Analysis of Conflict Distribution Characteristics between Motor Vehicles and E-Bikes at Roundabouts

Round 1

Reviewer 1 Report

Dear authors,

I had the opportunity to revise the paper “Analysis of Conflicts Distribution Characteristics Between Motor Vehicles and E-bikes at Roundabout”. In this study, the authors explore the spatial-temporal distribution characteristics of conflicts between motor vehicles and electric bikes (e-bikes) at signalized roundabouts in order to provide a theoretical basis for improving traffic safety at roundabouts. The content of the paper is interesting and the results achieved promising.

The paper needs however great improvements in order to make it more suitable for publication.

The reviewer would in particular suggest the following main actions:

·        The bibliography is poor. 23 papers cited are too few: relevant literature on roundabout safety analysis and surrogate safety parameters is practically ignored. In this way, the resulting state of the art cannot be considered exhaustive.

·        Improve the structure of the paper. In its present form the paper is rather unbalanced. As reported above, literature review occupies a too little section: it needs a good level of details.

·        Improve the quality of images in Figure2 and Figure 3.

·        Conflicts between vehicles and e-bikes at roundabouts normally show the same features extensively discussed in the literature for conflicts between vehicles and vulnerable users. Therefore, the two most representative kind of conflicts identified in this paper do not add novelty to what other researchers have already found on this topic.

·        As reported by authors in the conclusions, this research only gives a preliminary insight about conflicts between vehicles and e-bikes at a specific type of roundabout. Many issues need to be explored in further works considering, in particular, other different roundabout’s configurations.

·        An extensive editing of English language and style is required.

Author Response

Point 1: The bibliography is poor. 23 papers cited are too few: relevant literature on roundabout safety analysis and surrogate safety parameters is practically ignored. In this way, the resulting state of the art cannot be considered exhaustive.

Response 1: The bibliography has been updated in the revised manuscript. The papers cited in this research are added to 42 in total. Relevant literature on roundabout safety analysis and surrogate safety parameters is added as below:

1.1 Roundabout  management

The continuous evolution of automotive technologies has a profound impact to improve safety and mobility [12]. Some progress benefited by connected and autonomous vehicles also was made for roundabouts safety management. Mohebifard, R. using a two-dimensional point-mass model introduced a methodology to optimize the trajectories of connected automated vehicles at a roundabout. The trajectory optimization was formulated as an optimization problem that included collision avoidance and path constraints to minimize the total travel time and collision probability of vehicles. The results show that the travel times, average delays, and collision probability were reduced based on trajectory optimization [13]. Martin-Gasulla, M. developed and tested Roundabout Manager acts as a central controller, and manages conflicts within the roundabout. In this research seven strategies were proposed to coordinate CAV trajectories. The result shows that, under any of the strategies proposed, the system guarantees higher throughput with a lower average control delay compared to conventional vehicles’ operation [14]. Wang C. proposed a roundabout control strategy for connected and autonomous vehicles, which entails developing a roundabout flow control model to optimize merge-in flows, a merge-in decision model to generate vehicle passing sequence from the optimal flows, and a virtual platoon control model to achieve safe and stable vehicle operations in a circular roundabout platoon to enhance round-about performance while ensuring vehicle safety [2].

Mascio P. D. explored the results of first applying jointed plain concrete pavement on an Italian roundabout. Presented that the construction of concrete pavement in an urban roundabout will better manage and slow down fast vehicular flows, and increase traffic fluidity in an important junction, trafficked by heavy vehicles, reduced or avoid queues at roundabouts [15]. Sakshaug L. combined quantitative and qualitative methods in traffic conflict, interaction and behavioral studies to find out how interactions and conflicts differ between the two roundabout designs, and extensively explained separating cycle crossings or integrating cyclists with motorists is the safest roundabout design for cyclists [16]. Jensen S.U. discussed the safe roundabouts de-signed problems based on the conversions of intersections to 255 single-lane roundabouts in Denmark and found that the diameters and height of the central island will lead to a different safety level for a single-lane roundabout. Diameters of 20–40m are safer for cyclists compared with smaller or larger central islands. When the middle of the central islands is elevated 2m or more above the circulating lane is safer for cyclists than roundabouts with lower central islands [17]. Pilko H. forwarded a multi-criteria and simultaneous multi-objective optimization (MOO) model to optimize geometry, traffic efficiency (TE), and traffic safety (TS) of unsignalized single-lane roundabouts. In this model, they took the main roundabout geometry elements and demand flow rate as design parameters and regard acceptable levels of TE and TS as optimization objectives and constraints. Preliminary results suggest that the proposed model can optimize the geometry, TE, and TS of urban single-lane roundabouts [18].

Point 2: Improve the structure of the paper. In its present form the paper is rather unbalanced. As reported above, literature review occupies a too little section: it needs a good level of details.

Response 2: In the revised manuscript the literature review section has been amplified and be subdivided into three parts as follows:

1.1 Roundabout management

1.2 Safety problems of e-bikes

1.3 Traffic characteristics at roundabout

Point 3: Improve the quality of images in Figure2 and Figure 3.

Response 3: Original Figure 2 and Figure 3 have been reformed and updated in the revised manuscript as below:

Figure 3. Aerial view of roundabouts

Figure 4 -Trajectory extraction steps

Point 4: Conflicts between vehicles and e-bikes at roundabouts normally show the same features extensively discussed in the literature for conflicts between vehicles and vulnerable users. Therefore, the two most representative kind of conflicts identified in this paper do not add novelty to what other researchers have already found on this topic.

Response 4: In this research based on the video analysis and 85% percentile cumulative curve analysis method we proposed the definition and boundary of Serious Conflict and General Conflict. Then based on vehicles and e-bikes traffic movement classified out two kinds of most representative conflicts. Using AcrGis heat map analysis method and one green signal period presented the conflict spatial and temporal distribution characteristics between e-bikes and motor vehicles at multi-lanes roundabout.

Compared with discussed research in the literature, in this research, we first presented the conflicts spatial and temporal distribution characteristics between e-bikes and motor vehicles at the multi-lanes roundabout (conflicts occurred more frequently than single-lane roundabouts). Besides, those discussed literature’s conclusions mainly focused on the bicycle accident mechanism and influencing factors of electric are mainly focused on single-lane roundabouts, conventional cross-shaped intersections, and human-powered bicycles.

Point 5: As reported by authors in the conclusions, this research only gives a preliminary insight about conflicts between vehicles and e-bikes at a specific type of roundabout. Many issues need to be explored in further works considering, in particular, other different roundabout’s configurations.

Response 5: Conventional roundabouts in China are usually characterized as large radii, multi-lanes, circle central island, and four-way cross signalized intersections always with heavy e-bike traffic flow and more frequent conflict. Thus, in this research, we targeted three roundabouts with similar physical characteristics and developed a procedure to explore the conflict distribution characteristics between motor vehicles and e-bikes at these defined roundabouts types in our research. In the future, the conflicts between vehicles and e-bikes under different central island radii and control modes at different roundabout still needs further study.

Point 6: An extensive editing of English language and style is required.

Response 6: All the language and spelling issues revised in the manuscript have been marked in tracking mode.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript analyzed the safety impact of electric bicycles at the roundabout. The manuscript is valuable for improving the safety of e-bikes from the policy, the management of e-bikes, and road design. Several comments were presented as follows to help improve the manuscript.

1. A more detailed definition of the conflict between e-bikes and motorized vehicles in the roundabout would be welcome. A figure is helpful for the definition of the conflict. The interweaving area, exit road, and outer circulatory lanes need to be indicated clearly in the figure using boxes and numbers.

2. What are the space distance method, energy method, and time distance method? Why is the time distance method more scientific?

3. Why did the authors choose 1.7s and 2.9s as critical value to define the serious and general conflicts?

4. The circulatory lane drawn in figure 9 is too concisely. The number of circulatory lanes should be marked out and consistent with the three case roundabouts chose in this research.

5. The title of section 3.2 could be changed to conflicts temporal distribution characteristics make the structure and readability of the article better.

6. The language and spelling need to be further revised.

Author Response

Point 1: A more detailed definition of the conflict between e-bikes and motorized vehicles in the roundabout would be welcome. A figure is helpful for the definition of the conflict. The interweaving area, exit road, and outer circulatory lanes need to be indicated clearly in the figure using boxes and numbers.

Response 1: Traffic conflict is defined as an event involving two or more road users, in which the action of one causes the other to make an evasive maneuver (Hauer et al., 1988). In this research, the conflict between e-bikes and motorized vehicles in the roundabout is defined as motor vehicles or electric bicycles taking a maneuver of braking, turns, deceleration, etc. to avoid crashes at the interweaving area or outer circulatory exit lanes at roundabout.

Fig1. Area division of circulatory lanes in a roundabout

Point 2: What are the space distance method, energy method, and time distance method? Why is the time distance method more scientific?

Response 2: Space Distance method: Use the distance between both conflict parties as an indicator to judge the severity of the conflict. When the distance between the two is small, the probability of spatial conflict between the two is greater, and the corresponding conflict severity is higher, and vice versa.

Energy Method: If the vehicle still maintains its existing trajectory a collision will occur when the trajectory coincides at a certain moment, the degree of conflict is measured by the amount of energy generated at the time of collision. The greater the energy generated by a conflict between vehicles, the greater the severity of the conflict between the two, and in the same way, the less energy generated by a conflict between vehicles, the less severe the conflict between two people.

Time Distance Method: the time calculated based on the actual distance between the two conflict parties and their speed of them. When the time is bigger than the conflict threshold, it is considered that no conflict occurs.

The Space Distance method will cause a large error in the case of low speed, the Energy Method will be related to the model of vehicles. And the Time Distance Method is based on the speed and distance when the two vehicles collide, which is more scientific to determine the traffic conflict, and is mostly used at intersections. Therefore, the Time Distance Method is selected for conflict discrimination method in this research.

Point 3: Why did the authors choose 1.7s and 2.9s as critical value to define the serious and general conflicts?

Response 3: Based on the conflict cumulative frequency distribution shown in Figure 7 and Figure 8, using 85% percentile cumulative frequency value to determine the boundary of serious and non-serious conflicts, we could obtain the threshold of serious conflict is 1.7s, and the threshold of non-serious conflict is 2.9s.

Figure 8 - Cumulative frequency distribution of serious conflicts

Figure 9 - Cumulative frequency distribution of moderate conflicts

Point 4: The circulatory lane drawn in figure 9 is too concisely. The number of circulatory lanes should be marked out and consistent with the three case roundabouts chose in this research.

Response 4: Figure 9 has been updated in manuscript as below.

Figure 10- Conflict times and ratios of 4 conflict types

Point 5: The title of section 3.2 could be changed to conflicts temporal distribution characteristics make the structure and readability of the article better.

Response5: This title has been updated in the revised manuscript as suggested.

Point 6: The language and spelling need to be further revised.

Response 6: All the language and spelling issues in the revised manuscript have been marked in tracking mode.

Author Response File: Author Response.docx

Reviewer 3 Report

“The authors addressed the safety issue of e-bikes at roundabouts by analyzing the conflicts' distribution characteristics.

Below, some comments are provided with the aim of improving the paper and its readability.

- eq. 1. How were K and E evaluated? To what does the minimum sample size refer?

- eq. 2, please add the unit of measurement for d and v. The same also for eq.3

- may the authors provide info on how many e-bikes they observed/simulated? How did the authors set their frequency in the model?

- Please, add a definition for the levels of severity used in table 4.

- the paper lacks a discussion section. the discussion should contain a comparison of your results with those already available in the literature. 

- Recently, there is an increasing interest in proactive road safety assessments based on the examination of the characteristics of the road to identify the presence of risk factors (i.e., https://doi.org/10.1016/j.aap.2022.106858 was carried out for roundabouts). How may your study be of support for such safety approaches? Please add a comment in the discussion/conclusion section.

- The authors stated that their study could provide a theoretical basis for formulating improvement measures for roundabouts. Several prior research (i.e., https://doi.org/10.1177/03611981211034718) explored different safety countermeasures by the means of a driving simulator to evaluate which one is the most perceived by the drivers. What measures would you suggest? Please, add your thoughts in the conclusion section to be helpful for future research on driving simulator experiments on e-bikes.

Minor

- line 34, may be the world order  wrong in "in China urban the road design improvement"?

- line 41, please, add the reference for Papoutsi S.

- line 79, please replace the word sex with gender.

- please check the text for typos. i.e., singular vs plural as for the 294 roundabouts (line 89), the -ing form for as for "concluded a result show-ing that the diameter..." (line 98).

- Figure 3 should be improved in quality. The right part, indeed, is completely impossible to read.

- figure 9, please add a legend for green and red maneuvers to make the image self-explained.”

Author Response

Point 1: eq. 1. How were K and E evaluated? To what does the minimum sample size refer?

Response 1: From the statistics point of view, this population size of conflict is infinite. And we have got the tested standard deviation of 6.81. To find the minimum sample size we could use eq.1 to calculate it. In this equation, K is the reliability factor. It is a constant from the table of probabilities of the standard normal distribution for the desired confidence level when the confidence level is 95% K is 1.96.

E is the margin error, which is a statistic expressing the amount of random sampling error in the results of a survey. The larger the margin of error, the less confidence one should have that a poll result would reflect the result of a census of the entire population. In this research, the confidence level takes 95% then the margin error E is 1.43.

Point 2: eq. 2, please add the unit of measurement for d and v. The same also for eq.3

Response 2: In eq.2, d₁, d₂ are the distance of the two vehicles from the conflict point(m), and v₁, v₂ are the original moving speeds of the two vehicles(m/s). In eq.3, L_a, L_b are the body length of the vehicle and the e-bike(m). V_a, V_b in eq.3 are the speed of the vehicle and e-bike(m/s).

Point 3: may the authors provide info on how many e-bikes they observed/simulated? How did the authors set their frequency in the model?

Response 3: Based on the video analysis software Tracker, a total of 1045 times potential conflicts are identified according to their trajectory data, speed, and acceleration data. Because the target of this study is the number of conflicts and the conflict distribution characteristics. In the case of ensuring that the video observation time required to meet the minimum conflicts sample size is met, we mainly counted the number of conflicts and didn’t make detailed statistics on the vehicle flow and e-bike flow or frequency.

Point 4: Please, add a definition for the levels of severity used in table 4.

Response 4: Through the video observation by professionals, the judgment of serious conflicts was based on obvious braking, large turns, sudden deceleration, etc. of motor vehicles or e-bikes, and general conflicts are judgement based on their slight braking, small turns, slow deceleration, etc. If come forth inconsistent results of judgment, a further discussion will be carried out to get the number of serious conflicts and general conflicts finally determined.

Thus, the Serious Conflict could be defined as when motor vehicles or e-bikes take an obvious braking, large turns, sudden deceleration, etc. to avoid a potential collision. General conflict could be defined as when motor vehicles or e-bikes take a slight braking, small turns, slow deceleration, etc. to avoid a potential collision.

While Traffic Interference was eliminated from the general conflict list, defined as when the TTC is large than 2.9s according to the 85% percentile cumulative curve of general conflicts frequency distribution.

Point 5: the paper lacks a discussion section. the discussion should contain a comparison of your results with those already available in the literature.

Response 5: The Discussion section has been updated in the revised manuscript.

Point 6: Recently, there is an increasing interest in proactive road safety assessments based on the examination of the characteristics of the road to identify the presence of risk factors (i.e., https://doi.org/10.1016/j.aap.2022.106858 was carried out for roundabouts). How may your study be of support for such safety approaches? Please add a comment in the discussion/conclusion section.

Response 6: A safety assessments study of 50 urban roundabouts from Rome, Italy developed and validates a Safety Index (SI) for evaluating urban roundabouts, provided a Safety Index for a quantitative safety assessment of both roundabouts and roundabout approaches based on the presence of risk factors [41].

Point 7: The authors stated that their study could provide a theoretical basis for formulating improvement measures for roundabouts. Several prior research (i.e., https://doi.org/10.1177/03611981211034718) explored different safety countermeasures by the means of a driving simulator to evaluate which one is the most perceived by the drivers. What measures would you suggest? Please, add your thoughts in the conclusion section to be helpful for future research on driving simulator experiments on e-bikes.

Response 7:  Though the data of actual accidents is not obtained, the relationship between conflict and accidents is very related. By analyzing the two kinds of data, we can further excavate and avoid safety problems by setting up some traffic control devices at a roundabout. Such as designing an advanced warning signs, perceptual measures, or delineation treatments in serious conflict areas to warn vehicles and e-bikes of the high risk of potential conflicts [42]. Which will be an interesting topic worthy of further study.

Minor

Point 8: line 34, may be the world order wrong in "in China urban the road design improvement"?

Response 8: This sentence has been reorganized as “Nowadays, a lot of measures such as optimizing road geometry design and signalization have been carried out to improve the traffic safety at roundabout management” in the revised manuscript.

Point 9: line 41, please, add the reference for Papoutsi S.

Response 9:  The reference [19] is added for Papoutsi S.

Point 10: line 79, please replace the word sex with gender.

Response 10: This wording has been changed as suggested in the manuscript

Point 11: please check the text for typos. i.e., singular vs plural as for the 294 roundabouts (line 89), the -ing form for as for "concluded a result show-ing that the diameter..." (line 98).

Response 11: All the language or typos, and spelling issues revised in the revised manuscript have been marked in red.

Point 12: Figure 3 should be improved in quality. The right part, indeed, is completely impossible to read.

Response 12: Original Figure 3 has been updated in the revised manuscript as below.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Thanks for your responses to my comments.

Reviewer 3 Report

The paper has been improved and is eligible for publication.