Horizontal and Vertical Alignment The desirable intersection angle of

# Horizontal and vertical alignment the desirable

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Horizontal and Vertical Alignment. The desirable intersection angle of intersecting roads at a junction is about 90 degrees. When roads intersect at angles less than, say 60-70 degrees, consideration should be given to realigning the minor road. Also in siting an intersection, good practice requires that: an intersection should avoid a curve section, cut section of road and bridge approaches; the gradients at approaches of an intersection should be as gentle as possible (up 2.5% desirable); and, an intersection should not be located at the bottom of sag or apex of a crest vertical curve
Lecture 6 13/04/14 20 INTERSECTION DESIGN Sight Distance The intersection, and traffic on its approaches, should be seen and appreciated by approaching drivers. At major/minor priority junctions, the practice is for the major road through traffic to be provided with the desirable minimum stopping sight distance on the approaches to and through each junction. Drivers on the minor road should have unobstructed visibility to the left and right along the main road for a distance dependent upon the major road traffic speed. The visibility should be available on a line between two points 1.05m above the carriageway as follows: A point 9m along the centerline of the minor road from the continuation of the line of the nearer edge of the carriageway of the major A point at a length determined by design speed of main road (70m for 50mph), measured along the nearer edge of the major road carriageway from its intersection with the centerline of the minor road
Lecture 6 13/04/14 21 INTERSECTION DESIGN Basic Design Principles/Essence Channelisation In practice the highway engineer must also be guided by the following basic design principles to ensure safety of operations through channelisation : Minimize the carriageway area of conflict (reduce dead space) This principle is illustrated below as the large area in which possible conflicts can take place at a conventional Y-intersection can be modified. When elongated islands are placed appropriately in the intersection, the size of the uncontrolled area is significantly reduced so that there is less chance of confusion and potentially dangerous vehicle movements. Control angle of potential vehicle conflict: Where two opposing traffic streams meet at about a 30 degree angle collisions are very likely as the entering motorist from the minor road has particular difficulty in seeing to the right which increases the chance of conflict with traffic traveling in both directions on the major road. Furthermore, should an accident occur between opposing vehicles, the risk of a fatality is high since the relative speed is high.
Lecture 6 13/04/14 22 INTERSECTION DESIGN Provide protection for vehicles leaving or crossing the main traffic stream.

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