Undergraduate Research Project - Part 3 of 5: Literature Review, Knowledge Gap, Scope and Limitations

Literature Review

Studying the previous research work is a vital step in a research. The researcher must understand about the present status of the research area in order to move forward and discover new knowledge. In the previous articles, the initial steps such as background study, problem statement, aim and objectives were discussed. These steps lay the foundation for the research so that the way forward is reasonably clear. After the first stage, the research should advance to the second stage, which is the literature review (also known as the literature survey). This is the fifth step of the flowchart presented in the first article.

The purpose of conducting a review of literature is to search and evaluate the present knowledge. Here, the idea is not to develop any new arguments or theories, but to summarize what others have done without changing the original idea/findings. A properly conducted literature review will show the progress of the considered research area, means how people unraveled the knowledge throughout the history (major breakthroughs, inventions, turning points etc.). Eventually, the researcher will realize what the current status is and where it should be headed. In some cases, literature review reveals that the selected topic has been already studied. Therefore, the focus of the research will have to be changed.

The preparation of a methodology for the research is also done as per the findings of the literature review. The methods and techniques that previous studies have adopted have to be considered prior to the development of the methodology. Also, the drawbacks of those methods and techniques can be easily identified. The constraints, barriers, obstacles, problems, etc. the previous researchers had to overcome are also usually discussed in their publications. Those aspects will also assist the methodology development process.

The other benefits of the literature review are,

Knowledge gap or research gap can be identified
Changes or alterations to the topic/research area can be done if necessary (if the research has already been conducted or if there is a need to conduct more studies before proceeding into the considered topic)
Different approaches to the research can be found out
Limitations to the research can be identified along with the possible measures to overcome those
The scope and scale of the research can be defined properly
Resource requirements, time frame and external support needed can be finalized
The researchers will know the sources that should not be included in the study

Prior to conducting the literature review, identify the focus area and what is expected from the literature review. This helps organize the content and to build up a flow. It is important to remember that other people will be reading it so the idea has to be properly conveyed. It is also advised to use simple language with appropriate technical terms.

Prepare a breakdown for the content as follows,

Introduction: A brief idea about the research area and other relevant information

Body: A detailed discussion of all the information collected through various sources

Conclusions: An idea about the findings of the literature review and how the research should be proceeded

The process of writing a literature review has four stages,

1.0 Searching for information

The researcher must gather data relevant to the topic. This is basically collecting information through various sources to extract details about the research area. These are known as 'publications' or 'research publications'. In academic research, there are generally accepted types of information sources. Following is a list of such types,

Book: A reference book is intended to be a source of information on specific matters
Journal: A journal is an educational publication which contains articles written by researchers. They usually have a field of study
Conference proceedings: Conference proceeding is a collection of research papers published in an academic conference or workshop
Report: An official document prepared on a particular matter, by an appointed person or body
Web site: An official website which contains credible information (avoid Wikipedia, social media)
Interview: A session or a conversation with a person of public interest
Patent: A license providing the right or ownership of an invention
Periodical: A magazine, newspaper or any other publication issued at regular intervals.
Miscellaneous (press conference, video, etc.)

It is highly important that the information sources are pertinent and related to the study. Some resources might deem relevant at the beginning, but actually the focus areas of those will not be related to the study being conducted. Hence, the researcher must be cautious not to refer any irrelevant studies. Reading the abstract of a research paper or the executive summary of a report is an easy way to get an idea about the content of such a document. A list of widely referred websites to search for research publications is given below,

More details about referencing can be found in the following article: Referencing and citation.

2.0 Synthesizing the information

This means putting the collected information together and making a collective idea or a collection of ideas. Here, the researchers have the freedom to reorganize/reshuffle/combine the information found in the previous step. There are several ways of synthesizing,

Summarizing: Briefing all the collected information in a clear, orderly and concise manner with a few words
Paraphrasing: Expressing the same content using different words to make a more meaningful statement (summarizing is not necessary in paraphrasing)
Quoting: Repeating the same text using the same words from the original source (the source and authors should be properly acknowledged)
Copying: Reproducing or duplicating the same work without any change (identical to the original source)

In a literature review, it is not recommended to 'copy' from other researchers' works. 'Summarizing' is the most commonly practiced method. 'Paraphrasing' and 'quoting' can also be done where necessary. It is important to understand that a literature review is not just a summary of previous research work, but it should create a link between the past researches and the current research topic. Flow and the order have to be maintained when stating facts. A common mistake the students make is reviewing publication by publication. As an example, one research paper is selected and it is reviewed. Then another paper is selected and reviewed. Eventually, the student comes up with a collection of reviews combined together. This is not the approach for a literature review. First, all the collected information should be carefully read and understood. Then a structure or an order can be planned for the literature review according to the facts that will be discussed. Finally, the content can be prepared (written) using the above four synthesizing techniques. Some typical ways to structure the literature review are given below,

Chronological order: This means how the research area has progressed over time ('History', 'Recent past' and 'Present situation' can be the three sections of the structure)

Methodologies: Structure can be divided into different sections to discuss about different methodologies or approaches that have been used to conduct researches

Local and foreign: The research work done in your own country and others countries/regions can be the two sections of the structure

There are many more ways to form a structure and the researchers should be able to decide what the most suitable way is.

3.0 Analyzing the information

The researchers have the liberty to analyse the information in the literature review up to a certain extent. The purpose of this is not to evaluate the previous research work, but to identify the knowledge gaps, limitations, barriers, different points of view, contradictions etc. in the research area being studied. This helps formulate a rationale to the research as well as developing a methodology. Shaping up the future of the research can also be done through the analysis process. The researchers should also attempt to make conclusions or arguments throughout the literature review to support the ongoing research.

Note: Acknowledging the sources should be done when writing the literature review. This is done by 'citing' the source and displaying a list of 'references' at the end of the document. Read referencing and citation for more details.

4.0 Presenting the information

This refers to the documentation part of the literature review. The use of appropriate terminology & language, section breaks & paragraphs, structure of sentences & grammar, flow of the content, order of the facts stated, conclusions etc. should be well planned and presented. Professional writing styles should be adopted along with the formatting techniques.

Knowledge Gap

At the end of the literature review, it is advised to mention the knowledge gap that is intended to be filled through the research. The term knowledge gap means, the difference between the expected research outcomes and the current state of knowledge. The results and conclusions of the research will fill this gap.

Scope and Limitations

The scope of the research mainly defines how far the research will explore and what its boundaries are. This is determined by the researchers and it doesn't depend on external factors. Here, they can mention the particulars of the research so that a methodology can be properly developed. The facts such as sample sizes, scenarios considered, data collection & analysis methods, user cases, testing mechanisms, etc. are discussed under the scope. Scope is subjective to the topic and depends on the research area.

Limitations are external factors that affect the project. They can restrict the study in certain ways and act as constraints. The researchers don't have the control over limitations. Therefore, solutions or alternatives have to found to overcome such obstacles. Unavailability of resources, technological barriers, cost constraints, time limitations, human resource requirements, inadequate research facilities are some of the most common limitations. When developing a methodology, both scope and limitations must be incorporated.

Example

The second chapter of the example discussed in previous articles is given below (the section break of the first chapter is also shown). The literature review conducted on the topic 'Study of operational improvements of introducing electronic toll collection to expressways in Sri Lanka' is as follows,

Chapter 1: Introduction

1.1 Background

Refer the previous article Part 1 of 5: Introduction

1.2 Problem statement

Refer the previous article Part 2 of 5: Problem Statement and Objectives

1.3 Aim and objectives

Refer the previous article Part 2 of 5: Problem Statement and Objectives

Chapter 2: Literature Review

2.1 Local literature

The present toll collection system of expressways has not yet reported any severe congestion, therefore no party has paid attention towards this issue or has conducted a proper study to evaluate the current method and discuss its future. A study conducted on the efficiency of present manual toll booths with respect to the existing ramp lengths of the Southern expressway’s Kottawa to Pinnaduwa section (Kumari, 2015) has concluded that some of the interchanges like Kottawa, Pinnaduwa will not be able to serve the traffic demand in the year 2030 at a satisfactory level.

It’s an obvious fact that the traffic volume of the country is growing rapidly at the moment. So, the number of expressway users will also increase as a result. Since the manual toll collection system is inefficient, if the demand increases and goes beyond the toll plaza capacity, it will cause heavy congestion at such interchanges. So, the concern of this study has been the possible queue formations at entry and exit points which are going to affect the level of service. Due to the possibility of these queues propagating back to the expressway lanes, the adequacy of ramp lengths has also been addressed through this study.

The researchers have studied the layout, the number of booths, geometry, ramp lengths, peak times and the procedure of collecting the fee at the toll gate of each interchange to identify the critical ramps, incoming flow rates and service rates. Through that, the longest individual delays and maximum queue lengths have been determined to conduct the analysis and recommend the necessary improvements required.

The study has not used any simulation method, but only the theories on queuing analysis. So, the facts like vehicle composition, speed variations, car following models and driver behaviour can’t be incorporated in the analysis. Consequently, the results have deviated from the actual scenario. Many of the occasions except for the peak times, the interchanges like Pinnaduwa and Kottawa don’t work in their capacity. Only a few number of toll booths are put into operation. If a critical situation is to be analysed, then it should be the scenario where all the booths are in operation at the maximum service rate. This fact has also not been considered in this study. Collection of traffic data has not been done on peak days as well.

As conclusions, it has stated that the interchanges will not be able to serve the traffic demand in the future and has proposed to adopt electronic toll collection method. Even though the study doesn't contain an analysis at the required level, it supports the idea that improvements should be done to the present manual toll collection system to cater the increasing demand.

2.2 Foreign literature

Most of the developed countries which have experienced similar issues, have conducted many researches with regard to different toll collection systems. Those countries have either proposed modern technologies or have adopted different systems to improve the efficiency of toll plazas. Most of these researches have been done using micro-simulation methods to study various scenarios at toll plazas. In micro-simulation, traffic flow is modelled and analysed incorporating a wide range of details and parameters. In addition to this, facts like the number of toll booths in operation, lane discipline, lane changing behaviour, driver behaviour, peak volumes and different toll collection methods have been taken into consideration for the development of the models. All the studies have produced similar results and conclusions. There are some findings which can be used to develop a methodology for this research.

Korean highways have faced a situation where there was an imbalance between road capacity and automobile population (Chang, n.d.). As a result, massive congestion incidents have occurred incurring huge costs to the government. So, they have adopted the toll road concept. After doing a successful demonstration programme, they have concluded that the electronic toll collection system reduces both the queue lengths and waiting times. Presently, this system is being used in Korea. A similar scenario is likely to happen in Sri Lanka in the near future. Therefore, advancing into new tolling technologies such as electronic toll collection should be considered.

Malaysia has faced a traffic congestion issue with regards to toll plazas. A study (Hamid, 2011) has been conducted to evaluate different traffic operations and management configurations. The decade from 1990 to 2000 was considered as the period where Malaysia turned its attention towards toll roads. They have developed a significant number of toll roads in their cities and now it has become the most preferred choice of Malaysians. Having a poor public transport system was also a reason for this preference. This scenario is similar to the present situation in Sri Lanka.

With the growth of the total number of users, the Malaysian toll roads have not been able to accommodate the traffic demand. Heavy congestion problems have started to occur at the interchanges. Even with card payment methods, they haven’t been able to reduce the peak time traffic. Even in Sri Lanka, during holiday seasons, these kind of congestion can be seen. It has not yet gotten any relevant party’s attention since it doesn't happen frequently. This study has been done using a micro-simulation method. PTV Vissim was the software that has been used since it provides many options and also can be calibrated to suit local conditions. As outputs; average delay, total delay, average number of vehicles processed per booth and the total number of vehicles processed have been measured. Actual conditions such as layout, driver behaviour, restrictions and safety aspects have been set as much as possible. The volume of the traffic and the type of payment system have been assumed as the facts that are related to the toll plaza configuration in this case. Queuing area, toll booth arrangements and traffic access instructions are the main components of a toll plaza’s configuration. The study has been done for different toll plaza configurations and the results have been compared with actual values obtained by a field survey to validate the model. A preliminary design table has been proposed for the toll plazas indicating the required number of booths for each plaza and the suitable methods of toll collection. This study has not been extended up to a level which would predict any future scenario. There is no any statistical analysis done as well.

Istanbul; the largest city in Turkey has been using highways since 1950. Later it has caused major traffic problems. Bosporus is a bridge which has been built to provide access between the two continents; Asia and Europe. Since it has reached its capacity before the estimated time, they have built another bridge called Fatih Sultan Mehmet. This bridge has failed to accommodate the incoming traffic on certain days due to several reasons. So, it has been studied using a micro-simulation method with some performance measurement criteria like speed, delay, occupancy, travel time and vehicle counts. Vissim is the simulation software which has been used in this study as well (Aksoya, 2013).

The Fatih Sultan Mehmet (FMS) Bridge has 22 toll booths which are operated manually. Six lanes at the starting point diverge into these 22 toll booths. After the toll plaza, these 22 lanes converg back into four lanes. This bottleneck has caused traffic congestion at the exit and has reduced the capacity of this bridge. Through this research, they have studied the effect on delay due to the number of toll booths. Different scenarios where the number of toll booths in operation is 22, 10, 8 and 6 have been simulated in Vissim. Total delays have been obtained for each scenario to compare the results. It has been observed that having 6 toll booths will reduce the total delay within the considered stretch. Due to the bottleneck at the point where 22 lanes merge into four, queues form if the capacity of the toll plaza goes beyond the capacity of this bottleneck at the exit. Therefore, the optimum number of toll booths should be provided. The arrangement in Kottawa interchange is similar to the FMS Bridge. Eight tollbooth lanes merge into two lanes before connecting to the A4 road (High-level road) at the exit. So, the ‘provision of the optimum number of toll booths’ concept should be taken into consideration when analyzing the interchange. For forecasting purposes also this fact has to be incorporated.

There is a practical issue that the vehicles face (Komada, 2009) when exiting from a toll plaza which is selecting the correct exit lane. As there are two types of vehicles; electronic and manual toll-paying vehicles, they need to sort themselves into the respective lanes. If the vehicle density is high, the electronic vehicles sometimes won’t get the chance to go into the lanes with electronic toll booths because they cannot change their lane. The same thing will happen to manual vehicles. Therefore, more conflicts and jamming may occur in these mixed traffic flows if the interchange is not properly planned and designed. This fact should be considered when deciding the placements of the electronic and manual toll exit lanes. For the current study also this fact should be incorporated.

Toll facilities have many constraints and unique characteristics (Ceballos). Measures of effectiveness have been defined in some studies to explain the level of service experienced by the users. Most significant measures of effectiveness are the queue lengths and waiting times. This study, which has compared queuing models and simulation, describes how the micro-simulation should be done. Some of the options and capabilities that Vissim possesses are mentioned there and those can be used to simulate the toll plazas in a more realistic manner. Given below are such options found in the literature review,

Service Time Distribution: Either an empirical or a normal distribution of the toll booth service times can be input with this option. This is useful when the toll transaction time data are available. A more realistic representation of the toll booths’ service performance can be simulated rather than using a constant value for the service time.
Speed Reduction Zones: Different speeds can be specified on a particular travel lane. Therefore, the simulation of toll lanes which vehicles are not required to stop, such as ‘electronic toll collection’ can be done by setting a reduced speed area.
Driver Behaviour: Some parameters of Vissim can be adjusted in order to calibrate it to suit the local driver behaviour conditions. Aggressive and cooperative behaviours are available in Vissim. Car following model, lane change behaviour, headway etc. can also be calibrated as desired.

2.3 Knowledge gap
As per the previous research findings, the present manual toll collection system of Sri Lanka will not have the capacity to accommodate the increasing demand in the future. However, comprehensive studies haven’t been done yet to evaluate the current toll stations' operation mechanism. Different tolling technologies that reduce the congestion at interchanges are available in developed countries. Prior to introducing such systems to Sri Lankan expressways, more studies should be done to understand the improvements they bring to the toll plaza operations. Hence, it is a timely requirement conduct a study to explore this research area.

2.4 Scope and limitations

Scope

Only the electronic toll collection method is considered as a new tolling technology
Scenario selected: Kottawa interchange in the year 2020
The electronic toll booth at Katunayake interchange is chosen as the prototype
‘Per vehicle delay’ and ‘departure rate’ of the toll plaza are considered as measures of effectiveness for evaluation purposes

Limitations

The commercial version of the micro-simulation software application is highly expensive to be purchased (the student version is used)
The only available traffic data source is the expressway management unit's database
Forecasting the future traffic is a complicated process due to the uncertainty of the demand

Leave a comment if you have any questions!!

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