Horizontal alignment

Authored by: Keith M. Wolhuter

Geometric Design of Roads Handbook

Print publication date:  April  2015
Online publication date:  October  2015

Print ISBN: 9780415521727
eBook ISBN: 9781482288728
Adobe ISBN:

10.1201/b18344-8

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Abstract

The horizontal alignment comprises a series of horizontal curves and the tangents between them. It is the most long-lived of the various elements of the road, with the alignment of some roads built by the Roman Empire still in use today. It is also the first element to be considered in the design sequence of horizontal alignment, vertical alignment and cross-section. The design sequence, however, is not to be construed as being a linear process. It is, in fact characterised by ongoing feedback and refinement because each element of the design can and does impact on the others. The vertical alignment could generate a need to realign the horizontal alignment to avoid undesirably steep gradients. The location of an intersection may be so critical that everything else has to be realigned to accommodate it, although, customarily, the horizontal and vertical alignments would jointly dictate where an intersection could be located. Generously sized cross-sectional elements would create the expectation that high speeds could safely be maintained. The standards selected for the horizontal and vertical alignments would have to match this expectation. In short, the various elements and dimensions of the design have to be in harmony with each other.

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