Vehicle Speed Characteristics and Alignment Design Consistency for Mountainous Roads

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.