What is a traffic intersection?
Traffic intersections are the areas where two or more roads meets or converge or diverges. Intersections can be at same level i.e. all the roads are at same height/level/plane whereas in some cases there may be two or more levels i.e. with one over other. The purpose of having an intersection is the continuity in flow of traffic and avoiding deadlock of traffic. Transport engineers work hard on designing new intersections all the time to reduce the conflicting points for a smoother flow of traffic thus saving time. There are number of factors which are kept in mind while designing intersections but still some require control for making and keeping them effective. These intersection control are used depending upon the requirement which is generally volume of traffic and speed of the vehicles.
Intersection control can be of following types:
1. Passive Control
In this type of intersection control the driver is exempted from any control and the actions are dependent on driver’s will. Its upto driver to follow the rules.
As the name suggests no control measure is used, this happens at intersections where traffic is very less. In such cases driver is expected to obey basic rules or driving like driving in their own lane, obey rules as required at time of obtaining licence.
This control measure include use of boards and warning signs. These signs are placed along the roads, on boards, on advertisement boards etc. Driver is expected to read and understand these signs and obey them. Examples include give way control, two-way stop control and all way stop control. Give way control measure directs driver on a minor road to slow down and let the vehicle on major road to proceed.
Traffic signs plus marking
When a combination of traffic signs and markings are used these control measures are considered in this category. Markings include use of lines like stop line marking, yield lines, arrow marking or any other sign/marking on lane.
2. Semi Control
Semi control imposes only some rules or restrictions on driver which he/she is supposed to follow and they do generally follow because of the geometry and shape of control measures. These helps in reducing the conflict points by directing vehicle in a specific lane or direction.
This include use of physical barriers meant to direct the driver on a specified path. This can be done by raising a portion of road a little, by physical road marking, by use of traffic cones, traffic islands etc. These reduces conflict points thus easing the flow of traffic. By use of channelizers merging & diverging points are reduced significantly thus decreasing the chances of accidents.
These form of control are meant to direct the flow of traffic around them in one direction. The basic principle in this type changes the type of conflict points to merging then weaving and then diverging. These are most suited at places where four or more major roads are converging and through conflicts are to be avoided. In most of the cases right or the left curve is allowed as per the rules followed in the particular country.
3. Active Control
This type of intersection control is considered to be most effective and movement of vehicles is governed by the control measures used. There are heavy penalties imposed for violating these control measures. Driver is forced to follow the path as required by the control measure.
Traffic signals are the most widely used type of intersection control all over the world. Traffic signals are required on roads with large volume of traffic which cannot be managed by other measures. It is based on time sharing i.e. the part of intersection is used by different users at different time. Fixed time signals are most popular which provides fixed passage time for traffic coming from different arms. In some cases different phases may be provided and such a traffic signal is caused multiphase signal.
Traffic signals may also be managed actively by a human which is generally traffic police personnel which adjusts the time of green signal as per the requirement.
Grade separated intersections
These types of intersection are further of two types which can be grade separated (where roads meets at different levels) and at grade (where roads meet at same vertical level). These intersections are used when the traffic volume is exceptionally high and the movement of vehicles is not supposed to be restricted or hindered. Speed is given more priority and vehicles change their direction multiple times but usually not stopped. Construction cost of these types of intersection is exceptionally high especially while constructing grade separated intersections.
An intersection layout that permits crossing manoeuvres at different levels is known as grade-separated intersection. This corridor is used as the highest type of treatment for a road intersection.
The choice between at-grade and grade-separated intersection at a particular site depends upon factors such as volume of traffic, economy, safety, delay, etc. Construction of grade-separated junctions is more expansive initially and is justified in certain situations. These are:
- On high type facilities such as expressways, freeways and motorways.
- Certain at-grade intersection, which already had maximum at grade improvement, have reached maximum capacity and where it is not possible to improve capacity further by retaining the at-grade crossing.
- At certain locations which have proven record of bad accident history when functioning at surface crossings.
- At junctions where the traffic volume is heavy and delays and loss caused justify economically the provision of grade separation.
- At certain specific topography situations where it is logical to provide a grade separated structure rather than an at-grade intersection, which may involve considerable earth work or acquisition of land.
Criteria for provision of grade separated junctions in urban areas
Grade separation is to be provided if the estimated traffic volumes within next 5 years are in excess of the capacity of the intersections; when traffic projections show that volume within next 20 years will exceed the capacity of an at-grade intersection, the need for a grade separated facility should be kept in view for future construction.
Impacts of Grade-separated junction
- Grade separated junctions occupy much greater area than ordinary intersections.
- The locations of bus stops at grade-separated intersection are often such that commuters have to walk greater distances for changing bus routes.
- In addition, because of the increase in walking distance and road widths, pedestrians and commuters are exposed to greater delays and higher accident risks. This further discourages use of public transport by children, disabled and other vulnerable road users.
Grade separate junctions may encourage higher speeds, greater use of private vehicles and long trip lengths.