9.1.1 Grading Options
There are basically two grading options: the major road (e.g. freeway) over the minor road or the reverse, including the situation where either the freeway or the secondary road is depressed into the natural surface (fully or partially). Factors affecting the choice of options are summarised in Table 9.1.
|Topography||Desirably, the major road (freeway) should be designed to match the terrain and minimise visual intrusion. As the geometric standards used on the minor road are lower than on the major road, it is commonly easier and less visually intrusive to grade the minor road over the major road on flat terrain than the reverse.|
|Adjacent development||In developed urban areas, the major road (freeway) usually has to be graded to minimise disruption to local development. Overpass structures may be acceptable in industrial areas but in residential areas they may be considered by residents as visually intrusive and an invasion of privacy. A fully depressed major road avoids visual intrusion and provides some noise attenuation.|
|Trucks||The major road (freeway) depressed under minor road arrangement is advantageous where the number of trucks is significant because the downhill entry ramps shorten the distance required by the trucks to achieve freeway speeds. Also, uphill exit ramps assist truck braking.|
|Noise||In urban areas, a major road (freeway) located below the adjacent land and under a minor road may be preferable because major road noise is minimised. For the alternative arrangement, noise attenuation walls may be installed.|
|Construction staging at an interchange||An elevated major road (freeway) minimises disruption to traffic on the minor road during construction. Where the minor road passes over the major road the disruption may be minimised by building the overpass on a new alignment.|
|Economy||The relative economy of the two options depends on the topography, the number of lanes on the major road and the minor road and the effect on services.|
The minor road over major road option is generally cheaper for interchanges with roads that are not arterial roads. Assuming that the reservation width is based on the ultimate major road (e.g. freeway) cross-section, future additional major road lanes can be constructed without affecting the bridge.
Where the minor road (relatively) is a major arterial road, the freeway-over option could be cheaper because the cross-sections are comparable and it is less disruptive to existing traffic. In this case, allowance should be made in the initial structural design for the bridge widening required in the future.