Grade separation
In
In North America, a grade-separated junction may be referred to as a grade separation
Effects
Advantages
Roads with grade separation generally allow traffic to move freely, with fewer interruptions, and at higher overall speeds; this is why
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
Typically, large
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
Weaving
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
Railways
In general
In railway construction, grade separation also means the avoidance of
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
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
In the United States, a flying junction on the
In what is known as
Footbridges and subways
-
TheBP bridge in Chicagowas built for pedestrians.
-
Nineteenth-century pedestrian underpass in Central Park
References
- ^ City of Eureka Municipal Code 71.85 Archived 2012-02-12 at the Wayback Machine (California, US)
- ^ 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.
- ^ Texas Department of Transportation. "Braided Ramp". TxDOT Visual Dictionary. Archived from the original on 2020-03-06. Retrieved 2020-05-10.
- ^ OpenStreetMap Archived 2011-02-23 at the Wayback Machine