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Vehicle drivers continuously make decisions and execute them while driving on a roadway. It requires perception, comprehension and prediction of roadway conditions. Studies show that the decision-making process and driver performance greatly depends on situational awareness. Drivers intend to operate vehicles at safe speed based on the roadway geometric features. These features are characterized by gradient, horizontal curvature, length of curves and tangent sections, superelevation, etc. Perceptive and consistent geometric characteristics of the roadway can ease the decision making burden and thus minimize driver error. So far, majority of the studies on geometric design consistency are conducted on isolated curve sections in plain terrain for traffic with strong lane discipline. The effect of gradient is not significant in these terrains. Further, the driving behavior of traffic with weak lane discipline is not represented in these studies. Hence, the aim of this paper is to study geometric consistency of roadways on mountainous terrain for traffic with weak lane discipline characteristics using existing consistency based safety evaluation methods. Isolated horizontal curves with horizontal curvature, length of curve and gradient are considered for the purpose. Sixteen sites in a mountainous terrain with distinct horizontal alignment and vertical profile features are chosen on a four-lane divided National Highway 40 in India. The obtained results indicate that 79 % and 93 % locations exhibit good level of consistency as per Lamm's Criteria I and Criteria II, respectively. It is also observed that average vehicle operating speed at most of the locations exceeds the design speed. Keywords Geometric design consistency Á Horizontal curve Á Vertical gradient Á 85th percentile speed Á Mountainous terrain

Researchers have studied two-lane rural highways to predict the operating speed on horizontal curves and correlated it with safety. However, the driving characteristics of four-lane-divided highways are different. Weak lane discipline is observed in these facilities, which influences vehicle speed in adjacent lane or space. So, irrespective of its lane or lateral position, vehicles in four-lane divided highways are considered free flowing only when it maintains the minimum threshold headway from any lead vehicle. Examination of two conditions is proposed to ensure the free flow. Vehicles meeting both conditions, when tracked from the preceding tangent section till the centre of the horizontal curve, are considered as free flowing. The speed data of such free flowing passenger cars at the centre of eighteen horizontal curves on four-lane divided highways is analysed to develop a linear operating speed prediction model. The developed model depends on curve radius and preceding tangent length. The operating speed of passenger car in four-lane divided highways is influenced by horizontal curve of radius 360 m or less. Further, longer tangent would yield higher operating speed at the centre of the curve. Finally, two nomograms are suggested for conventional design, consistency based design and geometric design consistency evaluation of four-lane divided horizontal curves.

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