Shared spaces aim to reduce the dominance of motor vehicles by promoting pedestrian and 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 semiautomatically 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.

翻译：共享空间旨在通过促进行人和骑行者活动、减少道路使用者之间的隔离来降低机动车的支配地位。尽管其初衷是提高弱势道路使用者的安全性，但文献中仅有少数研究聚焦于实施前后的安全性评估，主要分析使用者轨迹与速度、交通流量及冲突数量的变化——这些方法虽有用，却无法唯一性地量化道路交通安全。本文提出一种基于替代安全指标与极值理论的更先进方法，用于评估共享空间实施前后的道路安全状况。其目的是在不同场景下生成事故风险估计，获得量化且全面的指标，为从业人员评估城市设计方案的安全性提供实用工具。通过真实案例研究展示了所提流程。在共享空间实施前后的两个单独日期采集视频数据，并通过半自动处理提取道路使用者轨迹。对交通流量、轨迹、速度及让行比例的分析揭示了两种场景下道路使用者的空间行为。利用创新性替代安全指标——避碰点时间（TTAC）识别交通冲突，进而估计洛马克斯分布，建模冲突与事故之间的概率关系，最终得出事故风险估计值。结果表明，所分析的共享空间能显著降低事故风险，这一发现与观测到的使用者速度及空间行为变化一致。该案例研究及其局限性凸显了方法论的主要特征，并为从业人员提出了实用建议。