13 - Chapter-13 - Geometric Design Manual-2002 Chapter 13...

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Chapter 13 Geometric Design Manual-2002 Grade - Separated Junctions Ethiopian Roads Authority Page 13-1 13 G RADE –S EPARATED J UNCTIONS 13.1 General The circumstances in which the use of a grade separated junction is warranted are usually as follows: ± An at-grade junction has insufficient capacity (see Figure 13-1) ± The junction is justified economically from the savings in traffic delays and accident costs ± Grade separation is cheaper on account of topography or on the grounds that expensive land appropriation can be avoided by its construction ± For operational reasons ± Where roads cross motorways In deciding on the location of a grade-separated junction, the following factors should be taken into account: Trip length (travel distance) Size of urban areas Predicted traffic volumes Cost of junction Congestion control The use of grade separation results in the separation of traffic movements between the intersecting roads so that only merging or diverging movements remain. The extent to which individual traffic movements should be separated from each other depends mainly upon capacity requirements and traffic safety aspects; it also depends upon the extent to which important traffic movements should be given free flow conditions. 13.2 Choice of Scheme From a study of conflicting traffic movements, it will generally be apparent which traffic streams must be grade separated, leaving the other streams to be dealt with by junctions at grade; the choice of these will depend upon the capacities needed. A study of the characteristics of various types of grade-separated junctions is necessary, and a number of alternative designs should be prepared. The final choice of scheme must satisfy capacity requirements, geometric standards, and operational needs, and represent an economical design. In some instances the choice of a particular design will be determined by the adoption of two-stage construction, e.g. constructing an at-grade junction first and providing grade separation later. 13.3 Geometric Standards The geometric standards given in this manual for roads and at-grade junctions also apply to grade separated junctions. However, the low design speeds of loops and other ancillary roads necessitate further standards to be given. These are described below:
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Chapter 13 Grade - Separated Junctions Geometric Design Manual-2002 Page 13-2 Ethiopian Roads Authority 13.3.1 D ESIGN S PEED The design speed for the through traffic movements shall be determined in accordance with Chapter 5. Stopping sight distances appropriate for the design speed should always be provided. Where a dual carriageway intersects with another dual carriageway, the junction between the
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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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13 - Chapter-13 - Geometric Design Manual-2002 Chapter 13...

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