What is an Intersection?

An intersection is a location where two or more roads or railways meet or join. The traffic movement is controlled at the intersection that depends on its design and type. It governs the operation, efficiency, speed, capacity, and safety of the highways, airways, and railways. The movement of the pedestrian causes hazards and delays at the intersection.

Types of Intersections

Depending on the grades of the roads, the intersections are of two categories

1. Intersection at grade
2. Grade Separated Intersections

Intersection at grade

This type of intersection is located where all the roads meet at more or fewer level grades. It involves merging, divergent and crossing traffic. The two factors that affect the design of intersections at grades are the relative speed and maneuver areas.
The intersection at grade involves the various categories-

1. Channelized Intersection

At this intersection, to channelize the moving traffic into suitable directions, islands are introduced. The island helps in reducing the conflict areas and helps in the smooth functioning of the traffic. The channelized intersection turns the traffic into a suitable direction, controls the angle of approach from the different paths, and reduces the relative speed of the moving vehicle. The only disadvantage is that the traffic volumes at the intersection increase. 

It serves the following advantages: 

• The major and the minor conflict areas decrease.
• The speed of the moving vehicles reduces. Thus, reducing the chances of conflicts and accidents. 
• The points of conflict of the vehicles from the different paths reduce. 
• It serves best for the proper installation of traffic signs and following the traffic rules. 

2. Unchannelized Intersection

It is the location of the intersection where there is no restriction to the vehicles for their movement. The vehicles use any path of the road. It is easy to construct but serves the most complex situations to control traffic movements. It is suitable for low traffic flow with proper traffic signs and signals for high traffic flow. It can be a plain intersection that provides no extra pavement for the turning movement of the vehicle. It can be a flared intersection that provides an extra pavement for the turning movement of the vehicles. 

3. Rotary Intersection

It is the location of the intersection where the moving traffic is allowed to move around a large central island in the same direction before they split into different directions radiating from the central area. It helps in reducing the crossing conflicts and the stopping of the vehicles for crossing. The journey of the travelers becomes smooth and comfortable. It can work smoothly without any traffic signals and signs. It reduces the chances of conflicts and accidents. Such an intersection has a capacity of 3,000 vehicles per hour to enter and depart at their paths.

The disadvantage of the rotary intersection is that it requires big space for construction and is costly for low traffic flow. It cannot control the traffic flow with more pedestrian traffic and mixed traffic of cyclists, pedestrians, and vehicles.

Design of Rotary Intersection

• Design Speed: When a vehicle enters a rotary intersection, its speed reduces compared to its design speed. The design speed for traffic rotaries in the rural area is taken as 40 kmph and in the urban areas as 30 kmph in India. 
• The shape of the central island: The shape depends on the layout of the roads. The shape can be elliptical, turbine, tangent, and circular depending on the traffic conditions and limitations. The circular rotary is suitable for the two roads that are crossing at right angles. The turbine-shaped rotary helps in reducing the speed of the moving vehicles while entering and exiting the rotary. 
• The radius of the central island and rotary roadway: In the rotary intersection design, neglect the value of superelevation and consider the friction factor ‘f’ only. The minimum radius of the curve is given by the relation- $R=\frac{V^2}{127f}$, where V is the design speed of the vehicle in kmph. According to the IRC Standards, the radius of the entry curve should be 20 – 35 m for the design speed of 40 kmph and 15 – 25 m for the design speed of 30 kmph. The minimum radius of the central island should be 1.33 times the radius of the entry curve.
• Weaving angle and Weaving distance: The weaving angle should not be less than 150 for smooth traffic flow. The weaving length should be 45 – 90 m for the design speed of 40 kmph and 30 - 60 m for the design speed of 30 kmph. 
• Width of the carriageway at entry and exit: According to the IRC, the width of the carriageway at entry and exit should be 6.5, 7.0, 8.0, and 13.0 for the radius 25 – 35 m. The width of the carriageway should be 7.0, 7.5, 10, and 15 m for the radius of the entry curve 15 – 25 m for the multilane rotaries. 
• Width of Rotary: The width of the non-weaving section of the rotary should be equal to the widest single entry of the rotary and less than the width of the weaving section. The width of the weaving section of the rotary should be one traffic lane wider than the mean width of the entry and non-weaving section. 
• The capacity of the rotary: It depends on the minimum capacity of the individual section. It can be calculated by using the equation -$Q_P=\frac{280W(1+{\displaystyle \frac{e}{W}})(1-{\displaystyle \frac{P}{3}})}{(1+{\displaystyle \frac{W}{L}})}$
   

Where

$Q_P$= Practical Capacity of rotary

W = Width of the weaving section

e = Average width of the entry and non-weaving section

p = Proportion of the weaving traffic

L = Length of the weaving section

4. Signalized Intersection

At this intersection, the moving traffic and pedestrian movement are controlled by traffic signs and symbols. The automatic traffic signals are used at this intersection that regulates the stop and goes of the traffic and helps to reduce the conflicts and accidents by time separation. It also helps to minimize the delay of the movements of the vehicles and is economically less in cost. It can provide the highest capacity at the intersection if used with proper planning and advanced signals.

Grade separated intersections

It is the intersection where the roads are located at different levels and are separated by a bridge to avoid conflict. Interchange facilities enhance the transfer of paths and turning. The grade separations can be a fly-over or under-pass. 

The advantages of the grade-separated intersections are-

• The traffic flow is smooth and uninterrupted.
• The comfort and smooth journey of the travelers are maintained that saves their time and fuel.
• Grade separators enhance the safety of the traffic flow for turning and interchange of the routes. 

The disadvantages of the grade-separated intersections are-

• The cost of construction and operation is high.
• Its construction is difficult and undesirable where there is unfavorable topography and right of way.
• They result in undesirable crests and sags in the flat or plain terrain.

Context and application

The intersection plays an important geometric feature in the design of highways. The intersection regulates the traffic flow by directing them into different paths and different directions. This helps to control the capacity of the traffic flow and the efficiency of the road system. The correct design of intersections affects the safety, speed, and cost of construction and design of the highways.

This topic is studied in courses like-

• Bachelors of Technology (Transportation Engineering )
• Masters of Technology (Transportation Engineering)

Practice Problems

Q 1. Which of the following statements is correct regarding the well-designed signalized intersection?

1. The cycle time is the sum of red phase and yellow phase.
2. The cycle time is the sum of red phase and green phase.
3. The total delay time is minimized.
4. The conflicts at the intersection point are increased. 

Answer- Option c

Explanation- The well-designed signalized intersection minimizes the total delay time.

Q 2. What should be the radius of the entry curve when the vehicle is moving at a speed of 40 kmph?

1. 45 m
2. 42 m
3. 38 m
4. 28 m

Answer- Option d

Explanation- According to the IRC Standards, the radius of entry curve should be 20 – 35 m for the design speed of 40 kmph.

Q 3. What is the drawback of the channelized intersection?

1. The major and the minor conflict areas are considerably increased.
2. The speed of the moving vehicles is reduced.
3. The points of conflicts of the vehicles from the different paths are increased.
4. The traffic volumes at the intersection increases. 

Answer– Option d

Explanation– The traffic volumes at the intersection are increased in the channelized intersection.

Q 4. Which is the effective shape of the rotary to reduce the speed of the vehicle when it is entering?

1. Turbine shaped
2. Tangent shaped
3. Circular shaped
4. Elliptical shaped

Answer- Option a

Explanation- The Turbine Rotary helps in the reduction of speeds of vehicles while entering the rotary. 

Q 5. Which of the following is not a type of intersection at grade?

1. Unchannelized intersection
2. Under-pass interchange
3. Channelized intersection
4. Signalized intersection

Answer- Option b

Explanation- Under-pass interchange is a type of grade separated intersection.