Traffic signal light

Friday, 12 January 2007

A traffic signal light is used to control vehicles and passengers so that traffic can flow smoothly and both drivers and passengers are safe. Traffic signal lights have been around for years and are used to efficiently control traffic through intersections. Although traffic signal lights are relatively simple and commonplace, they are critical for ensuring the safety of the driving public. The growing use of traffic lights attests to their effectiveness in directing traffic flow, reducing the incidence of accidents, and most recently to their utility in controlling the flow of traffic through large metropolitan areas when used in conjunction with computer driven systems. A traffic light is an important feature in traffic control today. Not only does the traffic light provide an efficient means of controlling traffic flow, it adds greatly the safety of drivers on the road. Traffic signal lights improve road safety and reduce congestion by providing for the orderly and predictable movement of traffic through intersections. Traffic control lights are provided for traffic control on streets and highways, especially at intersections. The traffic signals are cyclically displayed through a suitable timing and control mechanism. Such traffic control lamps are usually provided with green, red or amber lenses or sometimes with lenses having arrows to indicate direction. Typical traffic lights operate using at least red and green light phases, with traffic required to stop when the light is red, and permitted to pass through the intersection when the light is green. A yellow light phase may further be used to indicate that the light will change to red shortly. Driving through a red light without justification may be a citationable traffic offense.

Traffic signal lights are generally comprised of an enclosure that houses green, amber and red lights. The switching mechanism for controlling the activation of the lights may be located within the enclosure or at a remote site. Traffic lights have been used since the early 1900's to control vehicular traffic flow by advising drivers when and where to stop, proceed, turn, etc. Modern traffic light is no different today than it was decades ago. The basic light still employs red, yellow, and green filters over lamps that are sequentially turned on and off by an electromechanical timing switch or by more modern solid state traffic controllers. A regular traffic signal light is generally comprised of a reflective lamp shade, a light bulb mounted inside the lamp shade, and a lens covered on the front open side of the lamp shade. Traffic light assemblies are arranged in multiple groupings about central vertical post means. Typically, each traffic light assembly comprises a red stop light, a green "go" light, and a yellow caution light. The three lights of each assembly are arranged in vertical or horizontal array. Each multiple group of lights is commonly located at a traffic intersection so that one set of lights faces each oncoming stream of vehicular traffic. In the case of a four-way traffic intersection, each traffic signal system will include four sets of traffic signals facing the four directions of oncoming traffic. Three-way traffic intersections and two-way intersections will have three sets of signals and two sets of signals, respectively. As the world becomes a more crowded and busy place, there are an increasing number of automobiles, trucks, buses and other vehicles on the road. Very early in the development of our roadway system, the traffic light was developed to control the flow of traffic at intersections. The earliest traffic lights were simply controlled by timers, each light was on for an allotted period of time within a cycle which repeated over and over. Some level of sophistication was added when the traffic patterns at a particular intersection were studied at the timers, no computer controlled, varying the timing of the traffic lights according to the predicted average traffic load for different times of the day. Modern traffic control signals, or traffic lights, are commonly controlled by a computer based controller. Modern traffic signal systems include two major components: the controller and the display (lights). The technology of modern traffic signal controllers is quite evolved and includes modern computer technology which incorporates traffic flow sensors, timers, and the like. Many traffic intersections have a predetermined timing mechanism that sets a time for the traffic light's green light element, yellow light element and red light element to be activated. The various traffic lights at the intersection are coordinated by the timing mechanism to control traffic flow. Some traffic light systems have a variable timing mechanism that is responsive to the actual flow of traffic. These systems are far superior in terms of controlling traffic flow since the often inaccurate estimation of anticipated traffic flow can be eliminated and the actual traffic flow can trigger the timing of the traffic lights.

A conventional traffic light employs red, yellow and green filters over incandescent bulbs that are sequentially turned on and off by an electromechanical timing switch or by solid state controllers. Each bulb is electrically coupled to a conventional power source, such as the conventional power distribution network. A considerable cost factor with such traffic signal installations with conventional incandescent lamps is the relatively often required replacement of these lamps. Traffic lights using this technology have many drawbacks: the bulbs power consumption is high (each being between 100 W and 150 W), thus increasing the operation costs, the bulb lifetime is short and decreases with environmental conditions. For example, a light bulb's lifetime decreases with vibrations and temperature. These problems are of special concern because most traffic signals operate continuously. Another drawback with conventional traffic signals is that the masked lenses used to create the illuminated symbol images waste energy because they block light produced by the lamp in order to form the images. Therefore, a significant amount of light from the lamps is wasted as heat absorbed by the lenses instead of light being projected out to the traffic. Light emitting diode (LED) lamps have been developed to replace conventional incandescent or fluorescent lamps for reducing electrical and maintenance costs, and for increasing reliability. LEDs offer the considerable advantage of consuming significantly less power than incandescent lamps. An LED traffic light uses only a fraction of the electrical power a light bulb traffic light used and is thus less expensive for long term use. In most cases, an LED array will consume about one tenth the power that a filtered incandescent bulb will consume to produce the same light output. The life cycle costs of a traffic signal using an LED array in lieu of an incandescent bulb is also significantly reduced since incandescent bulbs used in traffic signals typically must be replaced once or twice a year. A well designed LED array could be expected to function for more than twenty years before requiring replacement. LED lamps also generally require less frequent replacement due to burn out than incandescent lamps. The LED array is more resistant to the elements and is more mechanically durable than an incandescent bulb. It is also possible to achieve a higher flashing rate with an LED array than with an incandescent bulb. LED lamps typically include a power supply and a plurality of LEDs mounted on a flat or curved surface. An LED array does not require a light reflector like the relatively large parabolical reflectors used with incandescent bulbs. The elimination of the reflector is an advantage because during certain seasons at certain times of day, sunlight can be reflected off the reflector in an incandescent bulb traffic signal and cause a confusing display.

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