Grade separation is the method of aligning a
junction of two or more surface transport axes at different heights (
grades) so that they will not disrupt the traffic flow on other transit routes when they cross each other. The composition of such transport axes does not have to be uniform; it can consist of a mixture of roads, footpaths, railways, canals, or airport runways. Bridges (or
overpasses or flyovers), tunnels (or
underpasses), or a combination of both can be built at a junction to achieve the needed grade separation.