Assessing the Impact of Roadside and Median Safety Barriers on Available Sight Distance on Croatian Motorways

Available sight distance (ASD) on motorways represents the unobstructed line of sight from the driver’s perspective to an obstacle on the carriageway, which must equal or exceed the required stopping sight distance (SSD). This study evaluates ASD across both two-dimensional (2D) and three-dimensional (3D) motorway alignments, encompassing right- and left-hand horizontal curves combined with different vertical alignments, as well as roadside and median barriers. 2D analyses were performed on predefined theoretical profiles, while 3D ASD evaluations were conducted on real-world models of existing motorway sections in Croatia. These sections were selected based on their diverse horizontal and vertical geometric alignments, design eras, traffic characteristics, and historical crash data regarding collisions with obstacles. The findings reveal that safety barrier placement on motorways can severely constrain sightlines, demonstrating that even radii exceeding the regulatory minimums are insufficient to guarantee necessary SSD requirements. Visibility restrictions from safety barriers are especially pronounced along barrier-restricted horizontal curves, primarily on right-hand curves and occasionally on left-hand curves, depending on the radius. In certain real-world sections, cut slopes also restrict the required visibility. Current Croatian guidelines rely on 2D analysis, which fails to account for safety barriers obstructing visibility. This research emphasises the critical necessity of conducting continuous 3D sight visibility simulations during initial design phases, while recommending further research that could be implemented to update regulatory frameworks regarding parameters for ASD analysis.