Abstract
Design of curves and their adjacent elements presents the greatest safety problem on rural two-lane roads. The use of the existing alignment consistency safety criteria (design, operating speed, and driving dynamic consistency) could have some shortcomings, especially in countries where the project or design speed is in use instead of (higher) operating speed. The consequence is that the designer should use smaller cross fall on curves than needed, while the calculated side friction is lower than in reality. Further, the existing graphs of adjacent curve radii do not take into account that there is a maximum operating speed achieved for a certain radius or long tangent above which it does not increase. This paper presents a methodology for determination of adjacent horizontal curve radii, with and without tangent between, based on the operating speed models which include dependence of operating speeds on tangents and curves on speed of adjacent alignment elements as well as maximum tangent and curve speed. The developed graphs of adjacent radii at the same time include the limiting values of driving dynamic consistency criteria, so the road designer does not need to calculate permissible and demand side friction for every combination of adjacent alignment elements.
Highlights
A good road design implies the selection of alignment elements enabling constant driving speed along homogeneous segments, providing efficient, comfortable and safe driving
The design speed concept for the determination of the minimum radius according to Equation 1 is practically outdated because the operating speed is much higher than design and project speed, which are used for the determination of cross slope and sight distance
Since the operating speed is higher than design and project speed, the designers use smaller cross fall on curves than needed
Summary
A good road design implies the selection of alignment elements enabling constant driving speed along homogeneous segments, providing efficient, comfortable and safe driving. The design of curves and their adjacent elements presents the greatest safety problem on rural two-lane roads. Baldwin in [7] showed that, while the accident rate increases as the radius of horizontal curve decreases, the accident rate for sharp curves decreases as the frequency of curves (per length of highway) increases. This shows that there is a connection between the traffic safety and consistency of alignment elements which ensures that successive road elements provide uniform driving speed in line with the driver’s expectations [8]
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