Quantifying Hidden Carbon Emissions Induced from Curbside Capacity Loss in Urban Freight Operations

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents an original and conceptually valuable framework linking curbside spatial design to operational performance and environmental externalities through the introduction of the Factor of Occupancy (Fo) and the Hidden Carbon Emissions (HCE) indicator. However, the paper exhibits several structural methodological inconsistencies between the physical curbside system and the analytical models employed:

1. Fo = L · Tmax (m·min) is treated as a design capacity indicator, but this formulation does not correspond to a physically meaningful service capacity in a discrete vehicle system. Vehicles are indivisible service units, and curbside operations are governed by discrete parking slots and vehicle lengths, not continuous spatial metrics. As a result, Fo represents a continuous abstraction that lacks a direct physical mapping to the maximum number of serviceable vehicles.
2. The use of an M/M/1/1 loss model assumes a single service position. However, the restored 14 m configuration can physically accommodate multiple vehicles simultaneously, implying a multi-server system. The appropriate formulation should therefore be M/M/c/c (Erlang loss with c servers), where c reflects the number of parallel curbside service positions. Using M/M/1/1 introduces a structural mismatch between the model and the physical system.
3. The model adjusts system performance by modifying the service rate μ (via weighted dwell time) to represent increased curbside length. This is conceptually incorrect: curbside length affects the number of simultaneous service positions (capacity), not the service time of individual vehicles. Service time is a behavioral/operational variable, whereas curbside length is a structural variable. This parameter substitution misrepresents the physical meaning of μ.
4. The conversion: Fo_diff (m·min) / weighted dwell time (min) = “available linear meters (m)” is dimensionally consistent but physically meaningless. It implies the existence of a “monthly linear meter stock” of curbside space, which has no real-world interpretation. This step breaks the physical interpretability of the capacity reconstruction process.
5. The “weighted vehicle length” is computed using the number of unloading operations as weights. However, spatial occupation is governed by vehicle length × dwell time, not frequency of use. The current method ignores the temporal dimension of spatial occupation, leading to systematic bias in capacity estimation.
6. The analysis assumes constant demand under different curbside configurations. In reality, improved curbside accessibility affects delivery scheduling, routing strategies, and arrival timing. This neglect of behavioral feedback mechanisms leads to an overestimation of service gains under restored curbside capacity (induced-demand effect).
7. A single EFkm value (EU new van average) is applied across heterogeneous vehicle categories (vans, chassis cabs, 7.5t trucks). This ignores order-of-magnitude differences in emission factors between vehicle classes, introducing systematic bias into HCE estimates rather than random error.
8. The same idling emission factor is applied to all vehicles, despite observed refrigerated vehicles requiring continuous auxiliary power. Refrigeration units significantly increase fuel consumption during idling, making the adopted EFmin structurally inaccurate for this vehicle subset.
9. All results are presented as point estimates (blocking probability, HCE, HCE/m, HCE/rej) without confidence intervals, sensitivity analysis, or uncertainty propagation. Policy-relevant indicators expressed in kgCO₂/m or kgCO₂/vehicle therefore lack statistical reliability and decision robustness.

Author Response

All comments and, where applicable, modifications have been reported in the attached file. We sincerely appreciate your time and expert comments, which undoubtedly enrich our article.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper introduces the "Factor of Occupancy" (Fo) and "Hidden Carbon Emissions" (HCE) indicators, combining a loss-based queuing model with empirical observation to systematically analyze the impact of curbside loading/unloading zone allocation on freight efficiency and the environment. The conclusions offer valuable insights for urban logistics planning and low-carbon transportation policy. The paper is generally well-structured, but the following points should be clarified and improved.

1. Figure 1 requires significant revision for clarity and proper integration with the text. Figure 1 is currently not mentioned within the manuscript, and a specific cross-reference to it is lacking in the whole article. Furthermore, label number four is presented in caption of Figure 1 but not explained.
2. Please carefully proofread and format your manuscript. Numerous formatting issues are observed, such as inconsistent or missing spacing, inconsistent font sizes for sub-headings and body text, lack of a space and period before "Figure 2", use of the abbreviation "MMA" without its full definition, and unusual font sizes in equations. The formula for Pblock appears non-italicized. Furthermore, descriptions for variables should be placed in subscripts/superscripts rather than concatenated into the main variable name. There are multiple issues with equation presentation; please revise and check them thoroughly.
3. In lines 566 to 583, variable symbols can be used to simplify expressions. However, please ensure consistency if the same symbols have been used earlier. It is recommended to number the equations and reference these equation numbers when explaining the derivation steps in the actual calculation, which will provide clearer and more explicit logic.
4. The font in Table 5 is problematic, and the equations appear not to have compiled correctly. Please revise these carefully.

Author Response

All comments and, where applicable, modifications have been reported in the attached file. We sincerely appreciate your time and expert comments, which undoubtedly enrich our article.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The literature review is comprehensive, though it might benefit from a deeper synthesis of studies focusing on space-time indicators for curbside management. Drawing more explicit comparisons with existing design metrics would help to further clarify the rationale behind introducing the Factor of Occupancy .

Regarding the methodology, the application of the M/M/1/1 queueing model is a reasonable choice for loss systems, yet some additional justification for the single-server assumption in the context of curbside zones would be welcome. It might also be useful to briefly discuss why alternative models were not selected.

The behavioral classification of rejected vehicles could use a more concrete operational definition to avoid ambiguity during data collection and to facilitate replication.

The Fo indicator, as currently formulated, assumes uniform vehicle lengths. Given the heterogeneity in vehicle sizes and their varying space requirements, a weighted Fo might provide a more accurate reflection of actual capacity utilization. The current approach could potentially lead to over- or under-estimation in mixed-fleet scenarios.

Additionally, calculating queueing model parameters as monthly averages may overlook temporal variability. This could mask short-term congestion patterns and limit the model's predictive power for dynamic operations.

The environmental analysis is a good start, though it is currently limited to tailpipe CO2 emissions. Expanding the scope to include other pollutants or indirect impacts would make the sustainability assessment more robust.

The discussion on regulatory compliance appears somewhat idealized. Addressing practical implementation challenges and their potential effects on HCE reduction would add realism to the analysis.

And the policy implications, while relevant, are a bit broad. Providing specific implementation recommendations or a cost benefit analysis of restoring curbside length would strengthen this section. The conclusion would also benefit from a more balanced discussion of the trade offs between nonlogistics curbside uses and freight needs, as this would enhance the overall policy relevance.

 

Author Response

All comments and, where applicable, modifications have been reported in the attached file. We sincerely appreciate your time and expert comments, which undoubtedly enrich our article.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Here are my comments:

1. the format of references in the text seems inappropriate. Please check carefully.
2. I suggest to present some pictures of curbside space in Section 1, to give readers a direct image about the research object of this study.
3. I feel that the first paragraph of section 2 is just a repetition of background introductino, so I suggest to delete this paragraph.
4. Figure 1 is not referenced in the text.
5. It seems that the authors put the emphasis on proposing a new method to measure the hidden carbon emission in the context of curbside space. Therefore, I strongly suggest to introduce the methodology first, and next present the data.
6. The equations in section 4 should be labeled with numbers.
7. Regarding the results, my main concern is how did the authors prove that the results are believable and consistent with the reality. I mean, did the authors carry out validation betweent the results and observed data (such as traffic flow?)

Author Response

All comments and, where applicable, modifications have been reported in the attached file. We sincerely appreciate your time and expert comments, which undoubtedly enrich our article.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I am happy with the current version and I think it can it accepted.

Author Response

Based on the reviewer's comments, no further changes are necessary. We appreciate the reviewers' time and dedication.

Reviewer 2 Report

Comments and Suggestions for Authors

I have examined the authors’ response and the revised manuscript. The authors have made efforts to address the previous round of comments, and the manuscript has been substantially revised. The current version represents an improvement over the original submission. I have no further substantive comments to offer on this revision.

Author Response

Based on the reviewer's comments, no further changes are necessary. We appreciate the reviewers' time and dedication.

Reviewer 3 Report

Comments and Suggestions for Authors

No comment

Author Response

There are no comments, so we appreciate the reviewer's input.

Reviewer 4 Report

Comments and Suggestions for Authors

Thanks for authors reply and revision. However, I am actually disappointed about the revision, especially those toward my comments 1, 5, 6, and 7 in the last round:

1. regarding the comment 1 in the last round, the format of references is still inappropriate. Please check carefully!
2. regarding the comment 5 in the last round, I don't think there is "a standard empirical structure" of a scientific paper. If the authors aim to propose a method, then the mentod should be introduced first and then use data to confirm the validity of the method.
3. regarding the comment 6 in the last round, there is still a equation that are not labeled in lines 492-493.
4. regarding the comment 7 in the last round, I think authors' reply is still too simple and quantified results could be presented.

Author Response

The file with the proposed corrections is attached. We appreciate the reviewers' time and dedication.

Author Response File: Author Response.docx

Round 3

Reviewer 4 Report

Comments and Suggestions for Authors

Thanks for authors' revision, however, I am still not satisfactory. 

1. Regarding the reference format, here I present an instance. In the manuscript, the authors say "Urban freight transport relies on a scarce and increasingly contested resource: curbside space, Valença et al. 2021 [1]." (lines 44-45). What I mean is that "Valença et al. 2021" should not be presented here.
2. Regarding the structure, I still insist on that method shoul be presented first then the "Study Area and Data Collection" section.

Author Response

The corrections made are explained in the attached file.

Author Response File: Author Response.docx