Urban safety performance functions are used to predict crash frequencies, mostly based on Negative Binomial (NB) count models. They could be differentiated for considering homogeneous subsets of segments/intersections and different predictors. The main research questions concerned: a) finding the best possible subsets for segments and intersections for safety modelling, by discussing the related problems and inquiring into the variability of predictors within the subsets; b) comparing the modelling results with the existing literature to highlight common trends and/or main differences; c) assessing the importance of additional crash predictors, besides traditional variables. In the context of a National research project, traffic volumes, geometric, control and additional variables were collected for road segments and intersections in the City of Bari, Italy, with 1500 fatal+injury related crashes (2012–2016). Six NB models were developed for: one/two-way homogeneous segments, three/four-legged, signalized/unsignalized intersections. Crash predictors greatly vary within the different subsets considered. The effect of vertical signs on minor roads/driveways, critical sight distance, cycle crossings, pavement/markings maintenance was specifically discussed. Some common trends but also differences in both types and effect of crash predictors were found by comparing results with literature. The disaggregation of urban crash prediction models by considering different subsets of segments and intersections helps in revealing the specific influence of some predictors. Local characteristics may influence the relationships between well-established crash predictors and crash frequencies. A significant part of the urban crash frequency variability remains unexplained, thus encouraging research on this topic.

中文翻译：

---

不同道路要素的城市安全性能函数的可变性：意大利案例研究

城市安全性能函数用于预测碰撞频率，主要基于负二项式 (NB) 计数模型。可以区分它们以考虑段/交叉点的同质子集和不同的预测变量。主要研究问题涉及： a) 通过讨论相关问题并调查子集内预测变量的可变性，为安全建模找到最佳可能的路段和交叉点子集；b) 将建模结果与现有文献进行比较，以突出共同趋势和/或主要差异；c) 评估除传统变量之外的其他碰撞预测器的重要性。在国家研究项目的背景下，交通量、几何、为意大利巴里市的路段和交叉路口收集了控制变量和其他变量，其中有 1500 起致命+伤害相关的碰撞（2012-2016 年）。开发了六个 NB 模型用于：单向/双向同质路段、三/四腿、信号化/非信号化交叉口。在所考虑的不同子集内，碰撞预测器差异很大。特别讨论了垂直标志对次要道路/车道、临界视距、自行车道口、路面/标记维护的影响。通过将结果与文献进行比较，发现了一些共同的趋势，但也发现了碰撞预测器的类型和效果的差异。通过考虑路段和交叉点的不同子集来分解城市碰撞预测模型有助于揭示一些预测因素的具体影响。局部特征可能会影响完善的碰撞预测指标和碰撞频率之间的关系。城市碰撞频率变化的很大一部分仍然无法解释，因此鼓励对这个主题的研究。