Sight distance restriction on highways’ horizontal curves: insights and sensitivity analysis

Bassan, Shy · 2016 · OpenAlex-citations

DOI: 10.1007/s12544-016-0208-6

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Summary

This study investigates the geometric design constraints imposed by sight distance restrictions on highway horizontal curves, specifically addressing how barriers and walls limit available stopping sight distance (SSD). The research aims to determine desirable horizontal radii by balancing the equilibrium of centrifugal forces against horizontal sight distance requirements. The problem is motivated by the frequent occurrence of obstructions, such as median safety barriers on divided highways or roadside barriers on two-lane roads, which intrude on the driver’s line of sight and compromise safety if not properly accounted for in design. The methodology employs a theoretical analysis comparing two scenarios: Case 1, where the SSD lies within the length of the horizontal curve ($L$), and Case 2, where the SSD extends beyond $L$. The study utilizes standard geometric formulas to calculate the horizontal sightline offset (HSO)—the lateral clearance required between the lane centerline and an obstruction—and relates it to the curve radius and SSD. A sensitivity analysis is conducted for Case 2 to examine the ratio between SSD and curve length ($L$). The analysis assumes a maximum superelevation of 8% and evaluates HSO values of 4.8 m (representing a 3.0 m shoulder plus half lane width) and 3.0 m. The study references AASHTO design standards for minimum radii and SSD values across design speeds ranging from 60 to 120 km/h. The results indicate that for Case 1, the minimum horizontal radius derived from centrifugal force equilibrium is often too small to satisfy SSD requirements when obstructions are present. For instance, at a design speed of 80 km/h with an HSO of 4.8 m, the available SSD at the minimum equilibrium radius is only 94 m, significantly less than the required 130 m. The sensitivity analysis for Case 2 reveals that the gap between available and required SSD, as well as the discrepancy between minimum and desirable radii, can be fully resolved when the ratio of SSD to curve length equals 4.5. However, the study concludes that these conditions are generally not feasible in practical horizontal curve design, particularly when the HSO is limited to 3.0 m. The significance of this work lies in highlighting the limitations of conventional design guidelines when obstructions restrict sightlines. The findings suggest that when SSD limitations are valid and obstruction heights exceed critical thresholds, highway engineers must consider trade-offs, such as increasing the horizontal curve radius or adjusting the design to accommodate the geometric constraints. The paper underscores that relying solely on centrifugal force equilibrium for radius selection is insufficient in obstructed environments, necessitating a more integrated approach to geometric design that prioritizes sight distance adequacy.

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• road geometry