Impact of Variable Speed Limits on Crash Frequency and Crash Rate: Stimulation by Flow Rate and Percentage of Heavy Vehicles

Al-Nuaimi, Atheer N.; Jameel, Abeer K.; Alsadik, Samer M. · 2025 · Crossref

DOI: 10.48084/etasr.9660

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Summary

This study investigates the impact of Variable Speed Limits (VSL) on crash frequency (CF) and crash rate (CR) on two-lane highways, addressing the high incidence of road accidents and economic losses associated with traffic collisions. The research focuses on the Mandali-Baqubah highway in Iraq, a strategic route prone to speeding and heavy vehicle traffic. The authors aim to model the relationship between VSL and safety outcomes while accounting for critical traffic characteristics, specifically Average Daily Traffic (ADT) and the Percentage of Heavy Vehicles (PHV). The novelty of the work lies in applying VSL modeling to two-lane highways, whereas previous studies primarily focused on multilane facilities. The methodology employed a case study approach using data collected from 2010 to 2019. The highway was segmented into 84 one-kilometer sections. Traffic data, including operating speed, ADT, and PHV, were gathered via video recording and sensor stations. Crash data were sourced from local traffic authorities and police records. To model the Safety Performance Functions (SPF), the researchers utilized Poisson-Gamma (PG) regression analysis to address overdispersion in the crash count data. The VSL strategy was simulated based on specific thresholds: a PHV of 11% and an ADT of 1490 vehicles per hour. Statistical validation included Variance Inflation Factor (VIF) checks for collinearity and Q-Q plots to verify distribution assumptions. The results indicate that ADT, PHV, and operating speed are positively and significantly correlated with both CF and CR. The PG models demonstrated good fit, with acceptable collinearity levels. Simulation of the VSL system revealed substantial safety improvements. When VSL was activated in response to high traffic flow (reaching the 1490 vehicles/hour threshold), the estimated average CF decreased by up to 31%, and the average CR decreased by up to 28%. Similarly, when VSL was triggered by high heavy vehicle presence (11% PHV), the average CF reduced by up to 28%, and the average CR by up to 27%. The study attributes these reductions to decreased overtaking maneuvers and rear-end crashes resulting from gradual speed limit reductions upstream of bottlenecks. The findings suggest that VSL systems can significantly enhance safety on two-lane highways by mitigating risks associated with high traffic volumes and heavy vehicle concentrations. The study concludes that implementing VSL is a viable countermeasure for reducing crash potential in such environments. However, the authors note that the effectiveness of VSL depends on driver compliance and behavior, recommending further investigation into enforcement strategies. Additionally, the study was limited to level terrain, and future research should explore VSL impacts on varied topographies and assess the cost-effectiveness of implementation.

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• Empirical Findings: crash risk outcomes