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10 - Chapter-10

# 10 - Chapter-10 - Geometric Design Manual-2002 Chapter 10...

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Chapter 10 Geometric Design Manual-2002 Ethiopian Roads Authority Page 10-1 10 P HASING OF H ORIZONTALAND V ERTICAL A LIGNMENT 10.1 Alignment Defects Due to Mis-phasing Phasing of the vertical and horizontal curves of a road implies their coordination so that the line of the road appears to a driver to flow smoothly, avoiding the creation of hazards and visual defects. It is particularly important in the design of high-speed roads on which a driver must be able to anticipate changes in both horizontal and vertical alignment well within the safe stopping distance. It becomes more important with small radius curves than with large. Defects may arise if an alignment is mis-phased. Defects may be purely visual and do no more than present the driver with an aesthetically displeasing impression of the road. Such defects often occur on sag curves. When these defects are severe, they may create a psychological obstacle and cause some drivers to reduce speed unnecessarily. In other cases, the defects may endanger the safety of the user by concealing hazards on the road ahead. A sharp bend hidden by a crest curve is an example of this kind of defect. 10.2 Types of Mis-Phasing and Corresponding Corrective Action When the horizontal and vertical curves are adequately separated or when they are coincident, no phasing problem occurs and no corrective action is required. Where defects occur, phasing may be achieved either by separating the curves or by adjusting their lengths such that vertical and horizontal curves begin at a common station and end at a common station. In some cases, depending on the curvature, it is sufficient if only one end of each of

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10 - Chapter-10 - Geometric Design Manual-2002 Chapter 10...

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