A comparison of roadside design approaches in terms of road safety to improve Turkish roadside safety standards

Betus, Yakup
When an errant vehicle runs off the road, crashing into objects/ reaching places in the vicinity creates “run-off-road (ROR)” accidents, which may result in severe injuries or fatalities. Roadside designs with proper Road Restraint Systems (RRS) ideally should cause neither hazards to the third parties nor threats to the vehicle users. The major components of the RRSs are the Vehicle Restraint Systems (VRS), including the safety barriers (i.e., guardrails) designed to i) either stop the errant vehicle fully or ii) slow it down for a safe return to the lane or less severe crash. There are national or regional standards for RRS (i.e., NCHRP-350, AASHTO MASH, and EN1317) mainly define the systems, classifications, and acceptance tests. However, there are no standardized methods/tools for the design and application specifications of RRS (i.e., type, location, length of need, etc.) since they are mostly correlated with the geometric design standards of the roads, which vary across countries, socio-economic characteristics of the road users and infrastructure planning principles. While intercity road design principles in Turkey are mostly adopted from the AASHTO codes in the US, the clear zone concept (verge/green zone in the UK) is not fully employed in Turkish road designs; it does not follow a standardized cross-section for different types/functions of roads as in the UK or Germany, either. Furthermore, Turkey's RRS design must follow the European region EN1317 standards. Current intercity roadside design in Turkey is a mix of the USA principles embedded into a rule-based approach, which is a simplified version of the German roadside design approach. This creates a unique challenge in the design and implementation of the road sides, especially in the absence of the Road Safety Audit (RSA) process. The high share of RORs in Turkey, in the order of 15%-20% of the fatal and injury accidents every year, so it is necessary to improve the roadside designs. This thesis compares current Turkish practice with a newly proposed roadside design approach adopted from the UK approach, which also follows the same EN standards for the RSS and relies on a systematic assessment procedure for major highways (speeds higher than 90 km/h and traffic volumes greater than 5,000 veh/days). The comparisons are made over a study corridor of 15 kilometers for the safety barrier along (the nearside of) the road. Both length of needs and locations of the applications are compared between alternative approaches to understand the differences better. The new approach required the N2 containment level for most sections of the corridor, which provides a lower containment level than the H1 required by the current practice. However, the total length of safety barriers needed by the new approach was almost 2.5 times the length of the current practice, showing significant differences between the two approaches. By considering the more objective evaluation criteria of the proposed approach, a series of recommendations was provided for improving traffic safety in Turkey via safer roadside designs.


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Citation Formats
Y. Betus, “A comparison of roadside design approaches in terms of road safety to improve Turkish roadside safety standards,” M.S. - Master of Science, Middle East Technical University, 2022.