|
Traffic signal timing is the technique which traffic engineers use to determine who has the right-of-way at an intersection. Signal timing involves deciding how much green time the traffic lights shall provide at an intersection approach, how long the pedestrian WALK signal should be, and many numerous other factors. ==Basic signal timing operation== To understand basic traffic signal timing fundamentals, one must also understand the different modes of operation for the traffic signal controller. Some traffic signal intersections have some sort of mechanism for detecting vehicles as they approach the intersection. One frequently used method is induction loops. There are many shapes that can be cut into or buried below the surface of the roadway. A typical installation could be approximately six foot square or five by twenty feet. The smaller size would have five to six 'turns' of wire the large size typically three turns. The preferred wire is stranded copper with an insulating cover which is then loosely surrounded by a polly cover. This assembly resembles an insulated wire floating inside a drinking straw. Electrically this is a 'flat' coil in the pavement structure that detects vehicles by changes in the inductive field of the coil when ferrous metal from a vehicle passes through the field. The electronic sensor component in the controller cabinet senses the delta L / L of the coil. The output from the sensor electronics is a 'switch' closure. This can be an electro-mechincial relay or solid-state. The 'switch' is normally closed (NC) in the de-engerized state but held open when power is applied to the circuit. This is called 'Fail-Call' so that if there is a failure in the sensor electronics the output will place a 'Call' to the controller as if a vehicle is present on the loop detector. Other detection methods are video (both normal and infrared) which use a change in contrast of the image detection zone, microwave detection, magnetometer, and most recently (circa 2005) digital radar. All except the Inductive Loop Detector and magnetometer suffer in accuracy as the result of the concept of occlusion, either optical for the cameras or electronic for the microwave and radar technologies. An intersection equipped with detection is said to be 'actuated'. If all approaches are actuated it is called fully actuated. An intersection without detection is called simply non-actuated and the amount of green-time that is displayed to each approach is fixed. There are different categories of actuated signals. To save money on maintenance, some agencies opt to design an intersection as semi-actuated. Semi-actuated means the intersection has detection on the minor street approaches and major street left turns only. The whole intersection is then programmed to operate a fixed time every cycle, but the controller will service the other movements only when there is a 'call' or demand. An arterial series of signals that are operating in a semi-actuated or fixed mode and importantly operating with the same cycle length can be made to function in a coordinated manner. During signal coordination, most signal systems typically are designed to operate in a semi-actuated mode. In fixed operation, a controller has a set programmed time to service all movements every cycle. The controller will service all movements whether or not there is vehicle demand. When a detector at an actuated signal breaks, that movement will then have to operate as fixed until the detector is repaired. There are three general ways for a traffic signal to operate, FREE, COORD, and FLASH operation. In FREE operation, the signal is running based on its own demand and timing parameters based on the information provided by its detectors. It is not operating under any background cycle length. In COORD operation, short for coordination, the signal is running a background cycle length. Non-major street movements are usually still actuated, and the controller will rest on the major street until the background cycle length is fulfilled. The final mode is FLASH operation in which all vehicle signal heads continuously display a flashing red, or the main street shows flashing yellow while others show flashing red. Pedestrian heads are dark. When the volume of vehicles at an intersection no longer warrants the signal to be active, the signal can switch to FLASH mode. When volume picks up again the signal switches back into either FREE or COORD operation. For example, the daily operation of a signal may involve it being in FLASH mode early in the morning, COORD during the day, FREE in the evening, and back to FLASH late at night. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Signal timing」の詳細全文を読む スポンサード リンク
|