Roundabouts are a special form of junction, which has been widely implemented during the last years, since for specific cases they are considered a suitable solution, in terms of safety and capacity, compared to conventional intersection types. Different types of roundabouts exist, and the design of each type follows specific principles, for reaching their design potential and avoiding negative implications. Although the roundabout types are common among several countries, the design principles and guidelines vary. This paper reviews the guidelines of six major countries (i.e., the United Kingdom, France, Germany, the Netherlands, the United States and Australia), which are considered front-runners in the design of roundabouts. For each national guideline, the main goal, the field of application and the estimation of basic roundabout geometric elements are identified. Through the comparison of national guidelines, points of convergence and divergence are identified and discussed. Recognizing these points can be a useful guidance for the development of guidelines for other countries, by identifying and transferring elements that best fit their road conditions. 

International Design Practices for Roundabouts

Shared spaces aim to reduce the dominance of motor vehicles by promoting pedestrian/cyclist activity and minimizing segregation between road users. Despite the intended scope to improve the safety of vulnerable road users, only few works in the literature focused on before-after safety evaluations, mainly analyzing changes in users’ trajectories and speeds, traffic volumes, and conflict counts, which, while useful, cannot univocally quantify road safety. Here, we propose a more advanced methodology, based on surrogate measures of safety and Extreme Value Theory, to assess road safety before and after the implementation of a shared space. The aim is to produce a crash risk estimation in different scenarios, obtaining a quantitative and comprehensive indicator, useful to practitioners for evaluating the safety of urban design solutions. A real-world case study illustrates the proposed procedure. Video data were collected on two separate days, before and after a shared space implementation, and were semi-automatically processed to extract road users’ trajectories. Analysis of traffic volumes, trajectories, speeds and yield ratios allowed to understand the spatial behavior of road users in the two scenarios. Traffic conflicts, identified with an innovative surrogate measure of safety called “time to avoided collision point” (TTAC), were then used to estimate a Lomax distribution, and therefore to model the probabilistic relationship between conflicts and crashes, eventually retrieving a crash risk estimate. Results show that the analyzed shared space was able to significantly reduce the risk of crashes, and these findings are consistent with the observed changes in users speed and spatial behavior. The analyzed case study and its limitations were useful in highlighting the methodology main features and suggesting practical prescriptions for practitioners. 

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Before-after safety analysis of a shared space implementation

Background: Roundabouts are considered one of the safest infrastructure typologies, when referring to motorized traffic. Due to their ability to reduce conflict points between vehicles, they have been largely spread, substituting signalized or unsignalized intersections. While the increase in safety for drivers has been largely tackled and demonstrated by researchers, and some efforts have been spent on the side of cyclists, pedestrian safety has not been extensively analyzed yet. Aim: The present paper aims at analyzing pedestrian safety at roundabouts set in two different locations, Italy and Slovenia. This research will highlight differences and similarities in the behavior of walkers at the same type of infrastructure, taking into account the effects risen by diverse road habits typical of the two countries. Methodology: Starting from video footages recorded at the two locations, behavioral analysis, pointing out pedestrian speed, acceleration and crossing time, and a proactive safety analysis, calculating surrogate safety measures for vulnerable road users, have been run. Descriptive statistics and additional statistical tests are developed to compare the two data samples. Conclusions: From the behavioral point of view, results show for both locations faster pedestrian paces than expected, with the Slovenian case having the highest speed values and lowest crossing times. As regarding the safety point of view, Time-to-Collision, Time Advantage and relative speed between oncoming vehicles and the crossing pedestrians permitted to objectively evaluate conflict severity. The calculated percentages of values overcoming the individuated thresholds for determining dangerous events underlines the need to find solutions from both the infrastructural side and pedestrian awareness about their safe behavior. 

Harish Nair

Papers

Pedestrian safety at roundabouts: a comparison of the behavior in Italy and Slovenia