06 - Chapter-6

06 - Chapter-6 - Geometric Design Manual-2002 Chapter 6...

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Geometric Design Manual-2002 Cross Section Elements Ethiopian Roads Authority Page 6-1 6C ROSS S ECTION E LEMENTS 6.1 Introduction A cross-section will normally consist of the carriageway, shoulders or curbs, drainage features, and earthwork profiles. These terms are defined in the Definition portion of the manual text; major elements are repeated here for clarity: ± Carriageway- the part of the road constructed for use by moving traffic, including traffic lanes, auxiliary lanes such as acceleration and deceleration lanes, climbing lanes, and passing lanes, and bus bays and lay-byes. ± Roadway- consists of the carriageway and the shoulders, parking lanes and viewing areas ± Earthwork profiles- includes side slopes and back slopes For urban cross-sections, cross-section elements may also include facilities for pedestrians, cyclists, or other specialist user groups. These include curbs, footpaths, and islands. It may also provide for parking lanes. For dual carriageways, the cross-section will also include medians. Typical Cross Sections are illustrated in Appendix E of this manual. Bus lay-byes, parking lanes, passing lanes, and viewing areas are presented in Chapter 14. Lane and shoulder widths should be adjusted to traffic requirements and characteristics of the terrain. The cross-section may vary over a particular route because these controlling factors vary. The basic requirements are, however, that changes in cross-section standards shall be uniform within each sub-section of the route and that any changes of the cross- section shall be effected gradually and logically over a transition length. Abrupt or isolated changes in cross-section standards lead to increased hazards and reduced traffic capacity and complicate construction operations. In certain cases, however, it may be necessary to accept isolated reductions in cross-section standards, for example when an existing narrow structure has to be retained because it is not economically feasible to replace it. In such cases a proper application of traffic signs and road markings is required to warn motorists of the discontinuity in the road. However, all such narrow structures must be widened or replaced however when the width across the structure is less than the adjacent carriageway width. 6.2 Lane Widths A feature of a highway having great influence on safety and comfort is the width of the carriageway. Lane widths of 3.65m are used for Design Classes DS1 and DS2. The extra cost of 3.65 m above that for 3.0 m is offset to some extent by a reduction in cost of shoulder maintenance and a reduction in surface maintenance due to lessened wheel concentrations at the pavement edges. The wider 3.65m lane also provides desired clearances between large commercial vehicles on two-way rural highways. Narrower lanes are appropriate on lower volume roads. Standards for carriageway widths

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This note was uploaded on 11/05/2010 for the course CIVIL ce-3020 taught by Professor Era during the Spring '10 term at Trinity College Dublin.

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06 - Chapter-6 - Geometric Design Manual-2002 Chapter 6...

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