Smart Traffic Light Control System for Urban Traffic Flows

Abstract. In this article, the analysis of existing traffic control systems is performed. The concept of building a traffic light control system is proposed. An algorithm for road regulation is developed.

Keywords: Traffic light, intersection, control, transport detector.

The growth of the car fleet and traffic volume leads to an increase in traffic intensity, that results in a transport problem in cities with historically developed buildings. Traffic delays are increasing, queues and obstructions are forming, which causes a decrease in the speed of communication, unjustified fuel overspending, and increased wear on vehicle components and assemblies.

When implementing traffic control actions, a special role is played by the introduction of technical means: road signs and road marking, traffic lights, road barriers and guide devices.

At the same time, traffic light regulation is one of the main means of ensuring traffic safety at intersections. The number of intersections equipped with traffic lights in the world's largest cities with a high level of motorization is constantly increasing. In some cases it reaches the ratio of one traffic light object for 1.5–2 thousand residents of the city.

Traffic is a complex, time-changing system. The system is a set of relationships between moving and stationary vehicles controlled by people and pedestrian flows. Each object of the system pursues its own specific goals when driving on the road and strives to carry them out with minimal time loss and sufficient degree of safety.

The main point of regulation is to require, prohibit, and recommend certain actions to drivers and pedestrians in the interests of speed and safety. It is carried out using the “Traffic regulations”, a set of technical means, administrative actions of traffic police officers.

There are different ways to control traffic depending on the degree of human participation in the control process, the degree of centralization, and the availability of feedback.

Automatic control of the traffic light object is carried out without the participation of a person according to a pre-set program. Automated control requires the participation of a human operator. The operator, using a set of technical tools to collect the necessary information and find the optimal solution, can adjust the program of automatic devices. Both in the first and in the second cases, computers can be used in the control process. And finally, there is manual control, when the operator, assessing the transport situation visually, exerts a controlling influence which is based on his experience and intuition. The automatic control circuit can be either open or closed.

In a closed circuit, there is feedback between the means and the control object (traffic flow). Automatically, it can be carried out by special devices for collecting information — transport detectors. Information enters automation devices. Based on the results of its processing, these devices determine the operation mode of traffic lights or road signs that can change their value on command (controlled signs). This process is called flexible or adaptive control.

When the circuit is open there is no feedback, traffic light control devices (road controllers) switch signals according to a pre-set program. In this case, strict software control is performed.

According to the degree of centralization, two types of control can be considered: the local and the systemic one. Both types are implemented in the ways described above.

In local control, the signal switching is provided by a road controller located directly at the intersection, while in system control, the intersection controllers usually act as translators of commands received via special communication channels from the control point. If the controllers are temporarily disconnected from the control center, they can also provide local control. In practice, the terms are used: local controllers and system controllers. The first ones do not have a connection with the control point and work independently, the second ones have this connection and are able to implement local and system management.

With local manual control the operator is located directly at the intersection, watching the movement of vehicles and pedestrians. With the systemic one he is located in the control point, i. e. away from the control object.

Local control is most used at separate or isolated intersection that has no connection to neighboring intersections either by control or by traffic flow. Changing traffic signals at such intersection is provided by an individual program, regardless of traffic conditions at neighboring intersections. The arrival of vehicles to this intersection is random.

The organization of coordinated change of signals at a group of intersections, carried out in order to reduce the time of vehicles movement in the specified area, is called coordinated control (on the principle of “green wave” — GW). In this case, system control is usually used.

This article proposes an adaptive control algorithm that implies coordinated automatic traffic control with feedback. The algorithm is based on analyzing the distance between cars arriving at the intersection. The distance is calculated using the time between the signals received from the transport detector about the passage of two cars. Their speed is assumed equal to the maximum allowed speed for this section. In this case, it is sufficient to use a single recording transport detector. The transport detector is used to determine the decrease in the flow density. It is useful when developing a network of connected intersections, because the algorithm replaces complex mathematical calculations and provides a variable phase shift.

Interactions between traffic detectors and traffic lights occur as follows:
1. The maximum (T_max) and the minimum (T_min) periods of the traffic light cycle and the phase shift time (T_shift) are set.
2. The maximum interval between cars (x_int) is set.
3. At each step, the distance between arriving vehicles is checked.
4. At the step when the traffic signal switching is coming, the following condition is checked:

x_i ≤ x_int ⇒ T_i = T_i + S_shift;
x_i > x_int = T_i = 0.

If T_i = 0 the traffic light switches to red.

This algorithm considers only clusters of cars arriving at the intersection. If the flow turns out to be homogeneous and there are no obvious breaks, the adaptive algorithm will switch to the fixed control mode.

Using the algorithm described above will significantly improve the road situation, namely:
— it will minimize traffic jams;
— it will increase the average speed of traffic flow;
— it will reduce fuel consumption and emissions to the environment.

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