Disadvantages
Grade-separated junctions are very space-intensive, complicated, and costly, due to the need for large physical structures such as tunnels, ramps, and bridges. Their height can be obtrusive, and this, combined with the large traffic volumes that grade-separated roads attract, tend to make them unpopular to nearby landowners and residents. New grade-separated road plans can receive significant opposition from local groups for these reasons.
Rail-over-rail grade separations take up less space than road grade separations; because shoulders are not needed, there are generally fewer branches and side road connections to accommodate (because a partial grade separation will accomplish more improvement than for a road), and because at-grade railway connections often take up significant space on their own. However, they require significant engineering effort, and are very expensive and time-consuming to construct.
Rail-over-road grade separations require very little additional space because no connections need be built, but they require significant engineering effort and are expensive and time-consuming to construct.
Read more about this topic: Grade Separation, Effects