What is it about?
Designing safe roads is not just about setting speed limits or placing signs. It also involves carefully shaping how roads rise, fall, and curve. Drivers experience the road in three dimensions, and the way horizontal curves (bends in the road) and vertical curves (hills and dips) are combined can greatly influence what they see while driving. When this coordination is poorly done, it may lead to dangerous situations where a driver cannot see far enough ahead to react safely—such as dips that hide oncoming vehicles or curves that appear suddenly after a hilltop. This review explores how these three-dimensional alignment issues affect visibility and driver perception, and ultimately, road safety. It highlights common design flaws, such as "sight-hidden dips" or "hidden beginnings of curves," which can confuse drivers and increase crash risk. The article also reviews current road design guidelines from around the world, pointing out where they succeed and where they fall short, especially in the context of new technologies like autonomous vehicles. By combining insights from engineering, perception science, and safety studies, this work calls for better tools and clearer standards to help engineers design roads that not only follow technical rules but also match how drivers actually see and react to the road ahead.
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Why is it important?
This research is important because it addresses a critical yet often overlooked factor in road safety: how the three-dimensional shape of a road affects what drivers can see and how they respond. Poor coordination between curves and slopes can create hidden hazards that contribute to crashes, especially on rural roads or at higher speeds. As transportation evolves, with increasing automation and higher safety expectations, understanding and improving 3D road design is essential to building safer, more predictable roads for both human drivers and autonomous vehicles.
Perspectives
Looking ahead, research on three-dimensional alignment coordination offers valuable opportunities to enhance road safety through more refined design standards, simulation tools, and risk assessment models. As autonomous vehicles become more prevalent, ensuring that road geometry aligns with both human perception and sensor capabilities will be increasingly critical. Future studies could focus on integrating real-world crash data with 3D modeling to establish clearer safety thresholds. There is also potential to develop user-friendly software that helps designers detect problematic alignments early. Ultimately, bridging engineering practice with human-centered design and emerging technologies will be key to creating safer, smarter road networks.
Dr. César De Santos-Berbel
Universidad Politecnica de Madrid
Read the Original
This page is a summary of: Safety Implications of Three-Dimensional Alignment Coordination in Highway Design: Sight Distance, Visual Perception, Design Methodologies, and Future Challenges, Journal of Transportation Engineering Part A Systems, September 2025, American Society of Civil Engineers (ASCE),
DOI: 10.1061/jtepbs.teeng-8852.
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