THE EFFECT OF LEFT-TURNING TRAFFIC ON THE PERFORMANCE OF THROUGH TRAFFIC AT A SIGNALIZED INTERSECTION: CASE STUDY OF ANLOGA INTERSECTION IN KUMASI, GHANA

ABSTRACT

Rapid vehicular population growth in urban centres in developing countries such as Ghana has resulted in increased volumes contributing to long delays at signalized intersections. At intersections where there is high volume of left-turning traffic, spill back and blocking of the through traffic has much effect on saturation flows. In many situations, a change of layout of intersection or even grade separation may be the option for improving delay and LOS but uneconomical to most developing countries due to lack of funds or inadequate right-of-way or both. The objective of this thesis was to evaluate the effect of left turning traffic on delay to through traffic at signalized intersection, evaluate the effect of left turning traffic on saturation flow for through traffic at a signalized intersection and propose a left-turn storage lane length that will improve the saturation flow and reduce delay for through traffic at a signalized intersectionusing micro simulation tool (VISSIM).An intersection characterized by long queue and delays during peak periods was chosen as the study site. Two hours video data collection was undertaken on a typical morning peak from which the traffic demand and turning movements were extracted. VISSIM was used for simulation analysis of the intersection. The model was calibrated with the traffic flow, queue length and delay data for the approaches and the result validated. After conducting a sensitivity analysis by increasing the approach volume from 1000pcu/h to 3000pcu/h and varying the left turn traffic percentage and left turn storage lane lengths, it was revealed that as the approach volume of signalizes intersection increases with a corresponding increase in left-turning traffic volume proportions, delay to through traffic movement increases and its saturation flow reduces. It was also found out that, for approach volumes of 1000pcu/h to 3000pcu/h and corresponding left turn traffic proportion 10% to 20%, 150m optimal storage lane is required to reduce delay and improve on saturation flow. For an approach volume of 1000pcu/h to 3000pcu/h with 30% to 50% left turn traffic proportion, 175m storage lane is required to reduce delay and improve on saturation.

CHAPTER 1: INTRODUCTION

1.1 BACKGROUND

Rise in the mobility demand of traffic within urban and suburban areas has resulted in rapid increase in growth rate of traffic in Ghana. A report published by the Driver Vehicle and Licensing Authority (DVLA) in 2012 indicates that the vehicle population in Ghana is close to a million, with much concentration in the two biggest cities, Accra and Kumasi. Research has shown that traffic movement within the streets of these cities is therefore becoming critical. One of these streets is the Osei Tutu II Boulevard in Kumasi.

It is an east-west major arterial of about 5.4 km length from KNUST junction to the Asafo Interchange traffic light (Nyantakyi, et al, 2013). The road is a 2-lane dual carriage and serves as the main route that leads into the Kumasi metropolis from the southern parts of Ghana. There are three signalized intersections on the road, namely, Anloga, Stadium and Amakom signalized intersections, (Nyantakyi, et al, 2013). There is a high demand at these intersections with regards to capacity especially during the peak periods making them fail in operation as traffic overflow normally occur.

Research conducted by Nyantakyi, et al, (2013), indicated that these intersections exhibit similar traffic characteristics and would reach critical traffic condition in five years.

1.1.1 Intersections

Intersection is any place where two or more roads intersect or meet each other at grade. A highway intersection is required to control conflicting and merging streams of traffic so that delay is minimized and this is achieved through choice of geometric characteristics and regulation that control and regulate the vehicle paths through the intersection. These determine priority so that all movements take place with safety.

1.1.2 Types of intersection

The three main types of intersections are:

1. Priority intersections, either simple T-junctions, staggered T-junctions or crossroads

2. Signalized intersections

3. Roundabouts.

All these are aimed at providing vehicle drivers with a road layout that will minimize confusion. The need for flexibility dictates the choice of most suitable junction type. The selection process requires the economic, environmental and operational effects of each proposed option to be evaluated.

The designer needs to assess the projected traffic flow at the location in question, termed the design reference flow. The range within which this figure falls will indicate a junction design which is both economically and operationally efficient rather than one where there is either gross over or under provision. Test should be conducted on different combinations of turning movements in order to check the performance characteristics of each junction option under consideration. The volume of traffic incident on the intersection together with the various turning, merging and conflicting movements involved are the main determinants of the intersection design.

1.1.3 Left- turn lane

A direct left-turn egress from a driveway must yield to all other movements since it has the lowest priority and it is the most likely movement to be delayed, especially under heavy traffic volume conditions, (Mahmoud Ahmed Taha, 2015). Left turns are a significant hindrance to the smooth flow of traffic in networks involving at-grade intersections. Therefore, separate turn phases are often used on the approach legs of the signalized intersections with heavy left turns, but this wastes capacity on the approach because some of its lanes cannot discharge during its green phases since left turns are a significant hindrance to the smooth flow of traffic in networks involving at-grade intersections, (Yiguang, et al., 2010). Left-turn lanes are therefore incorporated to separate the left turn traffic from the through traffic and this can lead to an improvement in the signalized intersection capacity.

1.2 PROBLEM STATEMENT

In Ghana, most urban areas such as Accra and Kumasi are constantly experiencing traffic growth. This development always results in undue delays at signalized intersections because of spillback of left-turning traffic blocking the movement of through traffic. The intersection capacity would be reduced due to inefficient utilization of the through movement green time, and the vehicle delay would increase because the blocked through vehicles spent more time in the traffic queues. Fuel consumption and emissions would also be increased since the queued vehicles would spend more idle time in the queues

1.3 PROJECT OBJECTIVES

The objectives of the project are to:

* Evaluate the effect of left turning traffic on delay to through traffic at signalized intersection.

* Evaluate the effect of left turning traffic on saturation flow for through traffic at a signalized intersection.

* Propose a left-turn lane length that will improve the saturation flow and reduce delay for through traffic at a signalized intersection.

1.4 JUSTIFICATION

To reduce traffic delays, most research by road and transportation engineers have suggested solutions such as addition of more lanes, grade separation (interchange), etc. However, such solutions are not always feasible for many at grade signalized intersections as a result of limited right-of-way or budgetary constraints. One of the possible solutions to reduce the overall delay and improve capacity of a signalized intersection is using exclusive left-turn lane which separates left-turning traffic from the through traffic.

The research has become necessary because of the following reasons:

Most of the arterial roads in the urban areas are connected by signalized intersections therefore their effectiveness in terms of saturation flow, capacity and level of service is of great concern especially as the proportion of left turner increase.

Ghana has on its drawing board designs to develop some of these intersections into interchanges at a very high cost. Since there are usually budgetary constraints, a better understanding of the effect of left turners will help to improve performance in the medium term.

The cost in implementing appropriate left turning storage lane to improve the performance of the signalized intersection is generally lower than changing the entire intersection design if left turning traffic is the main problem affecting the measures of effectiveness.

1.5 SCOPE OF WORK

The study was conducted to evaluate the effect of left turn traffic on the performance of through traffic for a signalized intersection. The signalized intersection’s performance indicators that were considered under this study were delay and saturation flow to through traffic.

General purpose: To find a simple inexpensive means other than an interchange and addition of more lanes for the reduction of delay and improving saturation flow. 

Subject matter: Effect of left turn traffic on the performance of through traffic; traffic delays and saturation flow of the through traffic were evaluated based on increase in approach volume and increase in left turn traffic percentage with variation of the left turn storage lane length.
Locale of the study: Anloga Intersection in Kumasi, Ghana.

Period of the study: The data collection was undertaken on15th March, 2016.

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Item Type: Ghanaian Topic  |  Size: 54 pages  |  Chapters: 1-5

Format: MS Word  |  Delivery: Within 30Mins.

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