12 - Chapter-12 - Geometric Design Manual-2002 Chapter 12...

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Chapter 12 Geometric Design Manual-2002 At–Grade Junctions Ethiopian Roads Authority Page 12-1 12 A T -G RADE J UNCTIONS 12.1 Introduction A junction, or intersection, is the general area where two or more roads join. A disproportionate amount of traffic accidents occur at junctions, and thus from a traffic safety aspect junctions require attention and careful design. Good junction design should allow transition from one route to another or through movement on the main route and intersecting route with minimum delay and maximum safety. To accomplish this, the layout and operation of the junction should be obvious to the driver, with good visibility between conflicting movements. Differing junction types will be appropriate under different circumstances depending on traffic flows, speeds, and site limitations. Types of junctions include: 1) T-Junctions 2) Cross-Junctions 3) Roundabouts 4) Grade Separation: Discussed in Chapter 13 This chapter describes the design for all at-grade junctions. A checklist for junction design is presented in Figure 12-12 at the end of the chapter. 12.2 Design Requirements The design of junctions must take account of the following basic requirements: Safety Operational comfort Capacity Economy A junction is considered safe when it is visible, comprehensible, and maneuverable. These three requirements can generally be met by complying with the following guidelines. 12.2.1 V ISIBILITY The junction should be sited so that the major road approaches are readily visible. The angle of skew of the junction should be no more than 20 degree from perpendicular. 12.2.2 C OMPREHENSION (i) The right of way should follow naturally and logically from the junction layout. (ii) The types of junctions used throughout the whole road network should be similar. (iii) The use of road signs is necessary. Road markings and other road furniture may also be required.
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Chapter 12 At-Grade Junctions Geometric Design Manual-2002 Page 12-2 Ethiopian Roads Authority 12.2.3 M ANEUVERABILITY (i) All traffic lanes should be of adequate width and radius for the appropriate vehicle turning characteristics. To accommodate truck traffic, turn radii shall be 15 meters minimum. (ii) The edges of traffic lanes should be clearly indicated by road markings. The operation of the junction depends principally upon the frequency of gaps that naturally occur between vehicles in the main road flow. These gaps should be of sufficient duration to permit vehicles from the minor road to merge with, or cross, the major road flow. In consequence junctions are limited in capacity, but this capacity may be optimized by, for example, canalization or the separation of maneuvers. 12.3 Selection of Junction Type The choice of a junction type requires knowledge of traffic demand, intersection performance and accident prediction. It is often difficult to determine the best intersection type of any particular situation, taking into account capacity, delay, safety and physical layout factors. Several alternatives may be possible at a junction. A guide for the selection of
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