Studying the effect of spiral curves and intersection angle, on the accident ratios in two-lane rural highways in Iran

Kamali, Mohammad Hossain Jalal; Monajjem, Mohammad Saeed; Ayubirad, Mohammad Sadegh · 2013 · OpenAlex-citations

DOI: 10.7307/ptt.v25i4.332

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Summary

This study investigates the impact of horizontal curve geometry, specifically spiral curves and intersection angles, on accident rates in two-lane rural highways in Iran. Motivated by the high frequency of accidents in highway curves—where rates are 1.5 to 4 times greater than on straight paths—the research aims to quantify how geometric design parameters influence safety. The authors address a gap in traditional highway design, which often lacks predictive safety criteria during the design phase, by utilizing simulation software to evaluate safety before implementation. The researchers employed the Interactive Highway Safety Design Model (IHSDM) version 7.0.1, developed by the U.S. Ministry of Highway and Transportation, to predict accident rates. They designed five specific curve configurations: two simple circle curves with radii of 565 meters (C565) and 716 meters (C716), and three clothoid-circle-clothoid spiral curves with a 565-meter radius and spiral lengths of 60, 85, and 115 meters. These curves were analyzed across eight inner intersection angles ranging from 90 to 160 degrees. The simulation parameters were based on Iranian standards for smooth topography, assuming a design velocity of 110 km/h, a 6% super-elevation, and a traffic volume of 14,300 vehicles per day. The geometric designs were created in AutoCAD Land and imported into IHSDM for analysis. The results indicate that spiral curves significantly reduce accident rates per kilometer compared to simple circle curves. Specifically, accident rates for clothoid curves ranged from 3.37 to 3.39 per kilometer, whereas simple circle curves exhibited rates between 3.39 and 3.42 per kilometer. The study found that increasing the length of the spiral curve further reduces accident rates, with the 115-meter spiral yielding the lowest rates. Additionally, for simple circle curves, accident rates increased as the inner intersection angle increased, attributed to drivers' difficulty in distinguishing curvature at sharper angles. In contrast, spiral curves mitigated this risk due to their gradual curvature change, which provides drivers more time to react. Curve fitting equations derived from the data confirmed these trends, showing strong correlation coefficients ($R^2 > 0.99$). The study concludes that incorporating spiral curves in highway design is essential for improving safety, particularly at larger intersection angles. The authors recommend using spiral curves to minimize accidents and suggest that if simple circle curves are used, larger radii should be selected to reduce risk. This research represents the first application of IHSDM for accident prediction in Iran, demonstrating the software's utility in evaluating geometric design safety. The findings provide actionable guidelines for highway designers to enhance safety through optimized curve geometry, potentially reducing the high accident rates associated with rural two-lane highways.

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Topics

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• intersection design