Exploring Urban Compactness and Greenhouse Gas Emissions in the Road Transport Sector: A Case Study of Big Cities in South Korea

Thank you for your valuable feedback regarding our paper. In response to your comments, we have thoroughly revised the paper to enhance its clarity and coherence. We have paid attention to articulating the conclusions more clearly. We believe these modifications offer a more precise and comprehensive overview of our study.

Comment

Response

1.     It is preferable that there are no citations in the conclusions chapter

We agree with your opinion. However, even if it is a conclusion, if an argument or academic proposition on a specific issue has been written by another researcher, we believe it should be cited. Therefore, we prefer not to write as much as possible about the part you reviewed, but we left the content we cited intact to comply with research ethics. Furthermore, we have removed citations that appeared to be unnecessary as they contained our own arguments Thank you for the review.

2.    The graphic component looks good, however, the way in which the correlations between the indicators and the population are made is not very clearly described.

As you mentioned, we confirmed that the derivation of the newly modified indicator is not clearly explained. Therefore, we have inserted Formula 5 and added the term 'adjusted' to Table 2 to make it more understandable (page 6 of 12).

3.    Also on the graphic component, I recommend the authors to create maps in which the links between the four previously mentioned parameters are highlighted.

The purpose of this study is not to elucidate the relationship among the three parameters and create a single compactness index that integrates them. Therefore, separate maps were generated for each index. A key finding of this study is that, in order to better represent the compactness of actual cities, the urban compactness index should consider normalized city size, which compares city size relative to others. However, as you mentioned, it would also be meaningful to understand their relationships and map them. We will consider this when further developing this study or conducting related research in the future. Thank you for your comments.

Thank you for your valuable feedback regarding our paper. In response to your comments, we have thoroughly revised the paper to enhance its clarity and coherence. We have paid attention to articulating the methods, and results more clearly. Specifically, we have restructured all parts including citations based on your comments. We believe these modifications offer a more precise and comprehensive overview of our study.

Comment

Response

1.     There is a very unclear procedure to analyze the correlation between GHG emission and UCIs. On raw data there is only one significant correlation. The authors adjust the values by multiplying UCIs with normalized city population. Since Gini and entropy index are smaller for larger city, there is difficult to interpret data, and relations are distorted. The conclusions are difficult to support.

We multiplied the normalized city size, which indicates the relative size of the population, with each UCI and compared this with the per capita greenhouse gas emissions. For large cities like Seoul, the UCI does not show a large or small value because they have a large population. Instead, per capita emissions appear small. This can be seen from the attached figure. If UCIs are used as they are, areas with smaller populations will show larger values than areas with larger populations. This is because when cities are divided into cells, areas with smaller populations have many areas where people do not live, which increases the value in the algorithm of the index calculation formula. This is because it has a mechanism that increases the value as the population difference increases. This study argues that this problem should be addressed and improved by correcting for the relative size of the city size by multiplying it with each index.

2.    The statistical analysis is very low for a scientific article. May be the linear regression is not the most suitable model.

In exploring the relationship between UCIs and greenhouse gas emissions, this study offers a method to interpret UCIs more accurately. Using UCIs directly in the same way as before may not show a clear linear relationship. However, using a corrected value that includes multiplying by city size, at least to some extent, the inverse relationship shows some linearity. This can be seen from the difference between Table 1 and Table 2. Additionally, dividing the data into groups based on city size reveals that compactness and greenhouse gas emissions are not unrelated. This, in itself, is considered a significant discovery.

3.    The citation is not successive (paper 29-31 are cited after 32 and 33).

We have reviewed and corrected it. Thank you for pointing it out.

4.    There is a limitation of the study due to a single year available data.

Yes, this is a clear limitation of the study. To more clearly explain the limitations of the study, we have included relevant content in the paragraph discussing limitations on page 10.

5.    It is not clear what is the source for Tier 1-3 emission models. What guidelines are used?

It is based on the '2006 IPCC Guidelines for National Greenhouse Gas Inventories – Energy – Mobile Combustion.' While the data being developed in South Korea's Greenhouse Gas Inventory Report (GIR) is currently at the tier 1 level, we have constructed ours at the tier 2 level. As you can see, we considered vehicle types and control technologies, which is a much more sophisticated method than the tier 1 GIR. It is worth noting that a more advanced method than our study is Tier 3, which requires activity data such as VMT.

6.    Lines 146-147 are redundant with line 138.

We have reviewed and deleted it. Thank you for pointing that out.

Comment

Response

1.    The title of the manuscript should be further refined. In my opinion, the title needs to be modified further. It may be like this: “Exploring Urban Compactness and Greenhouse Gas Emissions in the Transportation Sector: A Case Study of Big Cities in South Korea.”

[p1] We have revised the title of our study. To make it more concise and reflective of our specific focus, we have updated it to: “Exploring Urban Compactness and Greenhouse Gas Emissions in the Road Transport Sector: A Case Study of Big Cities in South Korea.” This modification from ‘Transportation’ to ‘Road Transport’ better represents our concentrated analysis on the road transport sector GHG emissions, rather than ones of the entire transportation sector.

2.    Abstract: The section needs to be better worked out. In my opinion, it lacks sufficient clarity. The content of the methods in the abstract is not sufficiently clear. I would suggest improving it. The result and conclusion need to be improved too. The authors need to answer these questions in the abstract section: Why did you do the study? What did you do? What did you find? What did you conclude?

[p1] As you mentioned, we modified the abstract. We elaborated our style with more clarity and clear sentences. Especially, we had focused on articulate the sentences about the result and conclusion based on the questions you suggested “Why did you do the study? What did you do? What did you find? What did you conclude?”

3.    Introduction: this section looks at insufficient relevant information. So, I suggest to the authors in the introduction section to tell the reader about the question your paper addresses, then summarise relevant literature relating to this issue (not a literature review), and then finish by telling your reader what the rest of the article is going to do.

[pp1~2] In accordance with your observations, we have acknowledged the presence of some insufficiently relevant information in the introduction. In this revised version, rather than listing each reviewed literature and prior study individually, we have restructured our approach to focus concisely on the central theme of our paper. We have now consolidated and summarised the reviewed literature and prior studies in a manner that directly relates to our subject matter. Additionally, we have refined the end of section of introduction to better guide the reader through the upcoming content of the paper, ensuring it is more understandable and accessible.

4.    I would like to see a clearer justification for undertaking the study. The paper's contribution to the literature needs to be better worked out, and the novelty of the paper should be presented clearly.

[pp1~11] We have reorganized and restructured the introduction to present a clearer justification for undertaking the study and our paper’s contribution to the literature clearly.

5.    Materials and methods: The section needs to be better worked out. In my opinion, it is hard to fully understand it. So, I would suggest presenting this part clearly. I recommend that the author divides the section into two parts. Firstly, present the data used for the study, and secondly, present the econometrics model.

[pp2~5] As you suggested, we have divided the 'Materials and Methods' section into two parts. In the 'Materials' part, we have listed the spatial and temporal scope of the study, as well as the subjects of analysis, and explained the data collection method for road-transport GHG emissions. In the 'Methods' part, we have described the analytical methods that are relevant to the purpose of this study.

6.    In Part 2, the clarity of the econometric model used in the study needs improvement. I recommend presenting this section in a clearer manner. Additionally, please provide explanations of the econometric model, covering both the model specification and your data specification. Furthermore, tell the reader why you used this model for the econometric analysis in this study.

[pp4~5] We have clearly explained the analysis process and methods in the '3. Methods' section. Since this study did not involve such a regression analysis, a separate model was not constructed. Apart from displaying basic statistics using scatter plots, we inferred the implications of the new urban compactness through simple bivariate correlation analysis. Therefore, to make it easier for readers to understand, we have added an explanation of this aspect to the paper.

7.    Result: In Part 3.1 and Part 3.2, the writing needs to be clearer and directly aligned with supporting the research question. However, I did not see a clear presentation of your findings in the manuscript that supports your research question. Therefore, I recommend that the authors clearly present their findings in the results section.

[pp5~11] Since we divided the ‘Materials and Methods’ section into two parts, previously labeled as ‘3.Results,’ we have moved the ‘Results’ section to ‘4.Results and Discussion.’ The main findings of this study clearly confirm the relationship between urban spatial structure and GHG emissions. Additionally, it is important to consider city size as a weighting factor for the three indicators of the urban compactness index. We have described these findings more clearly compared to the previous version.

8.    In Part 4, the Discussion section could be incorporated into Part 3 under the title 'Results and Discussion.' This would help reduce redundancy caused by repetitive words and improve the overall structure of the manuscript.

[pp5~11] We have merged the ‘Results’ and ‘Discussion’ sections into a single section. We have minimized repetitive arguments and expressions as much as possible.

9.    The conclusion must be enlarged, adding limitations and further research.

[pp9~11] We have incorporated this content into the '4.3 Discussion' subsection of the '4. Results and Discussion' section, and reorganized the paragraphs in the ‘Discussion’ for clearer understanding. The paper has been restructured to conclude with the limitations of the study and future research plans."

10. [Comments on the Quality of English Language] Readability needs to be improved, especially at certain points. I would suggest writing in a more “concise” manner at certain points too.

As per your suggestion, we have restructured and refined the English sentences by selecting clearer words and expressions to ensure easier comprehension for readers.

Comment

Response

1.    The spatial analysis does not sufficiently illustrate the application of geographic information systems and Big Data in the study of urban compactness and transport sector emissions. With the help of geographic information systems, it seems possible to obtain specific planning parameters that would clarify the proposed UCI compactness index. In addition, the mapping method and spatial analysis operations using GIS technologies could clearly demonstrate the results of assessing the spatial distribution of GHG emissions and the effectiveness of measures to reduce them.

We have fully understood your comments. It is indeed crucial to identify various components of cities and examine their relationship with greenhouse gas emissions, as evidenced by numerous previous studies. However, the aim of our study is not to derive such urban planning parameters. The primary focus of our study is to deeply investigate the relationship between three indices commonly used as measures of urban compactness—Entropy index, Moran’s I, Gini coefficient—and greenhouse gas emissions. Through this analysis, we found that by adjusting the Urban Compactness Index (UCI) with city size as a weighting factor, we were able to establish a significant linear relationship between urban spatial structure and per capita greenhouse gas emissions in the road-transport sector. While your feedback is indeed important, we will ensure to consider it thoroughly when conducting future research based on our study.

2.    When analyzing the planning structures of large cities, their types were not noted, combining elements of a regular, linear, radial-ring type, which increases traffic intensity, primarily in the central parts of the city.

In this study, we defined city size as population, as commonly understood in various urban planning literature. Therefore, we did not consider the physical form such as radial-ring type, linear type and so on of the city. It is known that in many studies related to urban form, when dealing with the three UCI, the physical form is not usually considered. However, I acknowledge that the aspect you have pointed out is indeed an important research topic. In future studies, we will consider incorporating this physical form as a variable in addition to urban physical form and urban spatial structure.

3.    In assessing greenhouse gas emissions, the city's fuel and energy balance method was used to identify emissions from the transport planning sector. The approach involves a transition from the emission value of the entire city as a whole, which can be obtained from aggregate statistical data, to individual territorial units that make up the city, taking into account their specifics, including differences in population density, types of development and intensity of traffic flows. The advantages of the method are the availability of open sources of initial data, as well as the absence of the need for the total amount of emissions. In our opinion, the applied method did not allow us to reflect the influence of planning parameters, but only takes into account the type of vehicle, type of fuel, etc.

Similarly to our response to your first comment, this study investigated the relationship between the three indicators of the Urban Compactness Index (UCI)—Entropy index, Moran’s I, Gini coefficient—and the per capita greenhouse gas emissions in the road-transport sector, which are closely related to urban spatial structure. Therefore, we intentionally excluded various urban components other than population and emissions. Additionally, the differentiation between types of fuel and vehicles represents more refined data than the greenhouse gas emissions data currently provided as open source in South Korea. However, understanding the urban components that influence greenhouse gas emissions is crucial, and we will ensure to consider this aspect thoroughly in future research. Furthermore, we have extensively revised and restructured the paper to make it more accessible to readers in this regard.

4.    . In this regard, the reliability of the data obtained for the period of one year - 2019 raises questions.

Due to the limitations in data availability, our analysis was conducted only for the year 2019. Although the Greenhouse Gas Inventory and Research Center of Korea (GIR) provides greenhouse gas emissions data for 2020, it is not sufficiently detailed at the regional level, making it challenging to use for our purposes. Furthermore, the road-transport sector greenhouse gas emissions data we constructed ourselves requires more specific information such as vehicle types and fuel types, which is only available up to 2019. Therefore, if we can obtain updated data, we plan to conduct future research on the relationship between urban components and greenhouse gas emissions using the most recent data available.

5.    The captions in Figures 1 and 5 are not informative. A transcript of the individual diagram images obtained should be given. Under what conditions are they obtained?

I have made improvements to Figures 1 and 5 to enhance their visibility. Figure 1 now depicts scatter plots showing the relationship between the three indicators of the Urban Compactness Index (UCI) and per capita greenhouse gas emissions in the transportation sector. Figure 5 presents box plots of per capita greenhouse gas emissions in the transportation sector, categorized into five groups based on city size.

6.    Perhaps in conclusion, more attention should have been paid to establishing the cause-and-effect relationships of the phenomenon being studied between urbanization and greenhouse gas emissions.

This study investigated the improvement of the Urban Compactness Index (UCI) by considering city size rather than urbanization, as well as the relationship between this enhanced UCI and greenhouse gas emissions. As you pointed out, I have revised and improved the conclusion section ('4. Results and Conclusion') with clearer sentences and vocabulary.

7.    Also, in our opinion, the list of published studies on the topic, including those published by MDPI, is insufficient.

I have added references including ‘Sustainability.’