What is intersection on the road?
Intersection is an area shared by two or more roads. This area is designated for the vehicles to turn different directions to reach their desired destinations. Its main function is to guide vehicles to their respective directions.
Traffic intersections are complex locations on any highway. This is because vehicles moving in different directions want to occupy the same space at the same time. In addition, pedestrians also seek the same space for crossing.
Drivers have to make split second decisions at an intersection by considering their route, intersection geometry, speed and direction of other vehicles etc. A small error in judgement can cause severe accidents.
It also causes delay and it depends on type, geometry and types of control. Overall traffic flow depends on the performance of the intersections. It also affects the capacity of the road.
Therefore, both from the accident perspective and the capacity perspective, the study of intersections is very important for the traffic engineers especially in the case of urban scenarios.
Conflicts at an Intersection
Conflicts at an intersection are different for different types of intersection. Consider a typical four-legged intersection as shown in figure. The number of conflicts for competing through movements are 4, while competing right turn and through movements are 8.
The conflicts between right turn traffics are 4, and between left turn and merging traffic is 4. The conflicts created by pedestrians will be 8 taking into account all the four approaches. Diverging traffic also produces about 4 conflicts.
Therefore, a typical four legged intersection has about 32 different types of conflicts. This is shown in figure (a). The essence of the intersection control is to resolve these conflicts at the intersection for the safe and efficient movement of both vehicular traffic and pedestrians.
Two methods of intersection controls are there: time sharing and space sharing. The type of intersection control that has to be adopted depends on the traffic volume, road geometry, cost involved, importance of the road etc.
Conflict in a traffic signal
Types of intersection and their Configuration
Intersection at grade
When the crossing road are on same level then the intersection formed is called intersection at grade.
Requirements of intersection at grade are listed below.
i) Adequate visibility should be available for all vehicles approaching the intersection.
ii) The area of conflict at intersection should be as small as possible.
(iii) Width of pavement and radius for turning should be sufficiently provided.
iv) Relative speed of approaching vehicles and angle of approach of vehicles from all roads should be kept small.
(v) Sudden change of path should be avoided.
(vi) There should be good lighting for night driving.
(vii) There should be proper sign road marking etc on the road approaching intersection to warn the driver.
(viii) If the number of pedestrians and cyclists are large, separate provision, should be made for their safe passage in intersections with high volume of fast moving traffic.
Now,let us talk about 3 types of intersection on the road one by one in detail.
3 types of intersection on the road
I) Unchannelized Intersections
The most common type of intersection is the unchannelized, consisting of the crossing of two roadways at the same elevation connected by radius returns to accommodate the wheel paths of turning vehicle.
Typical characteristics of unchannelized intersections are low turning movements and low overall traffic volumes. The driving through unchannelized intersection is unsafe as there is greater possibility of collision.
II) Channelized intersection
Vehicles approaching an intersection are directed to definite paths by islands, marking etc. and this method of control is called channelization. Channelized intersection provides more safety and efficiency.
It reduces the number of possible conflicts by reducing the area of conflicts available in the carriageway. If no channelizing is provided the driver will have less tendency to reduce the speed while entering the intersection from the carriageway.
If no channelizing is provided the driver will have less tendency to reduce the speed the speed while entering the intersection from the carriageway. The presence of traffic island, markings etc forces the drivers to reduce the speed and become more cautions while maneuvering the intersection.
A channelized island also serves as a refuge for pedestrians and makes pedestrians crossing safer. Channelization of traffic through a three legged intersection (figure-4) and a four legged intersection (Figure-5) is shown in the figure.
Advantages of channelized intersection are given below.
- The vehicles can be confined to definite paths by channelization.
- Points of conflict can be separated.
- Angles of the merging stream can be forced to be at flat angles so as to cause minimum disruption.
- Both major and minor areas of conflict within the intersection can considerably be decreased.
- Speed control devices can be installed to force the vehicle to reduce their speed before entering the intersection.
- Refuse or pedestrian islands can be provided for pedestrians within the intersection area.
- Angle between intersecting streams of traffic may be kept as desired in a favourable way.
III) Rotary Intersection
Rotary intersections are special form of at-grade intersections laid out for the movement of traffic in one direction around a central traffic island. ‘Essentially all the major conflicts at an intersection namely the collision between through and right-turn movements are converted into milder conflicts namely merging and diverging.
The vehicles entering the rotary are gently forced to move in a clockwise direction in orderly fashion. They then weave out of the rotary to the desired direction.
Advantages and disadvantages
The key advantages of a rotary intersection are listed below:
- Traffic flow is regulated to only one direction of movement, thus eliminating severe conflicts between crossing movements.
- All the vehicles entering the rotary are gently forced to reduce the speed and continue to move at slower speed. Thus, none of the vehicles need to be stopped, unlike in a signalized intersection.
- Because of lower speed of negotiation and elimination of severe conflicts, accidents and their severity are much less in rotaries.
- Rotaries are self governing and do not need practically any control by police or traffic signals.
- They are ideally suited for moderate traffic, especially with irregular geometry, or intersections with more than three or four approaches.
Although rotaries offer some distinct advantages, there are few, limitations for rotaries which are listed below,
- All the vehicles are forced to slow down and negotiate the intersection. Therefore, the cumulative delay will be much higher than channelized intersection.
- Even when there is relatively low traffic, the vehicles are forced to reduce their speed.
- Rotaries require large area of relatively flat land making them costly at urban areas.
- The vehicles do not usually stop at a rotary. They accelerate and exit the rotary at relatively high speed. Therefore, they are not suitable when there is high pedestrian movements.
Condition when traffic rotary is justified
Because of the above limitation, rotaries are not suitable for every location. There are few guidelines that help in deciding the suitability of a rotary. They are listed below.
- Rotaries are suitable when the traffic entering from all the four approaches are relatively equal.
- A total volume of about 3000 vehicles per hour can be considered as the upper limiting case and a volume of 500 vehicles per hour is the lower limit.
- A rotary is very beneficial when the proportion of the right-turn traffic is very high; typically if it is more than 30 percent.
Rotaries are suitable when there are more than four approaches or if there is no separate lanes available for right-turn traffic. Rotaries are ideally suited if the intersection geometry is complex.
I hope this article on “3 types of intersection on the road” remains helpful for you.
Happy Learning – Civil Concept