Grade separation


In civil engineering, grade separation is a method of aligning a junction of two or more surface transport axes at different heights 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, tunnels, 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 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, 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.

Disadvantages

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.
Rail-over-rail grade separations, though, take up much less space than standard road or highway grade separations. In part, this is because shoulders are not required for railroad operations, even at high speeds, and there are generally far fewer branches and side road connections to accommodate because a partial grade separation will yield more improvement than it would for a similar road project, on which the overall traffic flow is determined by its most congested sections, as a result of well documented phenomenon such as traffic waves.
However, highway, mixed and even railroad-only grade separation projects, especially when 'retrofitting' an active transit corridor built without traffic conflict mitigations to save on construction costs, nonetheless usually necessitates considerable engineering expertise and effort, and can be very expensive and time-consuming to construct, especially when multiple environmental and existing-traffic related impacts must be studied, determined and adequately mitigated, as is required by law for projects of this nature in most jurisdictions.
Grade-separated pedestrian and cycling routes often have a comparably modest footprint since they do not typically intersect with high intensity transit corridors that they would cross, without the safety provided by a grade-separated crossing. However, grade-separated pedestrian crossings with steps introduce accessibility problems and can potentially conflict with the Americans with Disabilities Act in the United States. Some crossings have lifts, but these measures can be time-consuming and inconvenient to use, and many of these footbridges and pedestrian underpasses lie unused, abandoned and fenced off.
Grade-separated roads that permit for higher speed limits can actually reduce safety due to 'weaving', the increased probability of collisions corresponding with induced demand as well as the demonstrably false sense of safety caused by the monotony of driving long distances at high speeds with little or none of the stimulation and activity provided at-grade by stop lights, pedestrian crossings, more frequent turns and intersections.

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 or slip roads 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.
File:Leipzig-Halle Airport Condor.jpg|thumb|right| A plane on a taxiway over the Autobahn at Leipzig-Halle Airport—a type of grade separation

Types

Fully separated

These junctions connect two freeways:
Image:M23-M25 Intersection - geograph.org.uk - 15455.jpg|thumb|4 level stack interchange between the M25 and M23 in the UK
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:
On roadways with grade-separated interchanges, weaving is a result of placing an exit ramp a short distance after an entry ramp, causing conflicts between traffic attempting to leave the roadway at the next junction and traffic attempting to enter from the previous junction. This situation is most prevalent either where the junction designer has placed the on-slip to the road before the off-slip at a junction, or in urban areas with many close-spaced junctions. The ring road of Coventry, England, is a notorious example, as are parts of the southern M25, the London orbital motorway, the M6/M5 junction north-west of Birmingham, and the A4/M5 junction west of Bristol. Weaving can often cause side-on collisions on very fast roads with top speeds of up to, as well as the problem of blind spots.
Where junctions have unusual designs weaving can be a problem other than on the main road. An example of this can be found at Junction 7 of the M6, where traffic joining the roundabout from the M6 Eastbound off-slip must weave with the traffic already on the roundabout wishing to use the M6 Westbound on-slip. This is as a result of the slip roads on the west side of the junction connecting to the roundabout on the inside of the eastern arc rather than the outside of the western arc as is normal. The two slip-roads are connected by a single lane on the inside of the roundabout, which traffic wishing to use the Westbound on-slip must join, and traffic from the Eastbound off-slip must leave.
Weaving can be alleviated by using collector/distributor roads or braided ramps to separate entering and exiting traffic.

Types

The weaving area can be categorized in to three categories based on number of lane changes required for each traffic flow:
  • Type A configuration: any weaving traffic must change lane at least once.
  • Type B configuration: one of the two weaving movements can be completed without a lane change; the other movement requires at most one lane change.
  • Type C configuration: one weaving movement has a "through" lane without lane change necessary; the other flow must make at least two lane changes.