INTRODUCTION

The traffic light, also known as traffic signal, stop light, traffic lamp, stop-and-go lights, robot or semaphore, is a signalling device positioned at aroad intersections. It use as the road signal for directing vehicular traffic by means of coloured lights. In most countries, the sequence is red (stop), green (go), yellow (prepare to stop). In some cases, traffic signals also indicate to drivers when they may make a turn.

     These signals may be operated manually or by a simple timer which allows traffic to flow on one roadway for a fixed period of time, and then on the other roadway for another fixed period of time before repeating the cycle. Other signals may be operated by sophisticated electronic controllers that sense the time of day and flow of traffic to continually adjust the sequence of operation of the signals.

    A modern traffic signal system consists of three basic subsystems: the signal lights in their housing, the supporting arms or poles, and the electric controller. The signal lights and housing are known as the signal light stack. A single stack usually consists of three lights : a green light on the bottom to indicate the traffic may proceeds, a yellow light in the middle to warn traffic to slow and prepare to stop, and a red light on the top to indicate the traffic must stop.

    Traffic light system controller gives sign of direction and warning, or it establishes a set of rule and instructions to road users to follow to avoid any road accident. Flexible traffic light controller is needed for maximizing the flow rate of vehicle at the junction. Development of flexible traffic light using verilog HDL is in two mode: normal mode that control the green light timer using switch, flex mode is using line sensor if sensor detect car on lane green timer use the value from switch and turn to red light  if detect no car.