Vehicular conflicts are manifest and with high expectancy for accidents At

Vehicular conflicts are manifest and with high

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Vehicular conflicts are manifest and with high expectancy for accidents. At-grade intersections also slow down vehicles and increase operating costs. When all these difficulties become great, grade-separation becomes an attractive option. Grade separated structures have very large initial costs and require high level of expertise to accomplish. The main situations which justify the very considerable extra expenditure can be summarised as follows: 7
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Lecture 7 13/04/14 INTERSECTION DESIGN At intersections with motorways The construction of a motorway with complete access control automatically justifies the use of grade separation in order to ensure the free movement of high-speed vehicles To eliminate existing traffic bottlenecks The inability of an important at-grade intersection to provide the necessary capacity is in itself a justification for grade separation on a major highway Safety Considerations Some at-grade intersections are accident prone, regardless of the traffic volumes they carry. Economic considerations At at-grade major road junctions very considerable economic losses can be incurred due to intersectional friction and resultant delays to traffic. Grade separation could result in long-term economic gains 8
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Lecture 7 13/04/14 INTERSECTION DESIGN Grade separated junctions may be with or without slip roads. Grade separation without slip roads: In this case a bridge or series of bridges are employed to lift the traffic streams of intersecting highways to cross over each other without any conflicts. Usually this is done when a relatively minor road crosses another much higher in the hierarchy as in rural areas. In urban/suburban conditions this type of intersection is used to reduce the number of intersections at-grade with major roads Grade separation with slip roads: This provides for interchanges between two or more roads on different levels. Requires bridges and slip or link roads. These come in two basic forms in terms of layout, T or X, but may have variations in between. T-interchange (Trumpet). These are most suitable for connecting two major roads with relatively low turning movements and in restricted spatial conditions. The lesser flow is constrained to use the semi-directional loop. 9
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Grade separated junctions/Interchanges Lecture 7 13/04/14 10
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Trumpet interchange Lecture 7 13/04/14 11
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Lecture 7 13/04/14 INTERSECTION DESIGN X-interchange (Cloverleaf, partial or full) This is a four-way, two level structure interchange having loop and slip roads for turning traffic. Internationally, this is often regarded as the ultimate answer to intersection problems by motorists. The greatest advantage is its simplicity and the fact that the eight turning movements are accomplished with no direct vehicle conflicts. It is has disadvantages, namely: They require an exceptionally large land area Vehicles doing left turns have to negotiate a 270 degree turn. This is a difficult design problem and vehicles merging from such loops have to weave their way through others exiting to another loop 12
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  • Summer '14
  • Freeway, Motorway, roundabout, Grade separation, At-grade intersection, Turbo Roundabouts

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