Developing safety performance functions to inform transport policies on urban two-lane roads

Abstract

Road safety is a critical concern in urban areas, where complex networks and high levels of interaction between road users contribute to severe crashes. This study explores how various risk factors influence crash likelihood in urban settings and discusses their implications for transport policies aimed at improving safety and sustainability. We present a Safety Performance Function (SPF) and related Crash Modification Factors (CMFs) tailored for Italian urban two-lane roads, covering 160 km across 10 provinces of the Tuscany Region, Central Italy. By analyzing 2519 Fatal and Injury (FI) crashes from 2008 to 2016, we identify risk factors such as traffic flow, road alignment, cross-section geometry, density of driveways, and density of intersections. Our findings reveal that non-compliance with infrastructure standards, such as too-wide roads, may contribute to higher crash risks. However, even compliance with regulations, such as the presence of frequent driveways or closely spaced urban intersections, could increase crash frequency due to traffic turbulence and heightened conflict points. These results highlight the necessity for evidence-based policies that not only enforce compliance but also reevaluate current standards to address residual risks. Targeted interventions, such as optimizing traffic flow and pedestrian mobility, regulating driveway density, and improving intersection design, can enhance urban road safety while aligning with broader sustainability goals. By addressing these specific risk factors, policymakers can create safer and more sustainable transport systems in urban environments.