Technical Correspondences __________________________________________________ A New Traffic-Signal Control for Modern Roundabouts : Method and Application”,”type”:”article-journal”,”volume”:”5″},”uris”:”http://www.mendeley.com/documents/?uuid=38ca5a42-bd84-484b-adba-82bc8530820f”},”mendeley”:{“formattedCitation”:”(Yang, Li and Xue, 2004)”,”plainTextFormattedCitation”:”(Yang, Li and Xue, 2004)”,”previouslyFormattedCitation”:”(Yang, Li and Xue, 2004)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Yang, Li and Xue, 2004). The conflicting points can result in congestion further more collisions.
2.4Roundabout Operational Performance Analyse
2.4.1Volume to capacity ratio (V/C)
(Odeleye, n.d.) Acknowledged by (A.Virukai, 2016) states that Volume/Capacity ratio (V/C) is one whereby the designed capacity traffic capacity of a certain roundabout is compared with the actual traffic volume within a specified time period. Volume/ Capacity ratio (V/C) ratio greater than 1 will result in congestion ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Of”,”given”:”Report”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Powered”,”given”:”High”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”On”,”given”:”Committee”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”0″},”title”:”REPORT OF HIGH POWERED COMMITTEE ON”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=ba787389-4d4d-4f84-8ad4-9f79d983abf8″},”mendeley”:{“formattedCitation”:”(Of, Powered and On, no date)”,”plainTextFormattedCitation”:”(Of, Powered and On, no date)”,”previouslyFormattedCitation”:”(Of, Powered and On, no date)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Of, Powered and On, no date) Hence it is one of the best measures on the operational analysis of the Mbudzi roundabout. Volume?to?capacity (V/C) ratios are the primary measure of effectiveness for evaluation against the City’s performance standards. V/C ratios for roundabouts should be calculated based on the entry demand and capacity for the most critical approach (i.e. approach with the highest v/c ratio) for single?lane roundabouts and the most critical lane (i.e. individual lane with the highest v/c ratio) for multilane roundabouts ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kittelson ; Associates”,”given”:”Inc.”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”April”,”issued”:{“date-parts”:”2010″},”title”:”City of Bend Roundabout Evaluation and Design Guidelines”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=66264976-d726-4778-b570-80c49178e87b”},”mendeley”:{“formattedCitation”:”(Kittelson ; Associates, 2010)”,”plainTextFormattedCitation”:”(Kittelson ; Associates, 2010)”,”previouslyFormattedCitation”:”(Kittelson ; Associates, 2010)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kittelson & Associates, 2010).
2.4.2QUEUING
Queuing estimates should be included with all near?term roundabout operational analyses (e.g., development applications, capital improvement projects). Depending on site?specific conditions and at the City’s discretion, queuing analyses may be required for long?term operational analysis ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kittelson ; Associates”,”given”:”Inc.”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”April”,”issued”:{“date-parts”:”2010″},”title”:”City of Bend Roundabout Evaluation and Design Guidelines”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=66264976-d726-4778-b570-80c49178e87b”},”mendeley”:{“formattedCitation”:”(Kittelson ; Associates, 2010)”,”plainTextFormattedCitation”:”(Kittelson ; Associates, 2010)”,”previouslyFormattedCitation”:”(Kittelson ; Associates, 2010)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kittelson & Associates, 2010).
2.4.3DELAY
To ensure a balanced comparison of alternative intersection forms, delay estimates should be developed when comparing alternative intersection forms to the roundabout ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kittelson ; Associates”,”given”:”Inc.”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”April”,”issued”:{“date-parts”:”2010″},”title”:”City of Bend Roundabout Evaluation and Design Guidelines”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=66264976-d726-4778-b570-80c49178e87b”},”mendeley”:{“formattedCitation”:”(Kittelson ; Associates, 2010)”,”plainTextFormattedCitation”:”(Kittelson ; Associates, 2010)”,”previouslyFormattedCitation”:”(Kittelson ; Associates, 2010)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kittelson & Associates, 2010).
Roundabout performance analyses usually consider two aspects:1) entrance capacity, 2) operational performance measures (Robinson et al., 2000). Entrance capacity, which is expressed as the maximum flow rate from an entrance approach, concerns the number of vehicles that can be accommodated at a roundabout. ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1016/j.jtte.2016.03.005″,”ISSN”:”2095-7564″,”author”:{“dropping-particle”:””,”family”:”Chen”,”given”:”Xuanwu”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Lee”,”given”:”Ming S”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”container-title”:”Journal of Traffic and Transportation Engineering (English Edition)”,”id”:”ITEM-1″,”issue”:”2″,”issued”:{“date-parts”:”2016″},”page”:”154-165″,”publisher”:”Elsevier Ltd”,”title”:”ScienceDirect A case study on multi-lane roundabouts under congestion : Comparing software capacity and delay estimates with field data”,”type”:”article-journal”,”volume”:”3″},”uris”:”http://www.mendeley.com/documents/?uuid=d937ff1a-351c-4ff5-b18a-f5f80da35419″},”mendeley”:{“formattedCitation”:”(Chen and Lee, 2016)”,”plainTextFormattedCitation”:”(Chen and Lee, 2016)”,”previouslyFormattedCitation”:”(Chen and Lee, 2016)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Chen and Lee, 2016) suggests that entrance capacity is strongly associated with circulating flow rate, which is the number of vehicles traveling inside the roundabout during the analysis period. On the other hand, operational performance measures, such as delay and queue length, gauge the effectiveness of roundabout service for users. Methods for capacity estimation can be divided into two groups: regression model and gap-acceptance model ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1016/j.jtte.2016.03.005″,”ISSN”:”2095-7564″,”author”:{“dropping-particle”:””,”family”:”Chen”,”given”:”Xuanwu”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Lee”,”given”:”Ming S”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”container-title”:”Journal of Traffic and Transportation Engineering (English Edition)”,”id”:”ITEM-1″,”issue”:”2″,”issued”:{“date-parts”:”2016″},”page”:”154-165″,”publisher”:”Elsevier Ltd”,”title”:”ScienceDirect A case study on multi-lane roundabouts under congestion : Comparing software capacity and delay estimates with field data”,”type”:”article-journal”,”volume”:”3″},”uris”:”http://www.mendeley.com/documents/?uuid=d937ff1a-351c-4ff5-b18a-f5f80da35419″},”mendeley”:{“formattedCitation”:”(Chen and Lee, 2016)”,”plainTextFormattedCitation”:”(Chen and Lee, 2016)”,”previouslyFormattedCitation”:”(Chen and Lee, 2016)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Chen and Lee, 2016). Parameters of the regression models are estimated with traffic flow data collected from actual roundabouts.
2.4.4Gap-acceptance
Gap-acceptance model estimates capacity at the entrance approach based on gap-acceptance theory, in which a gap is the headway between two consecutive vehicles circulating in roundabout. In the gap-acceptance theory, a driver who wants to enter the roundabout from an approach needs a gap that is large enough for him/her to enter the roundabout safely ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1016/j.jtte.2016.03.005″,”ISSN”:”2095-7564″,”author”:{“dropping-particle”:””,”family”:”Chen”,”given”:”Xuanwu”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},{“dropping-particle”:””,”family”:”Lee”,”given”:”Ming S”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”container-title”:”Journal of Traffic and Transportation Engineering (English Edition)”,”id”:”ITEM-1″,”issue”:”2″,”issued”:{“date-parts”:”2016″},”page”:”154-165″,”publisher”:”Elsevier Ltd”,”title”:”ScienceDirect A case study on multi-lane roundabouts under congestion : Comparing software capacity and delay estimates with field data”,”type”:”article-journal”,”volume”:”3″},”uris”:”http://www.mendeley.com/documents/?uuid=d937ff1a-351c-4ff5-b18a-f5f80da35419″},”mendeley”:{“formattedCitation”:”(Chen and Lee, 2016)”,”plainTextFormattedCitation”:”(Chen and Lee, 2016)”,”previouslyFormattedCitation”:”(Chen and Lee, 2016)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Chen and Lee, 2016). Libros (2003) suggests that when there is a significant volume of traffic on one road of an intersection, separate thoroughfares should be devoted to each direction of travel whenever possible, complemented by acceleration or deceleration lanes if necessary. The islands placed between the roadways for this purpose are also necessary for the placement of signals in many cases. ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Akçelik”,”given”:”Rahmi”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”May”,”issued”:{“date-parts”:”2005″},”title”:”Roundabouts with Unbalanced Flow Patterns”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=64a4f931-5c46-42e3-ba67-b8b10c459020″},”mendeley”:{“formattedCitation”:”(Akçelik, 2005)”,”plainTextFormattedCitation”:”(Akçelik, 2005)”,”previouslyFormattedCitation”:”(Akçelik, 2005)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Akçelik, 2005) suggests that the gap acceptance model can also be used as a function of roundabout geometry and traffic flow conditions using empirical conditions. Gap acceptance theory, therefore calculates the capacity of roundabouts by making use of data on entry vehicles.
Quantification of congestion is done by incorporating the volume and operational characteristics of traffic movement as well as the capacity of the roundabout. Bhargab Maitra, P.K.Sikdar and S.L.Dhingra, 1999 conducted a study on quantification of congestion. Modeling congestion provides a quantitative basis for understanding the contribution of the roundabout geometric design in the overall congestion.
2.5Geometric characteristics of the roundabout
Number of lanes in the roundabout
Width of the traffic lanes
Diameter of the roundabout
2.5.1Traffic characteristics
Volume to capacity ratio
Road side activities
Gap acceptance
2.6Current Situation at Mbudzi roundabout
The traffic growth has taken place at a pace far in excess of the rate of investment in suitably constructed and maintained urban transport infrastructure. This in turn has contributed to both widespread and location-specific congestion problems ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kibunja”,”given”:”Njoroge Joseph”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2009″},”title”:”TRAFFIC CONGESTION PROBLEMS IN NAIROBI : AN EXAMINATION OF”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=dba3161d-c5c1-45b4-8734-63b4b47974e6″},”mendeley”:{“formattedCitation”:”(Kibunja, 2009)”,”plainTextFormattedCitation”:”(Kibunja, 2009)”,”previouslyFormattedCitation”:”(Kibunja, 2009)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kibunja, 2009). Currently at Mbudzi roundabout vehicles are spending more hours trying to pass through the roundabout, in an article published by the Herald 12 September 2018 states that vehicles are spending hours at this place hence there is a need to come up with a solution to decongest this area. Mbudzi round about is a two lane roundabout meaning that the inner lane is for circulatory purposes whilst the outer lane is for entrance and exit.
left10795Increase in car ownership
Increase in car ownership
405765038735Long time spent at the roundabout
Poor traffic flow
High energy demand
Environmental impacts
Health implications
Increase in accidents
Long time spent at the roundabout
Poor traffic flow
High energy demand
Environmental impacts
Health implications
Increase in accidents
1114424248285
left350520Transport infrastructure (geometric design)
0Transport infrastructure (geometric design)
228600015240Increase in traffic congestion
Increase in traffic congestion
340995043815172402524765
110426591440left253365Traffic management and control
Traffic management and control
Fig 5. Source: Adopted and modified from Karlsson et al., (2007)
Chapter 3
3.1RESEARCH METHODOLOGY
When vehicles enter a four-way roundabout, there are four ways to exit: vehicles can turn right, continue straight, turn left, or turn around ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Alkhaledi”,”given”:”K”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”August”,”issued”:{“date-parts”:”2015″},”page”:”191-202″,”title”:”EVALUATING THE OPERATIONAL AND ENVIRONMENTAL BENEFITS OF A SMART ROUNDABOUT”,”type”:”article-journal”,”volume”:”26″},”uris”:”http://www.mendeley.com/documents/?uuid=0cfd2821-51db-4c13-8117-0226b4e8a38b”},”mendeley”:{“formattedCitation”:”(Alkhaledi, 2015)”,”plainTextFormattedCitation”:”(Alkhaledi, 2015)”,”previouslyFormattedCitation”:”(Alkhaledi, 2015)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Alkhaledi, 2015). This chapter elaborates on the ways used to determine the relationship between traffic congestion and the geometrical design of the Mbudzi roundabout. It also tries to explain how the modification of the roundabout is to be done so as to decongest the roundabout. Modelling of traffic demand pattern is important in optimizing the roundabout geometry including approach and circulating lane use ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Akçelik”,”given”:”Rahmi”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”May”,”issued”:{“date-parts”:”2005″},”title”:”Roundabouts with Unbalanced Flow Patterns”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=64a4f931-5c46-42e3-ba67-b8b10c459020″},”mendeley”:{“formattedCitation”:”(Akçelik, 2005)”,”plainTextFormattedCitation”:”(Akçelik, 2005)”,”previouslyFormattedCitation”:”(Akçelik, 2005)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Akçelik, 2005).
3.2Data collection
Primary data collection is done through physical counting of cars entering and exiting the roundabout using all the four ways. The counting of the cars will be based on the times proved by A.Virukai (2016) were congestion starts and also some other times will be used to collect data so as to fully determine the relationship between the geometry of the roundabout and traffic congestion. Also there is the measuring of the diameter of the roundabout. Secondary data collection is done by acquiring data from the ministry of transport, department of roads. A road’s capacity is a measurement of traffic flow that compares the number of vehicles using the road with the number of vehicles it is designed to accommodate ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Alkhaledi”,”given”:”K”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”August”,”issued”:{“date-parts”:”2015″},”page”:”191-202″,”title”:”EVALUATING THE OPERATIONAL AND ENVIRONMENTAL BENEFITS OF A SMART ROUNDABOUT”,”type”:”article-journal”,”volume”:”26″},”uris”:”http://www.mendeley.com/documents/?uuid=0cfd2821-51db-4c13-8117-0226b4e8a38b”},”mendeley”:{“formattedCitation”:”(Alkhaledi, 2015)”,”plainTextFormattedCitation”:”(Alkhaledi, 2015)”,”previouslyFormattedCitation”:”(Alkhaledi, 2015)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Alkhaledi, 2015) hence the data of interest to be collected is the geometric design of the roundabout that is the diameter of the roundabout, the width of the lanes and the expected number of vehicles that pass through the roundabout over a certain period when it is at its optimum operation.
Incoming Direction 0600hrs-0800hrs 1000hrs-1100hrs 1200hrs-1400hrs 1500hrs-1600hrs 1600hrs-1800hrs
North South East West Table 1: for average traffic volume and direction at different time intervals
Standard measurements from ministry of transport Field measurements
Inner radius Outer radius Number of circulating lanes Width of circulating lanes Number of entry lanes Width of entry lanes Table 2 showing Geometric elements of a roundabout, adopted and modified from ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Marinela”,”given”:”In??”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2017″},”page”:”1-8″,”title”:”Improving performance of roundabout intersections by optimizing traffic-flow speed”,”type”:”article-journal”,”volume”:”06003″},”uris”:”http://www.mendeley.com/documents/?uuid=1af0d6b1-a19b-4611-b867-043aea8ba54c”},”mendeley”:{“formattedCitation”:”(Marinela, 2017)”,”plainTextFormattedCitation”:”(Marinela, 2017)”,”previouslyFormattedCitation”:”(Marinela, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Marinela, 2017)
3.3Interviews
They are carried out so as to hear peoples view especially the road users’ ad what they think is the major cause of congestion. Also interviews are carried out at the department of roads gathering information about the specifications of the roundabout and its functionality.
3.4Models
As discussed earlier on, models are going to be used to check the origination and other parameters which are resulting in traffic congestion at Mbudzi roundabout as well as its effectiveness.
3.4.1Capacity for Multilane Roundabouts
Recognizing the need to provide a variety of lane configurations to accommodate a range of traffic patterns and the lane use imbalances that may result, multilane capacity analysis should be conducted on a lane?by?lane basis and reported for the most critical lane (i.e. lane with the highest volume) on each approach.
cpce = 1130?exp(?0.0007? vc, pce) (Equation 1)
where,
cpce = capacity, adjusted for heavy vehicles, (pc/h)
vc, pce = conflicting circulating flow rate, adjusted for heavy vehicles, (pc/h)
3.4.2QUEUING
Queue lengths should be estimated using the below equation.
where,
Q95 = queue length (veh)
x = volume?to?capacity ratio of the subject lane
c = capacity of the subject lane (veh/h)
T = time period (h)
Equation above will result in the 95th?percentile queue likely to occur during the peak of the hour being analyzed.
3.4.3DELAY
where,
D = average control delay (s/veh)
x = volume?to?capacity ratio of the subject lane
c = capacity of the subject lane (veh/h)
T = time period (h)
3.4.4Gap-acceptance
where
Ce = Entering Capacity (veh/h),
q = Circulating flow (veh/h),
tc = Critical gap of the circulating flow, s,
tf = Follow-up time of the entry flow, s.
The relationship between the roundabout geometry and the circulating flow
where
n = the number of the critical gap in the circle line,
V = the speed of the circulating flow under saturated condition (km/h),
tc = Critical gap of the circulating flow, s,
L = the length of the standard car ( m),
R = the radius of the roundabout (m).
The relationship between the circulating flow and the radius of the roundabout would then be
3.5Roundabout Performance Analysis Process
3524257360285Fig Roundabout Analysis Process, Source ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kittelson ; Associates”,”given”:”Inc.”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”April”,”issued”:{“date-parts”:”2010″},”title”:”City of Bend Roundabout Evaluation and Design Guidelines”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=66264976-d726-4778-b570-80c49178e87b”},”mendeley”:{“formattedCitation”:”(Kittelson ; Associates, 2010)”,”plainTextFormattedCitation”:”(Kittelson ; Associates, 2010)”,”previouslyFormattedCitation”:”(Kittelson ; Associates, 2010)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kittelson & Associates, 2010)
Fig Roundabout Analysis Process, Source ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Kittelson ; Associates”,”given”:”Inc.”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”April”,”issued”:{“date-parts”:”2010″},”title”:”City of Bend Roundabout Evaluation and Design Guidelines”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=66264976-d726-4778-b570-80c49178e87b”},”mendeley”:{“formattedCitation”:”(Kittelson ; Associates, 2010)”,”plainTextFormattedCitation”:”(Kittelson ; Associates, 2010)”,”previouslyFormattedCitation”:”(Kittelson ; Associates, 2010)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Kittelson & Associates, 2010)
2762256466840Step 9: Compute 95th?percentile queues for each lane.
0Step 9: Compute 95th?percentile queues for each lane.
right5714365Step 8: Compute the average control delay
00Step 8: Compute the average control delay
right4893310Step 7: Compute the volume?to?capacity ratio for each lane.
x = v/c
00Step 7: Compute the volume?to?capacity ratio for each lane.
x = v/c
right4017010Step 6: Convert lane flow rates and capacity into vehicles per hour.
v = vpce fHV c = cpce fHV fped
0Step 6: Convert lane flow rates and capacity into vehicles per hour.
v = vpce fHV c = cpce fHV fpedright3255010Step 5: Determine the capacity of each entry lane in passenger car equivalents.
00Step 5: Determine the capacity of each entry lane in passenger car equivalents.
right2569210Step 4: Determine entry flow rates by lane.
00Step 4: Determine entry flow rates by lane.
right1854835Step 3: Determine circulating and exiting flow rates.
00Step 3: Determine circulating and exiting flow rates.
right959485Step 2: Adjust flow rates for heavy vehicles.
vpce = v/fHV, fHV = 1/1 + PT (ET ?1), PT = Heavy Vehicle %, ET = 2.0 for Heavy Vehicles
00Step 2: Adjust flow rates for heavy vehicles.
vpce = v/fHV, fHV = 1/1 + PT (ET ?1), PT = Heavy Vehicle %, ET = 2.0 for Heavy Vehicles
40957564136Step 1: Convert movement demand volumes (V, veh/h) to flow rates (v, veh/h).
Hourly analysis: v = V
00Step 1: Convert movement demand volumes (V, veh/h) to flow rates (v, veh/h).
Hourly analysis: v = V
3.6Proposed solution (Round about design)
Roundabout shape is an important decision, because the shape can affect design elements that affect safety performance and operation of the roundabout ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“author”:{“dropping-particle”:””,”family”:”Design”,”given”:”Wsdot”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”July”,”issued”:{“date-parts”:”2018″},”page”:”1-26″,”title”:”Chapter 1320 – Roundabouts – Design Manual M 22-01″,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=e9333ed0-e052-4f53-a274-f1f4959983b8″},”mendeley”:{“formattedCitation”:”(Design, 2018)”,”plainTextFormattedCitation”:”(Design, 2018)”,”previouslyFormattedCitation”:”(Design, 2018)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Design, 2018). Roundabouts can improve safety by simplifying conflicts, reducing vehicle speeds and providing a clearer indication of the driver’s right of way compared to other forms of channelization and at the same time a good roundabout design should accidents, delay and costs.
3.7Methods to modify a roundabout (Mbudzi roundabout)
3.7.1Development of the Roundabout Layout
1. Identification of a limited number of dimensions through field measurements
2. Calibration of satellite images
3. Validation of parameters
The roundabout diameter and the circulation width were measured using field data that was collected. The remaining field measurements (entry width and the length of the splitter island) were compared with those of the calibrated roundabout image. Validated roundabout layouts were then used to obtain some of the other parameters of the roundabouts. Following this, detailed roundabout layouts were drawn up using Auto CAD, to obtain the other roundabout parameters ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.4038/engineer.v49i4.7237″,”author”:{“dropping-particle”:””,”family”:”Mampearachchi”,”given”:”Wasantha”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issue”:”November”,”issued”:{“date-parts”:”2017″},”title”:”Review of Roundabout Design Standards and the Development of a Roundabout Design Guideline for Sri Lanka”,”type”:”article-journal”},”uris”:”http://www.mendeley.com/documents/?uuid=330a7789-d8cc-4b61-b59b-2a260642f66e”},”mendeley”:{“formattedCitation”:”(Mampearachchi, 2017)”,”plainTextFormattedCitation”:”(Mampearachchi, 2017)”,”previouslyFormattedCitation”:”(Mampearachchi, 2017)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Mampearachchi, 2017).
3.7.2Roundabout Parameters
Roundabout parameters can be categorized in to three types as given below, based on the methods adopted to obtain them.
1. Parameters obtained through field measurements
2. Parameters extracted from validated roundabouts (satellite images).
3. Parameters derived using construction lines on AutoCAD drawings.
3.7.3Field Measurements
These are the parameters that can be directly obtained from the roundabouts and which can be easily measured. Examples of such parameters are the island diameter, circulation width and splitter island width. They can be measured easily in a roundabout even with heavy traffic.
3.7.4Extracted Measurements
Using direct measurements, a roundabout layout was drawn with the aid of satellite images. Using the corrected satellite images, the other important parameters given below, could also be extracted:
Entry width
Exit width
Entry radius
Exit radius
Circulating lane width
Other parameters include entry angle, exit angle and the approach path radius.
3.7.5Signalizing
A traffic light is setup to regulate the traffic ADDIN CSL_CITATION {“citationItems”:{“id”:”ITEM-1″,”itemData”:{“DOI”:”10.1016/j.physa.2007.05.014″,”author”:{“dropping-particle”:””,”family”:”Huang”,”given”:”Ding-wei”,”non-dropping-particle”:””,”parse-names”:false,”suffix”:””},”id”:”ITEM-1″,”issued”:{“date-parts”:”2007″},”page”:”603-612″,”title”:”Phase diagram of a traffic roundabout”,”type”:”article-journal”,”volume”:”383″},”uris”:”http://www.mendeley.com/documents/?uuid=d0d6eb25-2249-49bb-8442-e800d4a77bb8″},”mendeley”:{“formattedCitation”:”(Huang, 2007)”,”plainTextFormattedCitation”:”(Huang, 2007)”,”previouslyFormattedCitation”:”(Huang, 2007)”},”properties”:{“noteIndex”:0},”schema”:”https://github.com/citation-style-language/schema/raw/master/csl-citation.json”}(Huang, 2007).
