Variable speed limits (VSL) and road safety: A systematic review and guidance for VSL design and implementation

Abstract

Variable speed limit (VSL) systems are increasingly deployed as part of active traffic management, yet their safety impacts remain heterogeneous and context dependent. This paper presents a structured review of empirical and modeling studies evaluating the safety effects of VSL, with explicit distinction between crash-based and non-crash-based evidence. This systematic review used PRISMA-guided procedures to identify and synthesize 51 eligible studies that span a wide range of methodological approaches, from traditional before–after crash evaluations and simulation-based analyses to more recent data-driven and reinforcement learning–based VSL control frameworks. Crash-based evaluations generally report meaningful safety gains, with VSL associated with reductions in total crashes and in specific crash types, particularly rear-end, weather-related, and injury crashes, with the magnitude of these reductions varying depending on roadway, traffic, and environmental conditions. Complementary non-crash-based evidence shows that these reductions are plausibly mediated by lower speed variance, smoother traffic flow, and mitigation of congestion-induced shockwaves, although many studies rely on assumed compliance and are limited to simulated environments. At the same time, several evaluations highlight that poorly chosen activation conditions, aggressive speed steps, and low driver compliance can erode or even reverse safety benefits. Drawing on this mixed evidence base, the review synthesized practice-oriented guidance on where, when, and how VSL should be implemented. The findings indicate that VSL is most effective on corridors with recurrent congestion, adverse weather, or geometric bottlenecks that produce unstable traffic conditions, and that incremental, harmonization-oriented control strategies with short update intervals are preferable to large step-down speed reductions. The paper proposes indicative decision thresholds for VSL activation based on traffic-flow, weather, and safety triggers, and outlines agency readiness requirements. Overall, the review clarifies the conditions under which VSL is most likely to deliver robust safety benefits and provides a set of evidence-based recommendations to inform design and deployment.