Grade separation

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(Redirected from
Grade separated
)
An example of the potential complexity of grade separation, seen in the Jane Byrne Interchange in Chicago
Seven various overpasses for grade separation in Spain near Barcelona
Xiaoshan
, China
The concept of grade separation includes all transport modes, such as a simple pedestrian bridge over rail tracks.

In

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

, 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

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

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

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.

Leipzig-Halle Airport
- a type of grade separation.

Types

Fully separated

These junctions connect two freeways:

UK
.
  • 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:

Weaving

An example of weaving, where traffic drives on the left. The blue car entering the grade-separated road, and both the red and blue car exiting must both change lanes in the short distance provided.

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 (for example, the cloverleaf interchange), 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 200 kilometres per hour, 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[3]
to separate entering and exiting traffic.

Railways

In general

In railway construction, grade separation also means the avoidance of

street cars
.

Flying junction

Attempts have been made to increase the capacity of railways by making tracks cross in a grade-separated manner, as opposed to the traditional use of flat crossings to change tracks. A grade-separated rail interchange is known as a flying junction and one which is not a level junction.

In 1897, the

Southern Railway
later made extensive use of flying junctions on other parts of its busy former LSWR main line.

Today in Britain, the tightly grouped nest of flying junctions[4] to the north of Clapham Junction railway station—although technically a combination of many junctions—handle more than 4,000 trains per day (about one train every 15 seconds).

Virtually all major railway lines no longer cross (forming an 'X' shape) at flat level (although many diverge - i.e. 'Y' shape).

High-speed railways (200 km/h or 120 mph+)

On almost all

high-speed railway lines, the faster speed requires grade separation. Therefore, many high speed lines are elevated, especially in Taiwan and Japan
, where population density alongside high speed lines is higher than in France, Italy or Germany.

In the United States, a flying junction on the

.

In what is known as

former Soviet Union and other regions using the same gauge, the most complicated grade-separation railpoint is found at Liubotyn in Ukraine
.

Footbridges and subways

underpasses may be called tunnels
.

References

  1. ^ City of Eureka Municipal Code 71.85 Archived 2012-02-12 at the Wayback Machine (California, US)
  2. ^ Henry K. Evans (1950). "Read the ebook Traffic engineering handbook by Institute of Traffic Engineers". ENGINEERING HANDBOOK, Second Edition 1950. New Haven, Connecticut: Institute of Traffic Engineers. Archived from the original on 2018-10-13. Retrieved 2010-10-09.
  3. ^ Texas Department of Transportation. "Braided Ramp". TxDOT Visual Dictionary. Archived from the original on 2020-03-06. Retrieved 2020-05-10.
  4. ^ OpenStreetMap Archived 2011-02-23 at the Wayback Machine