Abstract
The AASHTO’s guideline for geometric design, also known as the green book, requires that the inside of horizontal curves be cleared of obstructions to sight lines in order to provide sufficient sight distances. Recently, innovative use of Euler’s spiral for determination of clearance offsets has been proposed. However, suitability of the offsets as minimum criteria has not been evaluated. This paper presents comparison between the proposed offsets and minimum offsets determined with the computational method suggested in the green book. Results of comparison show that offsets determined with innovative use of the Euler’s spiral are always longer than minimum values determined with the computational method. The differences in lengths of the two sets of offsets increase with decrease in curve radii. Therefore, on sites with large radii offsets determined through innovative use of the Euler’s spiral may be implemented in the field since the offsets are only slightly longer than minimum offsets. On sites with short radii some offsets on tangent sections are very long such that they result in extra cleared areas that will not accommodate sightlines. The areas that do not accommodate sightlines may result in unnecessary extra earthwork costs where highways are located in cut zones. Additionally, it has been suggested in this paper that designers also consider other curves, including elliptical arcs, for roadside clearance envelopes. One advantage of elliptical arcs is that they are flexible to align with boundaries of clear zones on tangent sections regardless of sizes of radii of horizontal curves. Besides, most offsets to elliptical arcs are comparable to those determined with the green book’s computation method. An example of design chart has been presented for practitioners to use. The chart is for minimum offsets needed to provide a given sight distance while gradually transitioning clearance from boundaries of clear zones on tangent sections.
Highlights
Part of design of safe highways is provision of sufficient sight distance
This paper presents comparison between the proposed offsets and minimum offsets determined with the computational method suggested in the green book
It is apparent that offsets to the innovative envelope that accommodate sightlines for 90th percentile speeds are even longer since they provide more than 100 ft more sight distance on the approach tangent than the envelope for minimum offsets [7], while sight distances provided by the envelope for minimum offsets [7] and by the clear zone are already greater than demanded stopping sight distances
Summary
Part of design of safe highways is provision of sufficient sight distance. On long tangent sections sufficient sight distances are naturally provided since sightlines are completely accommodated within travel lanes. The green book is flexible since it suggests that if designers feel that values obtained with Equation (1) are inapplicable they may use the computational method [2] or the graphical method to determine suitable clearance offsets. The graphical method and its analytical counterparts use curve geometry (i.e. radius and length) and stopping sight distance as input to directly output minimum offsets. These minimum offsets are used as criteria for whether or not to clear a roadside object of given offset. On the tangent sections significant lengths of sightlines near drivers are accommodated within lanes, making minimum offsets shorter than the available offset to the boundary (i.e. outside edge) of the clear zone. The shorter minimum offsets are usually discarded and the boundary of the clear zone automatically becomes part of the provided clearance envelope
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