Abstract
Sight distance is of the utmost importance for traffic safety. The consideration of three-dimensional (3D) available sight distance (ASD) in geometric design has been supported by several researchers. However, existing ASD estimation methods are two-dimensional (2D) in nature, which do not evaluate varying visibility conditions. This paper compares different methodologies of modelling the ASD. The ASD of 402 horizontal curves, located in twelve in-service two-lane rural highways, was analyzed. Three ASD estimation methods were used which include a 2D method and two different 3D methods. The ASD results obtained through 2D and 3D methodologies are compared. Also, the different conditions of the existing roadside features or geometric elements, under which the 3D ASD estimation is important, were identified. Next, reliability theory is utilized to evaluate the risk level (probability of noncompliance,Pnc) associated with limited sight distance for each ASD modelling method. The results of the comparison emphasized the importance of considering the 3D modelled sight distance when evaluating the associated risk either in highway design or during the service life. In addition, the results indicated that the ASD modelling approach can have a significant impact on the estimation of the safety of highway design.
Highlights
The importance of providing adequate sight distance for a safe and efficient traffic is recognized by researchers and most standard design manuals [1, 2]
The available sight distance (ASD) results obtained through 2D and 3D methodologies are compared
ASD estimation methods found in literature can be classified into two groups: detached 2D methods for horizontal and vertical alignment and 3D methods. 2D approaches have traditionally been used in geometric design and are presented in most current standards
Summary
The importance of providing adequate sight distance for a safe and efficient traffic is recognized by researchers and most standard design manuals [1, 2]. At any point on a highway, the available sight distance (ASD) should be sufficient to allow drivers to safely control their vehicles. Geometric design guides usually propose two-dimensional (2D) sight distance models, which fall short when the alignment is composed of complex sequences of three-dimensional (3D) elements. As those models may underestimate or overestimate the actual ASD, a 3D procedure to evaluate sight distance is preferable. The ASD depends on many parameters such as cross-section and roadside elements, besides the vertical and horizontal alignments. The Spanish standard on geometric design [6] provides separate 2D methods for the estimation of ASD on the horizontal and vertical alignment.
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