Abstract
Design guidelines require that high objects on the inside of horizontal curves be cleared so as to provide sufficient sight distance. An example of the guidelines that require such clearance is the AASHTO’s Green Book. The Green Book has an analytical model for determining minimum clearance for a given design sight distance. The model is well suited for middle sections of long curves. Applying such clearance to sections near beginnings and ends of the curves and to sections where there is reverse of curvature will result in over-clearance. Over-clearance implies extra cost of earthwork where highways pass in cut zones. To avoid such extra costs the guideline suggests using the graphical method to determine exact clearance offsets. The graphical method is accurate but it is also tedious and time consuming. This study developed analytical models for efficiently determining clearance offsets that match the offsets determined with the graphical method. The offsets are ordinates from driver paths to flat roadside spirals that make the boundary of the roadside area to clear. Mathematical equations for the spirals comprise of terms related to highway speed (in the form of design sight distance), curve features, and driver locations. In turn, these terms define magnitudes of the offsets to the spirals. Combination of the terms results in many parameters to the extent of making difficult development of design charts for offsets. However, examining suitability of published offset charts for simple curves as estimates of offsets for sites with reverse curves leads to finding that the charts are suitable as long as the reverse curves have common tangents that are at least as long as 25% of design sight distance. For reverse curves with no common tangents, offset charts have been developed and presented in this paper. Practitioners can use these charts or the derived equations to determine clearance offsets for new sites as well as for existing sites that are deficient in design sight distance.
Highlights
Geometric design guidelines require that sufficient sight distance be provided at all sections of highways
This study developed analytical models for efficiently determining clearance offsets that match the offsets determined with the graphical method
The first objective of this paper is to develop an analytical model for efficiently determining minimum clearance offsets that would otherwise be determined with the accurate but inefficient graphical method
Summary
Geometric design guidelines require that sufficient sight distance be provided at all sections of highways. This requirement is automatically met on straight sections of highways since sightlines from drivers to downstream objects are within travel lanes. To meet the sight distance requirement on the curved sections, design guidelines recommend that the inside of the curves be cleared of high objects that block the sightlines. Each guideline has a model for determining the extent of clearance that provides design sight distances. The design sight distance is the selected length of visibility of a highway section that allows drivers to perform specific maneuvers such as passing, stopping, or evasive maneuvers
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