Assessing The Capacity And Performance Of Kubiri Roundabout

ABSTRACT

Use of roundabouts as a viable traffic control measure of traffic flow at priority points is increasing in most of growing towns in Uganda. Their strength lies in their ability to reduce the number of vehicular conflicts at intersections and thereby enhance intersection capacity

and safety. There are additional intangible benefits of roundabouts such as their traffic calming effect, gateway feature and aesthetics. Increasingly, many roundabouts in growing urban towns i.e. Kampala city are becoming problematic due to capacity challenges resulting

from rapid traffic growth. Hence a micro-simulation model such as VISSIM was used to assess the performance, level of service of kubiri round about in order to minimize the uncertainties of increased; control delays , queue lengths experience during peak hours. The aim of this study investigated the viability of using micro-simulation model such as VISSIM in evaluation of operational performance, improved capacity, reduced delays and queue lengths of Kubiri roundabout as a case study. Field traffic volume studies for the entire roundabout’s

approaches were performed; travel time and delay studies for the subject approach (Gayaza approach) were also undertaken. Geometric data as well as data from the field studies were used to calibrate a model for the existing situation. The VISSIM model is an advanced microsimulation tool with capabilities of modelling both roundabout and traffic signals to acceptable level of detail. the model’s ability is to capture the effects of various geometric and traffic features like approach angle, inscribed circle, number of circulatory lanes, position

of the stop line and proportion of turning flows, and evaluates their impacts on the resulting capacities. Motion of an individual vehicle is simulated in small time steps and each vehicle is followed from the time it is generated into the network to the point of its exit. Vehicular behavior at roundabouts is modelled on the basis of a gap acceptance approach. Capacity, delay, queue length and degree of saturation were observed from the modal where signalization simulation options for signalized and existing un-signalized were determined.

The results indicated that the full signalization model produced the better parameter results as compared to the un-signalized model. However, there is an increase in the queue length and delay as well as a decrease in capacity on Gayaza approach attributed to alteration in the balance of traffic flow due to the signalization on all the approaches as well as the circulatory carriageway. Generally, there is a slight improvement in the flow of traffic after signalization however that the level of service is still low i.e. LOS F and degree of saturation is still high (greater than 1.00). This could be attributed to the fact that the total Pcus from all the approaches exceed 10000 Pcus/hr. which necessitates grade separation. Gradeseparation is one of the alternatives in solving congestion at Kubiri roundabout, but it is very expensive to implement.

CONTENTS

DECLARATION .................................................................................................. i

APPROVAL ........................................................................................................ii

ACKNOWLEDGEMENTS ..................................................................................... iii

ABSTRACT ....................................................................................................... iv

LIST OF FIGURES ........................................................................................... viii

LIST OF TABLES ............................................................................................... ix

CHAPTER 1.0 INTRODUCTION .......................................................................... 1

1.1 BACKGROUND ............................................................................................ 1

1.2 PROBLEM STATEMENT ................................................................................ 3

1.3 OBJECTIVES ............................................................................................... 4

1.4 JUSTIFICATION .......................................................................................... 4

1.5 SCOPE OF THE STUDY ................................................................................ 5

CHAPTER 2.0 LITERATURE REVIEW ................................................................... 7

2.1 INTRODUCTION ......................................................................................... 7

2.2 ROUNDABOUTS .......................................................................................... 7

2.3 CATEGORIES OF SIGNALIZED ROUNDABOUTS ........................................... 10

2.4 SIGNALIZATION AND PHASING ................................................................. 11

2.5 CAPACITY OF A ROUNDABOUT .................................................................. 12

2.6 KEY PERFORMANCE MEASURES OF A ROUNDABOUT ................................... 14

2.7 GAP ACCEPTANCE AND FOLLOW-UP HEADWAY .......................................... 16

2.8 TRAFFIC FLOW CHARACTERISTICS ............................................................ 17

2.9 LEVEL OF SERVICE (LOS) .......................................................................... 19

2.10 MICRO-SIMULATION SOFTWARE ............................................................. 21

2.11 VISSIM .................................................................................................. 21

2.12 CODING ................................................................................................. 23

2.13 MODEL CALIBRATION ............................................................................. 24

2.14 CONFIDENCE .......................................................................................... 25

2.15 GEH STATISTIC ...................................................................................... 26

vi

2.16 CAR FOLLOWING MODEL ........................................................................ 27

2.17 LITERATURE SUMMARY ........................................................................... 27

CHAPTER 3.0 METHODOLOGY ......................................................................... 28

3.1 SITE DESCRIPTION .................................................................................. 28

3.2 DATA COLLECTION FROM THE FIELD......................................................... 29

3.3 COMPUTER SIMULATION DATA ................................................................. 32

3.4 DATA ANALYSIS ....................................................................................... 39

CHAPTER FOUR ............................................................................................. 45

RESULTS AND DISCUSSION ............................................................................ 45

4.1 CALIBRATION RESULTS ............................................................................ 45

4.1.1 Throughput ........................................................................................... 45

4.2 PERFORMANCE RESULTS .......................................................................... 48

4.2.3 Graphical comparison of the two models .................................................. 53

CHAPTER FIVE ............................................................................................... 57

CONCLUSIONS AND RECOMMENDATIONS ........................................................ 57

5.1 Conclusions .............................................................................................. 57

5.2 Recommendations .................................................................................... 58

REFERENCES ................................................................................................. 59

APPENDICES .................................................................................................. 62

APPENDIX A Summary of the field traffic data by movement .............................. 62

APPENDIX B FIELD DATA ANALYSIS ................................................................ 68

APPENDIX C MOTORCYCLE VOLUME COUNT .................................................... 69

APPENDIX D MOTORCYCLE ANALYSIS ............................................................. 70

APPENDIX E DESIGN FLOW ............................................................................ 72

APPENDIX F TRAVEL TIME AND DELAY STUDIES .............................................. 73

APPENDIX G Geometric data of Kubiri roundabout ............................................ 75

APPENDIX H Throughput before Signalization ................................................... 75

APPENDIX J Delay before signalization ............................................................. 76

APPENDIX K Queue length before signalization ................................................. 77

APPENDIX L Throughput after Signalization ...................................................... 78