Rural Roads’ Passing Sight Distance Control Along Crest Vertical Curves
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis short article proposes an analysis of areas with PSD inadequacy on rural roads with crest vertical curves. This study search is based on the German (RAL, 2012) rural roads design guidelines, where PSD is currently dependent on the homogeneousness of the proposed road design classes and no longer on speed. This study set the required PSD for all the examined design classes at 20 to 600m.
In general, the article is interesting and well written.
The scientific approach is good, and some results are supported by data and formulas explained in the manuscript.
Figures and tables are clear.
However, in my opinion, only some improvements are required. My suggestions are given below and are related only to minor manuscript modifications.
- The reference list is too limited for a scientific article. In fact, it lists only 10 documents, of which 6 are guidelines and 4 are scientific articles. Therefore, a broader reference list is needed, introducing additional scientific articles on crest curves and models for estimating PSDs. For instance, authors may consult and/or cite the following article in which a long reference list about the crest vertical curves design method is presented:
“Two-lane highways crest curve design. The case study of Italian guidelines. https://doi.org/10.3390/app10228182”
- Considering the previous comments, I believe this additional information can also be included in the “Introduction” section.
- Please explain the meaning of the variable HK “crest vertical curvature rates “
Considering these minor suggestions, I support the publication of the article after the authors have improved the consistency of their manuscript
Author Response
This short article proposes an analysis of areas with PSD inadequacy on rural roads with crest vertical curves. This study search is based on the German (RAL, 2012) rural roads design guidelines, where PSD is currently dependent on the homogeneousness of the proposed road design classes and no longer on speed. This study set the required PSD for all the examined design classes at 20 to 600m.
In general, the article is interesting and well written.
The scientific approach is good, and some results are supported by data and formulas explained in the manuscript.
Figures and tables are clear.
However, in my opinion, only some improvements are required. My suggestions are given below and are related only to minor manuscript modifications.
- The reference list is too limited for a scientific article. In fact, it lists only 10 documents, of which 6 are guidelines and 4 are scientific articles. Therefore, a broader reference list is needed, introducing additional scientific articles on crest curves and models for estimating PSDs. For instance, authors may consult and/or cite the following article in which a long reference list about the crest vertical curves design method is presented:
“Two-lane highways crest curve design. The case study of Italian guidelines. https://doi.org/10.3390/app10228182”
The authors thank the reviewer for his valuable feedback. We appreciate his observation regarding the limited number of scientific references. In response, we have expanded the reference list by incorporating additional ten peer-reviewed papers related Passing Sight Distance (PSD) assessments and one statistical finding regarding overtaking related crash risk on Italian two-lane highways.
Considering the previous comments, I believe this additional information can also be included in the “Introduction” section.
The introduction section has been revised accordingly.
Please explain the meaning of the variable HK “crest vertical curvature rates “
The authors are not entirely certain about the intended meaning of the reviewer’s comment. If the comment refers to the differentiation between two types of crest vertical curvature rates (CVCR)—those designed to provide Stopping Sight Distance (SSD) and those designed to provide Passing Sight Distance (PSD)—the following clarification is offered:
For SSD, it is mandatory to ensure sufficient sight distance at every point along the alignment. This requirement typically governs the definition of CVCR in most design standards.
In contrast, providing PSD is not a mandatory criterion. However, if the goal is to ensure PSD throughout the CVCR, significantly larger curvature rates are required due to the longer distances involved. This often leads to substantial geometric, land-use, and economic constraints, which limit its practical application in many roadway designs.
The paper investigates areas with PSD adequacy for 2-lane rural road segments where crest vertical curvature rates are selected according to the existing road design approach of SSD provision.
The section under Figure 1 is revised accordingly.
Considering these minor suggestions, I support the publication of the article after the authors have improved the consistency of their manuscript.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript presents a thorough and methodologically sound analysis of the problem of Passing Sight Distance (PSD) control on rural roads with crest vertical curves, in line with modern German design guidelines (RAL 2012). The authors have developed a series of mathematical models that cover six distinct vehicle position scenarios in relation to the vertical curve, in order to accurately define sections of the roadway with insufficient sight distance.
However, the introduction and literature review would benefit from the inclusion of more recent studies. Additionally, the overall list of references should be significantly expanded to demonstrate a deeper engagement with previous research in the field and to better highlight the specific research gap addressed by this study.
The figure included in the introductory section lacks a proper title, source, and is not explicitly referenced within the main body of the text. This should be corrected for clarity and academic consistency.
Although the analysis is based on the German design guidelines, the paper does not discuss the applicability or adaptability of the model to other design standards, such as AASHTO (USA), DMRB (UK), or the Italian national guidelines. Discussing the universality of the approach would enhance the global relevance and applicability of the findings.
While the effect of roadway cross-slope (superelevation) has been examined, the authors acknowledge that its impact on PSD is minimal (<2.5 m). However, in extreme conditions, such as the presence of combined horizontal and vertical curvature, the influence may be more significant. A deeper discussion on this potential interaction would strengthen the technical completeness of the study.
In the conclusion, the authors correctly emphasize the need to link PSD adequacy to traffic volumes. Still, even a preliminary analysis of typical traffic conditions (e.g., how frequently passing maneuvers actually occur) would add practical value and contextualize the findings.
Finally, validation through a case study, simulation, or comparison with real-world road segments with known visibility problems is missing. Including or at least discussing this aspect would greatly enhance the credibility and practical relevance of the proposed models.
Author Response
However, the introduction and literature review would benefit from the inclusion of more recent studies. Additionally, the overall list of references should be significantly expanded to demonstrate a deeper engagement with previous research in the field and to better highlight the specific research gap addressed by this study.
The authors thank the reviewer for his helpful observation. In response, we have expanded the reference list by incorporating additional ten peer-reviewed papers related Passing Sight Distance (PSD) assessments and one statistical finding regarding overtaking related crash risk on Italian two-lane highways. These additions have been integrated into the introduction section to strengthen the background of the research, demonstrate broader engagement with existing work, and more clearly highlight the specific research gap addressed in this study.
The figure included in the introductory section lacks a proper title, source, and is not explicitly referenced within the main body of the text. This should be corrected for clarity and academic consistency.
The caption of Figure 1 was misplaced. It is corrected in the revised paper.
Although the analysis is based on the German design guidelines, the paper does not discuss the applicability or adaptability of the model to other design standards, such as AASHTO (USA), DMRB (UK), or the Italian national guidelines. Discussing the universality of the approach would enhance the global relevance and applicability of the findings.
The authors thank the reviewer for raising this critical comment. The applicability of the research was initially included in the Conclusions section of the manuscript. However, to enhance clarity and improve the overall structure, the relevant content has been relocated to the newly added Discussion section, where it is more appropriately contextualized.
While the effect of roadway cross-slope (superelevation) has been examined, the authors acknowledge that its impact on PSD is minimal (<2.5 m). However, in extreme conditions, such as the presence of combined horizontal and vertical curvature, the influence may be more significant. A deeper discussion on this potential interaction would strengthen the technical completeness of the study.
The authors sincerely thank the reviewer for highlighting this important point. The superelevation effect is indeed negligible in tangents.
Considering that overtaking is already prohibited on sharp curves where superelevation rates reach up to 7.0%, the influence of superelevation on PSD should instead be examined on wider curves with gentler superelevation rates, typically below 5%. For such cases, the analysis showed that PSD is only marginally affected—by approximately 6 to 9 meters—indicating a negligible impact on overall safety.
The above paragraph is added in the respective area of the revised paper.
In the conclusion, the authors correctly emphasize the need to link PSD adequacy to traffic volumes. Still, even a preliminary analysis of typical traffic conditions (e.g., how frequently passing maneuvers actually occur) would add practical value and contextualize the findings.
A respective paragraph is added at the newly added Discussion section.
Finally, validation through a case study, simulation, or comparison with real-world road segments with known visibility problems is missing. Including or at least discussing this aspect would greatly enhance the credibility and practical relevance of the proposed models.
The authors thank the reviewer for this valuable suggestion. We agree that incorporating validation through a case study, simulation, or comparison with real-world conditions would enhance the practical relevance of the proposed models. While such validation was beyond the scope of the current study, we have now included a discussion of this limitation and highlighted it as a key direction for future research (see Discussion section).
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsI would like to thank the authors for submitting the revised manuscript and emphasize that a large portion of the work has been done very well. The methodological approach is clearly defined, engineering-relevant, and practically applicable. I particularly appreciate the effort to develop a tool for engineers that can assist in the early identification of visibility issues on rural roads with crest vertical curves.
However, despite the high quality of the content and engineering accuracy, I note that the literature review lacks more recent references from the past few years. This may give the impression that the paper is not sufficiently anchored in the current research context. It would be advisable to supplement the literature with at least ten recent studies in order to strengthen the topical relevance and scholarly foundation of the work.
In this regard, I suggest that the authors consider including some recent publications that could contribute to enriching the literature and linking the paper more closely with ongoing research. I propose some potentially relevant areas and studies, which may be included if deemed appropriate for the topic: https://doi.org/10.56578/pmdf020201, https://doi.org/10.3390/land13122085, https://doi.org/10.56578/judm010104. Incorporating works with similar thematic frameworks and up-to-date analyses could enhance the theoretical grounding and demonstrate that the research findings are aligned with broader scientific discussions in the field of road safety and highway design.
Author Response
Our replies are highlighted in blue text below. The additions in the revised manuscript are highlighted in green.
I would like to thank the authors for submitting the revised manuscript and emphasize that a large portion of the work has been done very well. The methodological approach is clearly defined, engineering-relevant, and practically applicable. I particularly appreciate the effort to develop a tool for engineers that can assist in the early identification of visibility issues on rural roads with crest vertical curves.
However, despite the high quality of the content and engineering accuracy, I note that the literature review lacks more recent references from the past few years. This may give the impression that the paper is not sufficiently anchored in the current research context. It would be advisable to supplement the literature with at least ten recent studies in order to strengthen the topical relevance and scholarly foundation of the work.
In this regard, I suggest that the authors consider including some recent publications that could contribute to enriching the literature and linking the paper more closely with ongoing research. I propose some potentially relevant areas and studies, which may be included if deemed appropriate for the topic: https://doi.org/10.56578/pmdf020201, https://doi.org/10.3390/land13122085, https://doi.org/10.56578/judm010104. Incorporating works with similar thematic frameworks and up-to-date analyses could enhance the theoretical grounding and demonstrate that the research findings are aligned with broader scientific discussions in the field of road safety and highway design.
We would like to thank the reviewer for their thoughtful and encouraging comments. We truly appreciate the recognition of the methodological approach and the practical relevance of the proposed tool. We agree that strengthening the connection to recent research will enhance the scholarly depth of the manuscript, and we will carefully incorporate recent studies to better reflect current developments in the field.
However, we would like to kindly note that the number of recent studies specifically addressing visibility issues on rural roads with crest vertical curves remains relatively limited. Nevertheless, we have carefully reviewed the available literature and have included the most relevant and recent contributions to the extent possible, in order to further strengthen the scholarly foundation of the manuscript.
Thank you again for your valuable insights.
The following references were added:
Agina, S., Shalkamy, A., Gouda, M., & El-Basyouny, K. (2021). Automated Assessment of Passing Sight Distance on Rural Highways using Mobile LiDAR Data. Transportation Research Record, 2675(12), pp.676-688.
Abdulhafedh, A. (2023) A Novel GIS Approach for Locating No-Passing Zones and Assessing Passing Sight Distance on Two-Lane, Two-Way Highways: A Case Study of MO Route 5 in the State of Missouri, USA. Open Access Library Journal, 10, pp.1-15.
Author Response File: 
Author Response.pdf
