An intersection is the most dangerous place on any road, and the reason is simple arithmetic. On a straight road, vehicles travel in parallel and rarely meet. At a junction, streams cross each other — and every crossing point is a place where two vehicles can occupy the same spot at the same moment.
Intersection design is the business of reducing those conflict points, or removing them entirely. This post covers the two main types, traffic islands, and the difference between channelized and unchannelized junctions.
The Two Main Types
- At-grade intersection
- Grade-separated intersection
The distinction is about level. At-grade junctions bring roads together at the same height; grade-separated junctions keep them at different heights so they never actually meet.
At-Grade Intersections
All road intersections which meet at about the same level, allowing traffic manoeuvres like merging, diverging, crossing and weaving, are called intersections at grade.
The Four Manoeuvres
| Manoeuvre | What Happens | Risk Level |
|---|---|---|
| Diverging | A vehicle leaves one stream to join another | Lowest |
| Merging | A vehicle joins an existing stream | Low |
| Weaving | Merging and diverging combined over a length of road | Moderate |
| Crossing | One stream cuts directly across another | Highest |
Crossing is the dangerous one, and understanding why explains almost everything that follows in this chapter. In merging and diverging, vehicles move in broadly the same direction, so the relative speed between them is small. In crossing, paths meet at a large angle and the impact is a side collision at the full speed of both vehicles.
Nearly every intersection improvement you will study is an attempt to convert crossings into merges.
Three Sub-Types
At-grade intersections are further classified as:
- Unchannelized
- Channelized
- Rotary intersection
Grade-Separated Intersections
Intersection at grade is eliminated by using grade-separation structures that permit the cross flow of traffic at different levels without interruptions.
The phrase “without interruptions” is the whole point. Since the roads never occupy the same level, no vehicle ever has to stop for another. Conflict is not managed — it is removed.
Definitions to Know
| Term | Definition |
|---|---|
| Grade separation | A crossing of two highways, or a highway and a railroad, or a pedestrian walkway and a highway, at different levels |
| Overpass | A highway passing over an intersecting street, railroad or pedestrian facility |
| Underpass | A highway passing under an intersecting street, railroad or pedestrian facility |
Overpass and underpass describe the same structure from opposite viewpoints. Whether a crossing is called one or the other depends on which road you are asking about. Read the question carefully — it will specify the road in question.
Traffic Islands
Traffic islands are raised areas constructed within the roadway to establish physical channels through which vehicular traffic may be guided. Traffic islands often serve more than one function.
The word raised matters. A painted line asks for cooperation; a raised island physically prevents a vehicle from going where it should not. That is a far stronger form of control.
The Four Types of Traffic Island
- Divisional islands
- Channelizing islands
- Pedestrian loading islands
- Rotary
1. Divisional Islands
- Intended to separate opposing flows of traffic on a highway with four or more lanes.
- By dividing the highway into two one-way roadways, head-on collisions are eliminated and other accidents are generally reduced too.
- The width should be large if headlight glare is to be reduced during night driving.
- The kerb should be high enough to prevent vehicles entering the islands.
Note the two design requirements pulling in different directions: width is governed by night-time glare, while kerb height is governed by keeping vehicles out.
2. Channelizing Islands
- Used to guide traffic into the proper channel through the intersection area.
- Very useful as traffic control devices for intersections at grade, particularly when the area is large.
Why does a large area make them more valuable? Because a wide expanse of open pavement gives drivers no guidance at all. Everyone chooses their own path, and those paths cross unpredictably. Islands impose order on space that would otherwise be a free-for-all.
Five Uses of Properly Designed Channelizing Islands
- The area of possible conflicts between traffic streams is reduced. Introducing channelizing islands reduces both the major and minor conflict areas.
- They establish the desired angles of crossing and merging of traffic streams.
- They are useful when the direction of flow is to be changed.
- They serve as convenient locations for other traffic control devices.
- They serve as refuge islands for pedestrians.
3. Pedestrian Loading Islands
- Provided at regular bus stops and similar places for the protection of passengers.
- A pedestrian island at or near a cross walk, to aid and protect pedestrians crossing the carriageway, is termed a pedestrian refuge island.
- For crossing multilane highways, a refuge island after two or three lanes is desirable.
The refuge island turns one long, risky crossing into two short ones. A pedestrian deals with traffic from a single direction at a time and can wait safely in between — which matters enormously for the elderly, for children, and for anyone carrying a load.
4. Rotary Island
- The large central island of a rotary intersection.
- Much larger than the central island of a channelized intersection.
- The crossing manoeuvre is converted to weaving by providing sufficient weaving length.
There it is again — the recurring theme of this chapter. The rotary’s entire purpose is to take the dangerous crossing manoeuvre and turn it into weaving, which is far safer.
Unchannelized Intersections
The intersection area is paved and there is absolutely no restriction on vehicles using any part of the intersection area.
- Also called all-paved intersections.
- They are the lowest class of intersection and the easiest to design.
- When no additional pavement width is provided for turning movements, it is a plain intersection.
- When the pavement is widened at the intersection area by a traffic lane or more, it is a flared intersection.
The Problem With Them
Because the paths of turning vehicles are not restricted or controlled, the conflict area is quite large. One of the crossing vehicles has to stop while the other proceeds.
An unchannelized junction is essentially a paved rectangle where drivers negotiate among themselves. It is cheap and simple, which is why it remains common on low-volume roads — but it offers no help to the driver at all.
Channelized Intersections
Channelization is achieved by introducing islands into the intersection area, thus reducing the total conflict area available in the unchannelized intersection.
- The radius of the entrance and exit curves and the area are suitably designed to accommodate channelizing islands of proper size and shape.
- Channelization may be partial or complete, using divisional and directional islands and medians.
- From a traffic operation point of view, there is better control on traffic entering and leaving the intersection.
- Channelized intersections are therefore considered superior to the all-paved types.
Seven Advantages of Channelized Intersections
- Vehicles can be confined to definite paths.
- The angle of merging streams can be forced to be at flat angles.
- Both major and minor conflict areas within the intersection can be decreased.
- The angle between intersecting streams may be kept as desired in a favourable way.
- Speed control can be established over vehicles entering the intersection.
- Points of conflict can be separated.
- Refuge islands can be provided for pedestrians.
Why Flat Merging Angles Matter
Advantage 2 is worth dwelling on, because it explains the physics behind channelization.
When two streams merge at a flat angle, the vehicles are travelling in nearly the same direction, so the relative speed between them is small. Any contact is a glancing one. When they merge at a sharp angle, the relative speed is high and a collision becomes a serious side impact.
Islands force vehicles to approach at the angle the designer chooses, rather than whatever angle the driver happens to pick. Advantage 6 works alongside this — separating conflict points means a driver deals with one hazard at a time instead of several at once.
All Types Compared
| Type | Conflict Handling | Cost and Complexity |
|---|---|---|
| Unchannelized (plain) | Large conflict area, no control | Lowest |
| Unchannelized (flared) | Extra width for turning, still uncontrolled | Low |
| Channelized | Islands reduce and separate conflict points | Moderate |
| Rotary | Crossing converted to weaving | Needs large area |
| Grade-separated | Conflict eliminated entirely | Highest |
Quick Revision Notes
- Two main types: at-grade and grade-separated.
- At-grade allows merging, diverging, crossing and weaving; crossing is the most dangerous.
- At-grade sub-types: unchannelized, channelized, rotary.
- Overpass = highway passing over; underpass = highway passing under.
- Traffic islands are raised areas establishing physical channels.
- Four island types: divisional, channelizing, pedestrian loading, rotary.
- Divisional islands are for highways with four or more lanes and eliminate head-on collisions.
- Pedestrian refuge islands are desirable after two or three lanes on multilane highways.
- Unchannelized = all-paved, lowest class, easiest to design. Plain = no extra width; flared = widened by a lane or more.
- Channelized intersections are superior to all-paved types and have seven advantages.
Mistakes Students Commonly Make
- Confusing plain and flared intersections. Flared means the pavement is widened at the junction.
- Treating overpass and underpass as different structures. They are the same crossing described from different roads.
- Forgetting the rotary as one of the four island types.
- Saying divisional islands are used on two-lane roads. They are for four or more lanes.
- Missing that channelization reduces both major and minor conflict areas.
- Listing fewer than seven advantages of channelized intersections.
- Overlooking that merging at flat angles is what makes channelization safe.
Conclusion
Every intersection type in this chapter is an answer to the same question: what do we do about crossing conflicts? The unchannelized junction does nothing and leaves drivers to negotiate. The channelized junction uses raised islands to confine paths, flatten merging angles and separate conflict points. The rotary goes further and converts crossings into weaving altogether. And the grade-separated intersection removes the conflict completely by keeping the roads at different levels. Cost rises down that list, and so does safety.
Frequently Asked Questions
What are the two main types of intersection?
At-grade intersections and grade-separated intersections.
What is an at-grade intersection?
An intersection where roads meet at about the same level, allowing manoeuvres such as merging, diverging, crossing and weaving.
What are the sub-types of at-grade intersection?
Unchannelized, channelized and rotary intersections.
What is the difference between an overpass and an underpass?
An overpass is a highway passing over an intersecting street, railroad or pedestrian facility. An underpass is a highway passing under one. They describe the same grade separation from the viewpoint of different roads.
What are traffic islands?
Raised areas constructed within the roadway to establish physical channels through which vehicular traffic may be guided. They often serve more than one function.
What are the four types of traffic island?
Divisional islands, channelizing islands, pedestrian loading islands, and the rotary.
What is the difference between a plain and a flared intersection?
In a plain intersection no additional pavement width is provided for turning movements. In a flared intersection the pavement is widened at the intersection area by a traffic lane or more.
Why are channelized intersections superior to all-paved ones?
Because islands reduce the total conflict area and give better control over traffic entering and leaving the intersection, confining vehicles to definite paths.
What are the advantages of channelized intersections?
Vehicles are confined to definite paths, merging angles can be forced flat, both major and minor conflict areas are decreased, angles between intersecting streams can be set favourably, speed control is possible at entry, conflict points can be separated, and refuge islands can be provided for pedestrians.
Why is crossing the most dangerous manoeuvre?
Because the streams meet at a large angle, so the relative speed is high and any collision is a severe side impact. Merging and diverging involve vehicles moving in broadly the same direction at low relative speed.