Grade separation

In civil engineering (and more specifically, highway or railway 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.

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 and grade separation is typically a prerequisite for the implementation of meaningfully high-speed rail.

In addition, reducing the complexity of traffic movements can reduce the risk of accidents and further, reduce or preclude entirely the threat of vehicular homicide and fatal cyclist-vehicle collisions that becomes statistically inevitable with a large enough population of pedestrians or cyclists crossing even a modestly trafficked thoroughfare with reasonable posted speed limits. In the literal sense, only grade separation and the restriction of vehicle access to pedestrian spaces can actually and effectively reduce the probability of these deaths occurring regularly in any particular area to zero.

While much less common and generally easier to prevent than automotive and truck collisions with cyclists and pedestrians, vehicle-train, cyclist-train and pedestrian-train collisions are almost exclusively fatal, particularly when involving heavy or freight rail, and avoidable only on the end of the collision's victim in the absence of grade separation in most cases. Regardless of the competency and alertness of a train driver, there is nothing that the operator of a locomotive traveling at-speed can do to stop a train completely before reaching the most distant point on the tracks ahead of the driver that they were able to see at the point they first knew to apply the brake.

This is considerably less true in relation to light rail and trams, which frequently operate in mixed traffic and as such are comparably lightweight and responsive to braking, able to come to a halt at roughly the same rates as would a bus or lorry (truck), and usually stop in less time than a loaded semi-truck.

While trains overall are relatively predictable and pass far less frequently than automotive traffic, these collisions still occur with some regularity, particularly at grade crossings. As such, grade-separated crossings for railroads are both less challenging and expensive to implement, and similarly result in improved safety for all parties, at least when the comparably low rate of train collisions compared to road deaths is not taken into account.

With roadway junctions in particular, grade-separated interchanges 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.