05 - Chapter-5 - Geometric Design Manual-2002 Chapter 5...

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Chapter 5 Geometric Design Manual-2002 Design Controls & Criteria Ethiopian Roads Authority Page 5-1 5D ESIGN C ONTROLS AND C RITERIA 5.1 Introduction The choice of design controls and criteria is influenced by the following factors: the functional classification of the road; the nature of the terrain; the design vehicle; the traffic volumes expected on the road; the design speed; the density and character of the adjoining land use; and economic and environmental considerations. As these factors usually vary along a route of some length, the design does not have to be constant for the whole length of a road. On the contrary, changes in the design are usually required in order to obtain proper correlation between the road layout and the above factors, whilst maintaining construction costs at realistic levels. The design process is illustrated in Figure 5-1. The following text describes some of the factors affecting design controls and criteria. 5.2 Road Functional Classification and Numbering The functional classification in Ethiopia includes five functional classes. The following are the functional classes with their description. I. Trunk Roads (Class I) Centers of international importance and roads terminating at international boundaries are linked with Addis Ababa by trunk roads (see Table A-1). They are numbered with an "A" prefix: an example is the Addis-Gondar Road (A3). Trunk roads have a present AADT 1000, although they can have volumes as low as 100 AADT (see Table 2-1). II. Link Roads (Class II) Centers of national or international importance, such as principal towns and urban centers, must be linked between each other by link roads (see Table A-2). A typical link road has over 400 - 1000 first year AADT, although values can range between 50-10,000 AADT. They are numbered with a "B" prefix. An example of a typical link road is the Woldiya- Debre Tabor- Woreta Road (B22), which links, for instance, Woldiya on Road A2 with Bahir Dar of Road A3. III. Main Access Roads (Class III) Centers of provincial importance must be linked between each other by main access roads (see Table A-3). First year AADTs are between 30-1,000. They are numbered with a "C" prefix.
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Chapter 5 Design Controls & Criteria Geometric Design Manual-2002 Page 5-2 Ethiopian Roads Authority IV. Collector Roads (Class IV) Roads linking locally important centers to each other, to a more important center, or to higher class roads must be linked by a collector road. First year AADTs are between 25-400. They are numbered with a "D" prefix (see Table A-4). V. Feeder Roads (Class V) Any road link to a minor center such as market and local locations is served by a feeder road. First year AADTs are between 0-100. They are numbered with an "E" prefix and are presented in Appendix A. Roads of the highest classes, trunk and link roads have, as their major function to provide mobility, while the primary function of lower class roads is to provide access. The roads of intermediate classes have, for all practical purposes, to provide both mobility and access.
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05 - Chapter-5 - Geometric Design Manual-2002 Chapter 5...

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