Review Reports - The Design Vehicle Steering Path Construction Based on the Hairpin Bend Geometry—Application in Roundabout Design

Round 1

Reviewer 1 Report

Detailed comments:

1. Introduction - there should be a separate Literature review section, which should describe the previous research in the field of using computer simulation techniques for roundabout. A new paragraph in the Literature review section should be created to enhance the paper by some recent findings.

2. Methodology

• Line no. 133 – lack of justification: the outer radius of roundabouts (R) from 13 to 25 meters, with a 0.5-meter increment.

• Line no. 170 – lack of justification: the German guidelines of the vehicle.

3. Results

• Line no. 211-216 – The text does not add anything to the manuscript.

4. Drawings and charts – are clear and consistent.

5. Text editing – the International System of Units (SI) – BIPM to be applied

• m but not meters

• References – see Microsoft Word template file (https://www.mdpi.com/files/word-templates/applsci-template.dot) or LaTeX template files (https://www.mdpi.com/authors/latex)

Author Response

Dear Reviewer, thank you for your valuable input. We hope that we have successfully addressed all raised issues. Revisions made to the manuscript are marked up using the “Track Changes” function and highlighted in yellow.

COMMENT 1: 1. Introduction - there should be a separate Literature review section, which should describe the previous research in the field of using computer simulation techniques for roundabout. A new paragraph in the Literature review section should be created to enhance the paper by some recent findings.

ANSWER 1: Although important, the raised issue is not in the scope of this investigation. The relationship between the real trajectories of the vehicles and the theoretical ones hypothesized in the simulations was evaluated and discussed in our paper [13], where a comparison between the swept path widths determined using a vehicle movement simulation and the swept path widths determined using a precise GNSS device at the test site was given. Also, according to the Instructions for authors, a separate Literature review section is not advisable.

COMMENT 2: Line no. 133 – lack of justification: the outer radius of roundabouts (R) from 13 to 25 meters, with a 0.5-meter increment.

ANSWER 2: We edited this paragraph as follows:

“The applicability of approach A-2 was tested on 25 single-lane roundabout schemes designed in the Autodesk AutoCAD software. These schemes were constructed by varying the outer radius of roundabouts (R) from 13 to 25 meters, with a 0.5-meter increment. According to previous research given in [9], these outer radii are commonly used for single-lane roundabouts worldwide. An increment of 0.5 m was chosen to capture the dispersity of the results and to create a sample that is representative, manageable, and easy to present at the same time.”

COMMENT 3: Line no. 170 – lack of justification: the German guidelines of the vehicle.

ANSWER 3: As highlighted in the manuscript, German guidelines for the design vehicle lengths were selected for this analysis based on the results of the investigation conducted in [9]. The investigation results showed that the vehicle defined in German guidelines has the widest swept path when negotiating a roundabout for a tractor with a semi-trailer. We edited this paragraph as follows:

“For the simulations conducted in this investigation, a custom design vehicle was created based on the dimensions given in German guidelines [19]. German guidelines for the design vehicle lengths were selected for this analysis based on the results of the investigation conducted in [9]. This investigation included the comparison of turning envelope requirements for the design vehicles given in Austrian, German, Serbian guidelines, and Swiss norms as well as vehicles whose dimensions were collected from the catalogs of the most represented manufacturers on the European market. The investigation results showed that the vehicle defined in German guidelines has the widest swept path when negotiating a roundabout for a tractor with a semi-trailer. Due to this fact, this design vehicle was applied in the investigation of FACPP deviations presented in this paper, with the following customization: width of 2.55 m (Figure 4). The customization of the design vehicle width was made due to the following: …”

COMMENT 4: Line no. 211-216 – The text does not add anything to the manuscript.

ANSWER 4: Thank you for the input.

COMMENT 5: Drawings and charts – are clear and consistent.

ANSWER 5: Thank you for the input.

COMMENT 6: Text editing – the International System of Units (SI) – BIPM to be applied (m but not meters)

ANSWER 6: Thank you for the input. We have revised the manuscript according to your instructions.

COMMENT 7: References – see Microsoft Word template file (https://www.mdpi.com/files/word-templates/applsci-template.dot) or LaTeX template files (https://www.mdpi.com/authors/latex)

ANSWER 7: Thank you for the input.

Reviewer 2 Report

The design vehicle steering path construction based on the hairpin bends geometry

A brief summary

The paper proposes the investigation of the deviations of design vehicle steering paths for straight passage through the roundabout constructed according to two different approaches.

Comments

1. The idea of the research is interesting and presents enough novelty.

2. The paper should attract an audience in the field of Capacity Analysis of railway transportation systems.

3. The paper fits the topics of the journal.

4. The proposed method seems to be innovative and contains well-known hints of originality.

5. The study demonstrated that a larger offset of left-hand side design vehicle trajectories on the circulatory roadway could have a positive impact on the deflection around the central island of the roundabout.

Weakness of the paper:

1) Authors should anticipate some numerical results in the abstract to capture the readers’ curiosity.

2) The results of the paper should be better highlighted.

3) The paper should be revised paying attention to the position of the figures.

4) Briefly summarize in the conclusions (using at least one sentence) the research work proposed in the manuscript.

5) The most possible future developing of the proposed research should also be reported in the Conclusion

6) The authors should better explain the relationship between the real trajectories of the vehicles and the theoretical ones hypothesized in the simulations.

7) … no more weaknesses!

The overall merit of presented research works and findings can be taken into consideration for publishing after incorporating the above suggestions.

Author Response

COMMENT 1: Authors should anticipate some numerical results in the abstract to capture the readers’ curiosity.

ANSWER 1: We revised the Abstract as follows.

“… The results showed that the A-2, a faster and simpler approach, is applicable with caution, as resulting turning envelopes derived from this approach are more offset from the roundabout centers compared to the A-1 approach. 72 % of right-hand side path and resulting right-hand side design vehicle body and chassis turning envelopes deviations were between 0.00 and 0.15 m, 27 % were over 0.15 m, and 7 % were over 0.25 m. This results in larger roundabout entry and exit widths as well as the need for large lateral clearances for entries and exits on roundabouts where heavy pedestrian traffic on sidewalks is expected. The noted offset of left-hand side design vehicle trajectories on the circulatory roadway (which was mostly between 0.05 and 0.15 m) could have a positive impact on the deflection around the central island.

COMMENT 2: The results of the paper should be better highlighted.

ANSWER 2: We revised the Conclusions as follows.

“The investigation results showed that the front axle center point paths for straight passage through the roundabout constructed following the principles of hairpin bends geometry, i.e., with consecutive circular arcs, is applicable in the design of the right carriageway edge at single-lane roundabouts for design vehicle tractor with a semi-trailer, as only 7 % of right-hand side path and resulting right-hand side design vehicle body and chassis turning envelopes deviations exceeded 0.25 meters. At the same time, the offset of left-hand side design vehicle trajectories on the circulatory roadway (which was mostly between 0.05 and 0.15 meters) could have a positive impact on the deflection around the central island. Nevertheless, the following should be remembered when utilizing this approach: when compared to the path with short straight lines inserted between the adjacent arcs, the design vehicle body and chassis turning envelopes are more offset from the roundabout centers.”

COMMENT 3: The paper should be revised paying attention to the position of the figures.

ANSWER 3: We revised the paper accordingly.

COMMENT 4: Briefly summarize in the conclusions (using at least one sentence) the research work proposed in the manuscript.

ANSWER 4: We revised the Conclusions as follows.

“The investigation presented in this paper focuses on one of the elementary issues in transportation engineering: whether the traffic infrastructure will provide required traffic safety, i.e., how steering path geometry impacts turning movements for long articulated vehicles such as a tractor with a semi-trailer. Roundabouts are potentially hazardous locations due to the large turning envelopes required by long, articulated vehicles. When designing a roundabout based on the results of the design vehicle swept path analysis, parameters for the construction of the design vehicle turning envelope are not specified. Therefore, FACPPs need to be constructed solely based on the roundabout outer radius, approach alignments, and the selected shape of the splitter island. Two approaches can be applied when constructing the path for the straight passage. The first one (A-1) implies the application of short straight lines inserted between the adjacent arcs. The second one (A-2), which is faster and simpler to use, implies the application of the principles of hairpin bends geometry. In this paper, the investigation of the deviations for paths constructed according to these approaches was presented. The results of the investigation can help to identify an adequate vehicle path construction approach and the range and combination of path geometric elements needed for the simulation of design vehicle through movements at single-lane roundabouts with outer radii varying from 13 to 25 meters.”

COMMENT 5: The most possible future developing of the proposed research should also be reported in the Conclusion.

ANSWER 5: According to the Instructions for authors, future research directions should be highlighted in the Discussion section. Based on the comments of Reviewer 4, we edited this section to propose additional future research directions.

“The limitations of this investigation are as follows. The investigation was conducted for single-lane roundabouts whose approach leg axes are placed on the straight that passes through the geometrical center of the roundabout (center of the circular island). In the real world, this ideal position of the approach leg axes is sometimes not possible to achieve due to the spatial constraints, as well as the requested number and the alignment of the approach legs. This results in different locations of points A, B, and C on the paths for straight passage due to the offset of the approach leg axes from the center of the circular island and different angles between the approach legs. Furthermore, long vehicles that pose a challenge in roundabout design are not only tractors with semi-trailers, but also three-axle buses, which are not included in this investigation. Another issue that should be addressed is the influence of approach A-2 application on personal vehicle speed. Therefore, to investigate the potential effect of these factors, future research should include the investigation of turning envelopes of three-axle buses, investigation of different approach leg alignments, and investigation of fastest paths for personal vehicles.”

COMMENT 6: The authors should better explain the relationship between the real trajectories of the vehicles and the theoretical ones hypothesized in the simulations.

ANSWER 6: Although important, the raised issue is not in the scope of this investigation. The relationship between the real trajectories of the vehicles and the theoretical ones hypothesized in the simulations was evaluated and discussed in our paper [13], where a comparison between the swept path widths determined using a vehicle movement simulation and the swept path widths determined using a precise GNSS device at the test site was given. Nevertheless, considering your comment, we edited the Introduction as follows.

“The input parameter necessary for defining the swept path at roundabouts is the design vehicle steering path, i.e., front axle center point path (FACPP) [8,9]. FACPP can be drawn “freehand” (i.e., as manual hand drawing) or with geometric elements such as straight lines and circular arcs. Freehand drawing results in a path that is close to real-world conditions, but it requires considerable engineering experience and skill. Paths composed of straight lines and circular arcs can be drawn by simply offsetting the roundabout outer radius, approach alignments, and the selected shape of the splitter island. Mutual relations of straight lines and circular arcs depend on the direction of movement of the vehicle through the roundabout: the path for the right turn is composed of an entry branch (straight line) followed by a circular arc and an exit branch (straight line), while the path for the straight passage is composed of entry and exit branches and three consecutive circular arcs in the opposite direction.”

Reviewer 3 Report

With interest, I read the manuscript. It is appreciated that the manuscript is easy to follow and not too long. The message is clear and of interest to the community. The authors proposed a paper titled "The design vehicle steering path construction based on the hairpin bends geometry". The proposed paper seemed to be promising in terms of computational simplicity and classification accuracy. I would accept it in this present form.

Author Response

Dear reviewer, we would like to thank you for your time and effort in reviewing the manuscript. We highly appreciate your positive feedback.

Reviewer 4 Report

The paper is about geometry of roundabouts. Unfortunately, the title does not reflect this fact, it is too general. I would recommend using the word “roundabout” in the title.

Most of the roundabouts are not as simple as the examples in the paper. The authors are discussing the limitation of the research around line 400. However, there is no mention about 5^th legs, and angles different from 90 degrees which are quite frequent elements. Please elaborate on the potential effect of these factors.

A frequent problem of roundabouts is the high speed of passenger cars due to the generous geometry. The paper deals exclusively with heavy vehicles and passenger vehicles are mentioned in the introduction only. Please elaborate on the impact of the proposed geometry on passenger vehicles.

In my view, Appendix 1 could be deleted.

Author Response

The paper is about geometry of roundabouts. Unfortunately, the title does not reflect this fact, it is too general. I would recommend using the word “roundabout” in the title.

The design vehicle steering path construction based on the hairpin bends geometry – application in roundabout design

COMMENT 1: Most of the roundabouts are not as simple as the examples in the paper. The authors are discussing the limitation of the research around line 400. However, there is no mention about 5^th legs, and angles different from 90 degrees which are quite frequent elements. Please elaborate on the potential effect of these factors.

ANSWER 1: We have edited this part of the Discussion as follows.

“The limitations of this investigation are as follows. The investigation was conducted for single-lane roundabouts whose approach leg axes are placed on the straight that passes through the geometrical center of the roundabout (center of the circular island). In the real world, this ideal position of the approach leg axes intersection is sometimes not possible to achieve due to the spatial constraints of the surrounding space, as well as the requested number and the alignment of the approach legs. This results in different locations of points A, B, and C on the FACPPs for straight passage due to the offset of the approach leg axes from the center of the circular island and different angles between the approach legs. Furthermore, long vehicles that pose a challenge in roundabout design are not only tractors with semi-trailers, but also three-axle buses, which are not included in this investigation. Another issue that should be addressed is the influence of approach A-2 application on personal vehicle speed. Therefore, to investigate the potential effect of these factors, future research should include the investigation of turning envelopes of three-axle buses, investigation of different approach leg alignments, and investigation of fastest paths for personal vehicles.”

COMMENT 2: A frequent problem of roundabouts is the high speed of passenger cars due to the generous geometry. The paper deals exclusively with heavy vehicles and passenger vehicles are mentioned in the introduction only. Please elaborate on the impact of the proposed geometry on passenger vehicles.

ANSWER 2: Thank you for the input. As you already stated, it is a well-established fact that wide roundabout entrances and exits as well as the smaller deflection of the vehicle path around the central island increase personal vehicle speed. Therefore, the proposed geometry could result in a larger personal vehicle speed, which is emphasized in the revised discussion. As this research did not include the fastest path investigations for personal vehicles, we included this as a limitation of the study and a future research direction (in Discussion section).

“This shows that in general, the A-2 approach results in wider entrances and exits compared to the A-1 approach. Wide entrances are unfavorable in terms of traffic safety as they enable higher entry speed, especially for personal vehicles [8,14].

(...)

Another issue that should be addressed is the influence of approach A-2 application on personal vehicle speed. Therefore, to investigate the potential effect of these factors, future research should include the investigation of turning envelopes of three-axle buses, investigation of different approach leg alignments, and investigation of fastest paths for personal vehicles.”

COMMENT 3: In my view, Appendix 1 could be deleted.

ANSWER 3: According to the Instructions for Authors, the Materials and Methods should be described with sufficient details to allow others to replicate and build on the published results, i.e., all materials and data associated with the publication must be available to readers. We believe that deleting Appendix 1 would limit the possible replication of the investigation.