Types of highway intersections | At grade intersection | Grade separated intersection

 Highway intersections

When two or more roads intersect or cross, road users must adjust their movement to prevent a 'collision;' this adjustment could be in the form of a speed reduction or a change in the path of their motion.

"The broad region where two or more roadways join or cross, within which are included the carriageway and roadside design characteristics that support orderly traffic flows in that area," according to the definition of an intersection. "That segment of any one of the roadways radiating from an intersection that is outside of the region of the intersection proper," says an intersection leg.

The driver of an approaching vehicle must execute particular traffic manoeuvres in the intersection area depending on the direction or intersection leg in which he or she wishes to move.

The area shared by the joining or crossing of two or more roadways is known as a junction. Because the primary purpose of a crossroads is to allow a road user to choose a route, it is referred to as a decision point. As a result, the issues that motorists face when going through an intersection must be identified, and the design should be such that the driving task is as simple as possible.

Aside from being a point of decision, intersections are also a source of a huge number of important conflicts. These collisions could be caused by vehicles travelling in opposite directions meeting paths. The magnitude and nature of conflicts are minimised, and the driving route selection process is simplified, resulting in good intersection design.

Intersection Classification

Intersections are divided into two categories based on how they handle crossing conflicts:

1.       At Grade Intersection

2.       Grade Separated Intersection.

 

1.     At Grade Intersection

It is a bridge that, by vertically separating roadways in space, eliminates crossing issues at intersections. Interchanges are a type of grade separated intersection. Grade separated junctions are safer and less time consuming than grade intersections. Interchange structures consisting of ramps facilitate route transfer at grade separations. Direct, Semi-Direct, and Indirect interchange ramps are the three types of interchange ramps. The patterns of the different turning lanes or ramps are used to characterise interchanges. The interchange layouts are designed to suit the traffic requirements of flow, operation on the crossing facilities, physical requirements of the topography, adjacent land use, kind of controls, right-of-way, and movement direction while being economically viable.

By merging, diverging, and weaving at low relative speeds, grade separated crossings can eliminate all grade crossing issues and accommodate additional intersecting operations. The relative speed of the opposing vehicle streams has a significant impact on the severity of a conflict. There are two advantages to allowing for low relative speed. First, events unfold more slowly, giving you more time to make decisions; second, in the event of an impact, the amount relative energy to be absorbed is lower, resulting in less harm. Furthermore, when relative speed is low, the normal motorist will accept a shorter time gap between vehicles to make his move. The capacity of the road is increased as a result of this condition.

2.     Grade Separated Intersection Classification

The difference between a directed and a non-directional interchange is one of the distinctions made in interchange types. Ramps that tend to follow the natural direction of travel are known as directional interchanges. Non-directional interchanges necessitate a deviation from the natural flow of traffic. There has yet to be devised a complete classification scheme for grade separated intersection design that incorporates all potential geometric patterns. Each of the primary interchange types' design and operating characteristics are listed below and described in the sections that follow.

·         Underpass

·         Overpass

·         Trumpet Interchange

·         Diamond Interchange

·         Cloverleaf Interchange

·         Partial Cloverleaf Interchange

·         Directional Interchange

·         Bridged Rotary

 

1.     Underpass

An underpass or tunnel is a completely enclosed underground route with just openings for admission and egress, usually at either end. A tunnel can be used for pedestrian or vehicular traffic, as well as rail transit. A Subway is a pedestrian and/or cyclist-only underpass built beneath a road or railway to allow them to safely cross to the other side. When pedestrians must cross a railroad or a limited-access highway, these structures are built. Subways could be built for the benefit of wildlife as well.

2.     Overpass

An overpass, also known as a flyover, is a bridge, road, railway, or other similar structure that spans the gap between two roads or railways. A pedestrian overpass allows pedestrians to cross congested highways safely and without causing car congestion. As a safer option, railway overpasses are utilised to replace at-grade crossings. Overpasses separate rail traffic from automobile and pedestrian traffic, allowing for uninterrupted rail traffic movement. Many overpasses make comprise a stack interchange.

3.     Trumpet Interchange

Where one route ends and another begins, trumpet interchanges have been employed. These have at least one loop ramp that connects traffic entering or exiting the terminating expressway with the continuous highway's far lanes. These interchanges are excellent for both highways and toll roads because they condense all incoming and exiting traffic into a single stretch of road where toll booths may be placed. Trumpets are appropriate in areas where the side road is just on one side of the motorway and traffic is light. At each end of each entrance and exit are acceleration or deceleration lanes.

It only requires one bridge and is the most common method of dividing a three-way intersection by grade. The main benefits are low construction costs and suitability for motorways and toll roads. However, using trumpet interchanges has drawbacks, such as leaving a redundant stretch of land within the loop and making it difficult to navigate for those travelling in the loop's direction. Furthermore, reducing the interchange's capacity typically leads to more dangerous traffic congestion caused by articulated lorries that have toppled over.

4.     Diamond Interchange

The diamond interchange is the most basic type of grade separated intersection. A bridge structure eliminates problems between through and crossing traffic. This crossroads features four one-way ramps that run parallel to the primary thoroughfare. By removing the conflict with traffic moving in the other direction, left turn crossing movement issues are greatly decreased. At the end of each ramp, all remaining left turn conflicts, merging and diverging manoeuvre conflicts take place. The operation of these terminals determines the design's limitations in applicability. As a result, it is appropriate for sites with a low number of left turn traffic.

The diamond interchange takes up the least amount of space and is cost-effective to build. A diamond interchange also involves less out-of-the-way traffic and has lower vehicle operating expenses than most other interchange types. The ease of signing is aided by the single point of exit from the major highway. This interchange uses the least amount of right-of-way. Diamonds appear to be the optimal answer to an intersection problem because of these benefits. However, there is a risk of conflicts arising where ramps meet a grade separated cross street, which should be considered in the case of significant ramp volumes. Signal timings at cross intersections may be improperly designed, resulting in insufficient capacity for particular flows.

5.     Cloverleaf Interchange

By utilising weaving portions, the whole clover interchange removes all crossing movement issues. Cloverleaf pattern is incomplete without this part of weaving. It replaces a crossing conflict with a merging, which is followed by a diverging conflict a short distance away. Each through roadway has two places of entry and exit. Before the crossroad construction allows right turn movements, there is a first exit. The second exit, located directly after the crossroads structure, allows for left-hand turns. Between the structure's exit and entry points, a weaving portion is built. There must be enough length and capacity to allow for a seamless merging and diverging operation.

Only one bridge is required in the cloverleaf design. In this regard, it is the cheapest option, as it eliminates all grade crossing manoeuvres. Although full cloverleaf interchanges eliminate the inconvenient crossing movements of diamond interchanges, they have the drawbacks of longer travel distances, higher operating costs, difficult merging sections, circuity of travel, large areas for loops, sight distances to exits on the other side of the bridge, confusion caused by turning right to go left, and large rights-of-way caused by the radius requirements necessary for satisfactory ramp speeds.

The cloverleaf with collector-distributor roads is a variant of the cloverleaf arrangement. The major roadway operations for the collector-distributor lane are similar to those for the diamond interchange. There is a single exit point and a single entrance point for each direction of travel. The collector-distributor road can be constructed to support bigger relative speed disparities or encourage smaller ones, and it can handle speed changes, precise exit directional signage, and the storage and weaving issues that come with a cloverleaf. Despite the fact that this layout improves the operational characteristics of a cloverleaf interchange, the disadvantages of longer travel distances and the need for additional right-of-way remain.

At intersections between a freeway and an expressway, where a diamond interchange would not properly accommodate traffic demand, a cloverleaf with collector-distributor routes is appropriate.

6.     Partial Cloverleaf Interchange

This is a different take on the cloverleaf pattern. A partial clover leaf, also known as a parclo, is a modification that combines some parts of a diamond interchange with one or more cloverleaf loops to eliminate only the most significant turning conflicts. This is the busiest freeway-to-arterial interchange in the city. Parclo is typically used when crossing roadways on a minor route does not pose an unacceptable level of risk or delay. On the freeway, it provides additional acceleration and deceleration room.

7.     Directional Interchange

Direct channels for left turns are provided by a Directional interchange. There are ramps for one or more direct or semi-direct left turning actions at these interchanges. Direct ramps, which have higher speeds and capacities than loop ramps, are typically used at the intersections of two highways or interchanges with one or more very heavy turning motions. Some designers dislike merging traffic entering from the left lane, which is typical of most direct-connection bridges. When compared to diamond interchanges and, in some situations, cloverleaf interchanges, the main disadvantages of this style of interchange are the greater construction costs and the demand for a relatively big quantity of land.

For some settings, various combinations of directed, semi-directional, and loop ramps may be appropriate. They are the most basic patterns that take up the least amount of area, have the fewest or simplest structures, reduce internal weaving, and are suitable for most terrain and traffic circumstances.

 

 

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