How do the geometric characteristics of horizontal curves on rural highways affect the likelihood of traffic crashes?
By Meghna Chakraborty
Did you know that fatal crash rates on curved road segments are approximately three times higher than those occurring on straight segments? Data shows that more than 25 percent of fatal crashes are found to be associated with horizontal curves and the majority of fatal crashes occur on two-lane highways with almost twice fatality rate on rural highways compared to its urban counterpart. Unfortunately, there are very few studies on the safety performance of horizontal curves, especially on rural county highways. This is concerning as highways owned and maintained by counties are an important part of rural road system in several states, especially those in the Great Lakes region of the U.S. including Michigan.
Researchers (Dr. Meghna Chakraborty and Dr. Timothy Gates) at Michigan State University extensively studied the effects of horizontal curve design on traffic crashes and found a significant association of several curve characteristics (including geometric features such as curve type, curve radii and their design speeds, curve direction, tangent distance preceding a curve or inner curve distance) with traffic crashes. They analyzed eight years of non-animal single-vehicle crashes on a total of 15,754 rural two-lane undivided highway curves across the State of Michigan.
A significant finding from the study was that compound curves (multiple curves of different radii but in the same direction) and reverse curves (S-shaped curves or multiple curves of different radii but in the opposite direction) contributed to much greater crash occurrence when compared to simple curves (with only one radius). Additionally, curves with design speeds lower than the speed limit had higher crash occurrences than curves with design speeds at or above the speed limit, especially on county highways. This study also found that the left-turning curves experience greater crash occurrence compared to right-turning curves, probably due to the fact that left-turning curves position vehicles closer to the roadside on the outside of the curve, thereby increasing the likelihood of roadway departure crashes.
Some interesting findings from their study could also be used to improve highway geometric design from a safety perspective. For example, an increase in the inner-curve distance between successive compound or reverse curves was found to decrease crash occurrences. Thus, designing highway geometry with elongated inner-curve distances could improve road safety. At the same time, crash occurrence was found to increase with an increase in tangent distance approaching the first of a series of curves. Collectively, these findings suggest that elongated straight segments reduce crashes within a series of curves, but have the opposite effect on the approach to an isolated curve, likely due to the reduced expectancy as a curve becomes increasingly isolated along an uninterrupted highway segment. As curve realignment is typically cost-prohibitive, enhanced warning signage, such as speed feedback signs and/or flashing beacons, should be considered at isolated sharp curves.
Article Details
Relationship between Horizontal Curve Geometry and Single-Vehicle Crash Occurrence on Rural Secondary Highways
Meghna Chakraborty & Timothy J. Gates
First Published November 23, 2023
DOI: 10.1177/03611981231208901
Transportation Research Record: Journal of the Transportation Research Board
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