Modeling the Urban Freight-Transportation System Using the System Dynamics Approach

Round 1

Reviewer 1 Report

Purpose: must be improved. It's very vague. 

Introduction: What is the problem under study? Why is this a research problem? What is the novelty? The goal must be presented in the introduction. 

Literature review: Should focus on the use of the method or the research problem.

Research design: How is the consumer related to the birth/deaths/ (Figure 1)? The structure of consumer behaviour is flawed. The authors should basis it on the literature. 

Include the entire structure of the SD model in the paper, with all relations. Explain all connections based on the literature.

Results: 1. Describe the data: Source; limitations. 2. The increase in the population increases the food demand. Therefore, increasing food demand implies to increase the number of freight trips/vehicles, etc. What is the novelty of this paper? 3. Describe the scenarios and the importance of analysing them in more detail.

Conclusion: Improve the conclusion and make clear the purpose and principal findings. What are the limitations? What is the suggestion for future work?

Author Response

While appreciating your valuable comments and careful consideration, we would like to write down some issues that we took into account in the article revision. We carefully reviewed the article and wherever it was needed, we add more detailed materials to the text of the article. Based on the importance of the research, we improved the literature review in two aspects. In the first aspect, the previously reviewed sources were further elaborated, and in the second aspect, more articles were reviewed and added to the literature review. The research questions, objectives and hypothesis of the research were clearly stated. More explanations about research scenarios were provided and added to the text of the article. In the results section, the relationship between the objective, covering the research gap in the literature and the main findings of the article were explained. Now, there is more discussion about the findings of the research. In relation to the method, more explanations were added for justification of results, limitations, and research area. Abstract, list of references, figures and English text were reviewed and improved. The written text was checked from the point of view of citation adequacy and citations were added to the required sections.

In the following, the responses to the comments and suggestions were presented and the corrections made in line with each one were explained.

• The purpose of the article was improved and clearly added in the abstract, introduction and conclusions sections. Some of the mentioned sentences are as follows.

1. "The main goal of this article is to model the urban freight transportation system to identify the factors affecting the urban freight transportation demand and capacity." Mentioned in abstract.
2. "Accordingly, the main goal of this article is to model the urban freight transportation system to identify the factors affecting the amount of urban freight transportation demand as well as urban freight transportation capacity and also to identify the causes of the problem in the system and to adopt policies which solve the problem of capacity-demand mismatch in urban freight transportation system." Mentioned in paragraph 4 of introduction.
3. "Considering this problem, the main goal of this article is to create a balance between freight transportation capacity and freight transportation demand, at an urban level, by identifying influencing variables, formulating dynamic relationships and simulating the system behavior." Mentioned in paragraph 1 of conclusions.

• The research problem and the novelty were presented in the abstract, introduction, and conclusion sections. The research problem and novelty are as follows:

About research problem:

1. "One of the main problems of this system is the mismatch between the freight transportation capacity and the total freight transportation demand." Mentioned in abstract.
2. "In general, one of the main problems of the urban freight transportation system is the mismatch between the freight transportation capacity and the total freight transportation demand." Mentioned in paragraph 4 of introduction.
3. "Considering the effects of urban freight transportation system and economy on each other, one of the most important challenges of managing this system is to create a balance between freight transportation capacity and freight transportation demand, in order to respond to the need to transport freight despite the optimal use of resources." Mentioned in paragraph 1 of conclusions.

About research novelty:

1. "The novelty of the present research can be seen in the two areas of modeling the urban freight transportation in a macro way with a holistic and systemic approach, which is not observed in the past researches, and using a dynamic method that examines the interactions of all the influencing variables of the system quantitatively and is capable of scenario planning in the urban area by using real data." Mentioned in last paragraph of introduction.
2. "Considering the lack of sufficient studies in the field of macro and quantitative modeling of this system, this article are modeled the urban freight transportation by quantitative scenarios for the first time." Mentioned in paragraph 1 of conclusions.

• The literature review was reviewed by focusing on the method used and the research problem. On the one hand, the previously reviewed sources were further elaborated with this point of view, and on the other hand, several sources were deleted and several sources were added and explained.
• Regarding the relationship between the consumer and the death rate, the followings are important:

1. Death and birth rates affect the population and the interaction of the population and the household size results in the number of households. On the other hand, considering the GDP variable, household income is obtained. Therefore, the variables of household number and household income are used for input data to other loops. For example, from the interaction of these two variables and variables such as the fruit price and the fruit consumption proportion, the consumption fruit freight demand is determined.
2. In the last paragraph of section 3.1.1, a sentence was presented to explain this issue. "Socioeconomic structure, household structure, and economic variables of urban households, determine the consumption demand of each group of fruit, food, agricultural, construction, and industrial goods."
3. This sentence has been added to the last paragraph of section 3.1.1 for clearer explanation: "In fact, the variables in socioeconomic structure and consumption behavior, such as the number of households and household income, have been used as inputs for the estimation of the consumption freight in each of freight groups."

-    Due to the extensiveness of the model and its lack of integrated presentation, the model was designed using shadow variables in different subsystems in different views in Vensim software. Also, in order to understand and relate the variables to each other, more explanations were provided in the text. In figure 7, the urban freight transportation demand is presented. Each variable, such as consumption fruit freight (figure 2), consumption food freight (figure 3), consumption agricultural freight (figure 4), consumption construction freight (figure 5), and consumption industrial freight (figure 6) are entered into the model with its distribution factor. Variables are presented in pervious figures. Figure 8 also specifically deals with the freight transportation capacity modeling.

- The data sources of this research are the database of the "Iran Statistics Center" and the database and the results of studies on "the Transportation Master Plan of Shiraz city". These cases were clearly added in the second paragraph of the results section. Also, regarding data limitations, the following items can be mentioned, which were specifically added to the explanations of the second paragraph of the results section.

1. Since some of the mentioned variables, Such as per capita household income, price of consumer goods, the share of each group of goods in the household expenses, and vehicle depreciation rate, are extracted on average for the entire country and do not exist specifically for the investigated city, the average of the entire country is used in this article. Also, due to the fact that inflation was not available separately for the product groups used in this article, the average annual inflation rate was used for the so-called groups. This explanation was added to the text.
2. As you know: one of the most important requirements of any field is modeling the existing realities of a system and simulating its behavior. In this way, the control variables of each system can be known and the impact of adopting policies can be evaluated. On the other hand, due to the mutual effects of freight transportation and economic, political, demographic and environmental issues, knowing freight transportation system is very important. These interrelationships make the complexity of quantitative modeling of the urban freight transportation system very difficult, as quantitative modeling has not been done in this field in the past. Therefore, this research, for the first time, aims at quantitative modeling of urban freight system and simulating its behavior based on real data. By knowing the control variables of the system, formulating mathematical relationships and designing several scenarios, this article deals with the detailed evaluation of the effectiveness of each control variable and the results of urban policies. The developed model can be used in all cities using their data. The results of this model help city managers to adopt appropriate policies with knowledge of their results. Also, this article provides solutions to maintain the balance between supply and demand, which, as a result of their implementation, prevents the wastage of resources due to the large capacity of moving and the creation of problems in the proper functioning of the movement of goods due to the lack of capacity. The literature review also shows some researchers have modeled a part of the road freight transportation system in a micro view, and some other researchers have done a macro view of the area of freight in other modes of transportation such as rail, maritime, and air transportation. In other words, according to the knowledge of the authors, so far no researcher has studied the field of urban freight transportation in a quantitative and macro view.
3. The scenarios and importance of analyzing them were described in more detail and added to the text of the article.

- Based on your valuable opinion, the conclusions section was reviewed and edited in general, and the main purpose and principal findings of the research were clearly stated. Limitations and suggestions for future research are added as follows in the results section:

"Due to the limitations in order to maintain the focus of the research and the boundary of the model, some variables affecting the system have been added exogenously to the model. Therefore, it is suggested to add these variables as endogenous and develop specific sub-models in future research. Generally, the designed model can be used to de-sign different quantitative scenarios in order to optimize the freight transportation system’s performance. Therefore, it is suggested to define other combined scenarios in accordance with the specific challenges of each city. This study can help policymakers to manage the urban freight transportation system more efficiently. It is suggested that the proposed model be calibrated and used with the real data of each city".

- The article was checked and edited for the correctness of the English text.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper aims to identify the influencing variables of urban freight transportation and to model their relationships by the use of the system dynamics approach. The authors use real data from an urban area of Iran and formulate scenarios. A subject that has been approached by an interesting perspective.

Section 2 Literature Review, needs to be enriched with more citations on the subject and to emphasize the research gap and the main objective of the paper.

In page 3, first paragraph “… several researches focused mainly on road freight transportation…” but in the next paragraph the authors write “…limited primarily to the rail and air transportation modeling.” These are two contradictory findings. Please explain.  

In Section 3, some more theoretical material for the system dynamics needs to be added (before subsection 3.1).

Page 5, last paragraph of section 3.1.2 “Passing fruit freight …” please explain.

Page 6, section 3.1.4, “…their profitability decreases according to the existing demand (Loop B8)” and “…As a result, their establishment rate reduces (loop B10)”. Please enter the correct loops.  

Section 4.1, pages 10 and 11, please enter the name of the y-axis for all figures (figure 10, 11 and 12). The same for the next sections.

Page 11, last paragraph, please correct the error.

Section 4.2, all references to the figures are not correct, e.g., page 12, “… exceeds the amount of freight demand until 2027(Figure 9) …”, “… freight demand has been investigated (Figure 10) …”, page 13, “… The amount of the excess supply gap is equal to 2.6% (Figure 11) …”, etc., until the end of page 14. Please correct them all.  

In Figure 14, the red line for freight capacity base run is not visible.

Conclusions are like a summary of the text. It needs additions with the key points and to highlight the importance of the research

The Abstract needs to be shortened. It does not seem like an abstract.

Also, some more remarks:

Page 3, first paragraph “… system [13] [14] [15] [16] and supply chain management [17] Several researches …”, please add a full stop after [17].

Page 7, “… The freight demand for to the repair-reconstruction of building…” please delete “to”.

Page 9, Section 4, “To effect of changes in the endogenous variables of freight transportation…” meaning unclear.

The references listed in the References section, need to be reviewed carefully. Example: “Aschauer, G.J.; Steyr, L. The Development of Sustainable Transports through the Right Logistics Strategy – a System Dynamics Approach”, please add all the necessary information (e.g., date, journal, etc.).

Minor editing required.

Author Response

While appreciating your valuable comments and careful consideration, we would like to write down some issues that we took into account in the article revision. We carefully reviewed the article and wherever it was needed, we add more detailed materials to the text of the article. Based on the importance of the research, we improved the literature review in two aspects. In the first aspect, the previously reviewed sources were further elaborated, and in the second aspect, more articles were reviewed and added to the literature review. The research questions, objectives and hypothesis of the research were clearly stated. More explanations about research scenarios were provided and added to the text of the article. In the results section, the relationship between the objective, covering the research gap in the literature and the main findings of the article were explained. Now, there is more discussion about the findings of the research. In relation to the method, more explanations were added for justification of results, limitations, and research area. Abstract, list of references, figures and English text were reviewed and improved. The written text was checked from the point of view of citation adequacy and citations were added to the required sections.

In the following, the responses to the comments and suggestions were presented and the corrections made in line with each one were explained.

• Based on your comment, the literature review was totally revised and edited, and several related articles were added to this section. The previously reviewed sources were further elaborated and more articles were reviewed and added to the literature review. Also, in the last paragraph of the literature review, a summary of the reviews was added, in which the existing research gap and the main objective of the article have been emphasized. It is worth noting; based on many investigations and the knowledge of the authors, this article is novel in the field of urban freight modeling and quantitative assessment of the scenario, and no research was found that matches the purpose of this article.
• The meaning of the two sentences you mentioned is that some researchers have modeled a part of the road freight transportation system in a micro view, and some other researchers have done a macro view of the area of freight in other modes of transportation such as rail, maritime, and air transportation. In other words, according to the knowledge of the authors, so far no researcher has studied the field of urban freight transportation in a quantitative and macro view. However, in the main text of the article, the sentences were edited so that the meaning should be clearly conveyed to the readers. Also, in the last paragraph, this issue was considered and clearly mentioned in the summary.
• Theoretical materials of the system dynamics were added in section 3.
• More explanations were added in section 3.1.2.
• All items have been corrected.
• Necessary corrections were made in all sections.
• Cross reference error fixed.
• All references to the figures have been corrected.
• The freight capacity base run line became visible.
• The conclusions section was reviewed and edited and the key points and importance of the research were clearly stated.
• The abstract section was reviewed and edited.
• All 3 remarks have been corrected.
• The list of references was reviewed and modified in terms of the completeness of the necessary information of each article.
• The article was checked and edited for the correctness of the English text.

Author Response File: Author Response.docx

Reviewer 3 Report

There are gaps in the article that need to be filled:

1) the background to undertaking the research

2) justification of the methodology in view of the universalisation of results

3) a literature review not only on modelling and forecasting methods, but also in terms of the general and managerial scope of the research

4) a justification for the choice of areas for research - why only these were taken into account, the basis for the choice and the limitations

5) there is a lack of discussion 

6) what social, managerial implications the research results have 

7) the research objectives and hypotheses were not sufficiently highlighted

8) the research results should be commented on 

Minor editing of English language required

Author Response

While appreciating your valuable comments and careful consideration, we would like to write down some issues that we took into account in the article revision. We carefully reviewed the article and wherever it was needed, we add more detailed materials to the text of the article. Based on the importance of the research, we improved the literature review in two aspects. In the first aspect, the previously reviewed sources were further elaborated, and in the second aspect, more articles were reviewed and added to the literature review. The research questions, objectives and hypothesis of the research were clearly stated. More explanations about research scenarios were provided and added to the text of the article. In the results section, the relationship between the objective, covering the research gap in the literature and the main findings of the article were explained. Now, there is more discussion about the findings of the research. In relation to the method, more explanations were added for justification of results, limitations, and research area. Abstract, list of references, figures and English text were reviewed and improved. The written text was checked from the point of view of citation adequacy and citations were added to the required sections.

In the following, the responses to the comments and suggestions were presented and the corrections made in line with each one were explained.

1. Some more explanation about this case was added to introduction and literature review. For describing the background we could say that the literature review shows some researchers have modeled a part of the road freight transportation system in a micro view, and some other researchers have done a macro view of the area of freight in other modes of transportation such as rail, maritime, and air transportation. In other words, according to the knowledge of the authors, so far no researcher has studied the field of urban freight transportation in a quantitative and macro view. As you know, one of the most important requirements of any field is modeling the existing realities of a system and simulating its behavior. In this way, the control variables of each system can be known and the impact of adopting policies can be evaluated. On the other hand, due to the mutual effects of freight transportation and economic, political, demographic and environmental issues, knowing freight transportation system is very important. These interrelationships make the complexity of quantitative modeling of the urban freight transportation system very difficult, as quantitative modeling has not been done in this field in the past. Therefore, this research, for the first time, aims at quantitative modeling of urban freight system and simulating its behavior based on real data. By knowing the control variables of the system, formulating mathematical relationships and designing several scenarios, this article deals with the detailed evaluation of the effectiveness of each control variable and the results of urban policies. The developed model can be used in all cities using the data of the same city. The results of this model help city managers to adopt appropriate policies with knowledge of their results. Also, this article provides solutions to maintain the balance between supply and demand, which, as a result of their implementation, prevents the wastage of resources due to the large capacity of moving and the creation of problems in the proper functioning of the movement of goods due to the lack of capacity.
2. More explanation was added in the first paragraph of the Materials and Methods section: "The justification of implementing the system dynamic method in this article can be mentioned from two points of view. Firstly, complexity and dynamism are the most important features of urban freight transportation system, and it can be evaluated and classified as a complex and evolved system. These features have made it challenging to understand the system behavior and formulate effective solutions to achieve optimum performance. Such system’s characteristic differs from simple and linear relations, and naturally, their analysis also requires the use of new and creative approaches. To face such problems, it seems more necessary to develop approaches that can create a deep and comprehensive understanding of the system. It is necessary to have system thinking, so that one can develop the boundaries of their mental models and use compatible tools which can be used to understand the structure of complex urban freight transportation system and its behavior. Secondly, since the system dynamics provide a comprehensive framework to analyze a particular system, it makes other researchers confident about the generalizability of the model design across various context and conditions. This is be-cause system dynamics captures the dynamic of real-world phenomena in a particular system, which can be captured in other similar systems to observe how different variables interact to produce certain outcomes. In this case, the model should be validated with the real data of each particular system."
3. The literature review was totally reviewed by focusing on methods and scope of the research in terms of the general and managerial. The previously reviewed sources were further elaborated with this point of view, and also, several sources were added and explained. Some more explanations about the general and managerial scope of the context were added to clarify this point of view.
4. More explanation was added in the first paragraph of the Materials and Methods section: "The justification of implementing the system dynamic method in this article can be mentioned from two points of view. Firstly, complexity and dynamism are the most important features of urban freight transportation system, and it can be evaluated and classified as a complex and evolved system. These features have made it challenging to understand the system behavior and formulate effective solutions to achieve optimum performance. Such system’s characteristic differs from simple and linear relations, and naturally, their analysis also requires the use of new and creative approaches. To face such problems, it seems more necessary to develop approaches that can create a deep and comprehensive understanding of the system. It is necessary to have system thinking, so that one can develop the boundaries of their mental models and use compatible tools which can be used to understand the structure of complex urban freight transportation system and its behavior. Secondly, since the system dynamics provide a comprehensive framework to analyze a particular system, it makes other researchers confident about the generalizability of the model design across various context and conditions. This is be-cause system dynamics captures the dynamic of real-world phenomena in a particular system, which can be captured in other similar systems to observe how different variables interact to produce certain outcomes. In this case, the model should be validated with the real data of each particular system."
5. More explanations were added to explain all the scenarios, and in the last paragraph of the results section, more details were added to further describe the discussion.
6. More explanation about the social and managerial implications of research was added to introduction, literature review, results, and conclusion. For describing the managerial implication we could say that this research, for the first time, aims at quantitative modeling of urban freight system and simulating its behavior based on real data. By knowing the control variables of the system, formulating mathematical relationships and designing several scenarios, this article deals with the detailed evaluation of the effectiveness of each control variable and the results of urban policies. The developed model can be used in all cities using the data of the same city. The results of this model help city managers to adopt appropriate policies with knowledge of their results. Also, this article provides solutions to maintain the balance between supply and demand, which, as a result of their implementation, prevents the wastage of resources due to the large capacity of moving and the creation of problems in the proper functioning of the movement of goods due to the lack of capacity. The literature review also shows some researchers have modeled a part of the road freight transportation system in a micro view, and some other researchers have done a macro view of the area of freight in other modes of transportation such as rail, maritime, and air transportation. In other words, according to the knowledge of the authors, so far no researcher has studied the field of urban freight transportation in a quantitative and macro view.
7. The objectives of the article was improved and clearly added in the abstract, introduction and conclusions sections. Some of the mentioned sentences are as follows.

• "The main goal of this article is to model the urban freight transportation system to identify the factors affecting the urban freight transportation demand and capacity." Mentioned in abstract.
• "Accordingly, the main goal of this article is to model the urban freight transportation system to identify the factors affecting the amount of urban freight transportation demand as well as urban freight transportation capacity and also to identify the causes of the problem in the system and to adopt policies which solve the problem of capacity-demand mismatch in urban freight transportation system." Mentioned in paragraph 4 of introduction.
• "Considering this problem, the main goal of this article is to create a balance between freight transportation capacity and freight transportation demand, at an urban level, by identifying influencing variables, formulating dynamic relationships and simulating the system behavior." Mentioned in paragraph 1 of conclusions.
• Also, the dynamic hypotheses research was added in the materials and methods and conclusions sections:

Materials and methods:

"In order to verify the problem solving scenarios, general dynamic hypotheses have been developed based on the interaction of the main variables of the system. A dynamic hypothesis is an idea about what structure might be capable of generating behavior like that in the systems main variables. A dynamics hypothesis can be stated verbally and in this case it can be expressed as follows:

• An increase in freight transportation demand leads to an increase in the discrepancy with the freight transportation capacity and as a result, reinforces the development process of freight transportation capacity.
• If freight transportation capacity is more than freight transportation demand, due to the excess supply in the system, the process of freight transportation capacity development becomes slower in order to balance supply and demand in the system.
• An increase in the main influencing variables of freight transportation capacity sub-system, such as travel length, number of freight trips, and the numbers and average capacity of transportation fleet, in accordance with the existent freight demand in the system, leads to an increase in the amount freight transportation capacity.
• Optimizing the urban freight distribution system, by reducing the distribution coefficient, leads to a decrease in total freight transportation demand."

Conclusions section:

"The main dynamic hypotheses of the research have been developed based on the interaction of the main casual loops. It is based on goal-seeking behavior of freight transportation capacity which balances the capacity with demand through interaction loops between these two variables. By changing the control variables, capacity will undergo changes that match it with the demand."

8. More explanations were added to results, and in the last paragraph of the results section, text was written and added to further describe the results.

The article was checked and edited for the correctness of the English text.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I thank the authors to include my suggestions to improve the quality of the paper. Best regards

Reviewer 2 Report

No further cmments.

Reviewer 3 Report

The revised version is better. 

no