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The importance of considering three-dimensional (3D) sight distance in geometric design has been demonstrated by several researchers. However, little progress has been made in the use of 3D analysis in the design of highways. This can be mainly attributed to the complexity of the computations required for 3D analysis. This paper presents a new algorithm for calculating the 3D sight distance. The algorithm is considered more efficient, less computationally intensive, and more flexible than previously developed approaches. The algorithm is based on a parametric representation of the roadway and roadside features without any implicit approximation of the roadway surface. The paper presents analysis results of various alignment configurations to demonstrate the use of the algorithm and to examine the influence of different geometric elements on the 3D sight distance. The paper also provides an analytical tool that can investigate and determine the conditions under which a 3D analysis is considered necessary. [ABSTRACT FROM AUTHOR]

In British Columbia, many highways are located in mountainous terrain and the costs of highway construction in these areas are high. One method of controlling construction costs is narrow medians but the safety consequences of using narrow medians were never determined. Recent research on safety in geometric design has focused on establishing quantitative relationships between collisions and cross-sectional elements using collision prediction models (CPMs) and collision modification factors (CMFs). In some situations, such as the use of narrow medians, it is difficult to find CPMs and CMFs that adequately describe the design scenario. In other instances, it is difficult to measure the safety in terms of collision reduction because of a lack of data or difficulty isolating the impact of a single design element on collision frequency. In these situations, reliability analysis can be used to evaluate the risk associated with a particular design feature. Reliability analysis is not intended as an alternative to quantify safety using collision frequency but represents a complementary method of measuring safety in terms of risk. In this paper, reliability analysis was completed on a series of horizontal curves with varying horizontal sight distance restrictions and the probability of being unable to stop within the available sight distance was calculated. The results of the study found that narrow medians combined with tight horizontal curves did not provide sufficient sight distance that vehicles would be expected to stop if an object was in a vehicle’s path. The analysis was applied to a case study of two constrained alignments in mountainous terrain of British Columbia. [ABSTRACT FROM AUTHOR]