In civil engineering (more specifically highway engineering), grade separation is a 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, also called flyovers), tunnels (or underpasses), or a combination of both can be built at a junction to achieve the needed grade separation.
In North America, a grade-separated junction may be referred to as a grade separation or as an interchange – in contrast with an intersection, at-grade, a diamond crossing or a level crossing, which are not grade-separated.
Effects
Advantages
Roads with grade separation generally allow traffic to move freely, with fewer interruptions, and at higher overall speeds; this is why speed limits are typically higher for grade-separated roads. In addition, reducing the complexity of traffic movements reduces the risk of accidents.
Disadvantages
Grade-separated road junctions are typically 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. For these reasons, proposals for new grade-separated roads can receive significant public opposition.
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.
Grade-separated pedestrian and cycling routes often require modest space since they do not typically intersect with the facility (such as a highway) that they cross. However, grade-separated pedestrian crossings with steps introduce accessibility problems. Some crossings have lifts, but these can be time-consuming to use.
Grade-separated roads that permit for higher speed limits can actually reduce safety due to 'weaving' (see below) as well as a perceived sense of safety.
Roads
Overview
The term is most widely applied to describe a road junction in which the direct flow of traffic on one or more of the roads is not disrupted. Instead of a direct connection, traffic must use on and off ramps (United States, Australia, New Zealand) or slip roads (United Kingdom, Ireland) to access the other roads at the junction. The road which carries on through the junction can also be referred to as grade separated.
Typically, large freeways, highways, motorways, or dual carriageways are chosen to be grade separated, through their entire length or for part of it. Grade separation drastically increases the capacity of a road compared to an identical road with at-grade junctions. For instance, it is extremely uncommon to find an at-grade junction on a British motorway; it is all but impossible on a U.S. Interstate Highway, though a few do exist.
If traffic can traverse the junction from any direction without being forced to come to a halt, then the junction is described as fully grade separated or free-flowing.
Types
Fully separated
These junctions connect two freeways:
Stack interchange (two-level, three-level, or four-level stack, depending on how many levels cross at the central point) Cloverleaf interchange Trumpet interchange Cloverstack interchange Directional T Semi-directional T Turbine (whirlpool) interchange Windmill interchange Various incarnations of spaghetti junction
Partially separated
These junctions connect two roads, but only one is fully grade-separated, i.e. traffic on one road does not have to stop at yield lines or signals on one road, but may have to do so when switching to the other:
Diamond interchange
Partial cloverleaf interchange
Single-point urban interchange
Roundabout interchange
Compact grade-separation, whereby the two roads are linked by a compact "connector road", with major-minor priority junctions at each of its ends; usually a variant of the cloverleaf type interchange, but only involving two quadrants rather than four