Grade separation

Grade separation refers to separating two items that cross each other by placing them on different levels, or at different heights, to each other. Thus a bridge is one of the simplest forms of grade separation, allowing railways (railroads), canals, and rivers, for example, to pass above or below roads. A lack of grade-separation is referred to as at-grade (or level, in the case of road/rail level crossings outside the U.S.).

Road junctions

The term is applied most widely in the process of aligning a junction between roads such that the straight-through flow of traffic on one or more of them is not disrupted. This is typically attained by making the roads cross one another on different levels, with slip roads carrying traffic crossing from one road to another. Where a full spaghetti junction or cloverleaf style junction is not present, the interchange points of the junction will still be on a different level from at least one of the major roads. In some cases, an entire roundabout may be separated from the more major route, being either above or below it, with sliproads and the lesser road connecting to it.

Typically such junctions are used on motorways or similar major routes (especially dual carriageway).

Other uses

Grade separation is also used in personal rapid transit to describe a system-wide principle of keeping the transit guideway at a different level (usually elevated) from that of normal road traffic.