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Engineering, Public Works, or Traffic Operations
What is the purpose of a traffic signal? Traffic signals are electronically operated traffic control devices which alternately direct traffic. Traffic signals are designed to ensure an orderly flow of traffic, provide an opportunity for pedestrians or vehicles to cross an intersection, and help to reduce the number of conflicts between vehicles entering intersections from different directions. Will traffic signals prevent intersection crashes? Traffic signals do not prevent crashes. Side-angle (T-bone) crashes can be reduced by installing a traffic signal. However, rear-end collisions may increase since traffic is not used to stopping at the intersection. Does a traffic signal control speed? No. In fact, traffic signals sometimes result in greater speeds as drivers accelerate to try to get through the signal before it turns red. Additionally some drivers will speed between signals to make up for lost time at the signals. What is the difference between a traffic signal and a ramp meter? A ramp meter is a traffic control device located at "on ramps" for freeway access. A traffic signal is a traffic control device for two or more intersecting roadways. How do traffic signals work? Traffic signals change the lights according to the amount of traffic in each direction. Most signals use sensors to detect the number of vehicles and automatically adjust the length of the green time to allow as many vehicles as possible through the intersection before responding to the presence of vehicles on another approach. How does a traffic signal know when a vehicle is present? The controller (computer) of the traffic signal receives electric signals from the vehicle detection system, which is typically a loop detector in the roadway. Other types of detection systems are video, sonic and microwave. Why do traffic lights take so long to change? The length of the wait time at a traffic signal depends on the signal cycle length. Frequently on major corridors the cycle lengths are longer to accommodate higher volumes of traffic by providing more green time in order to move more vehicles through an intersection. However, increasing cycle lengths also causes secondary movements to experience longer delays. What is traffic signal coordination? Traffic signal coordination is when two or more traffic signals are working together so that vehicles moving in a group, or platoon, will make the least number of stops possible. When traveling on some streets, red lights are hit on a regular basis? Can't they coordinate them? Yes, all traffic signals along major streets are typically coordinated in peak hours to minimize stops and delays. Traffic engineers examine each corridor and design a "best-fit" timing plan to address the circumstances. However, these streets cannot be perfectly timed due to varying traffic speeds, congestion, the distance between signals, etc. Why isn't there enough green time at a signal to get the traffic through at some approaches? The amount of green time programmed for each movement at a signal varies by traffic demand and increasing green time for one movement requires decreasing the amount of green time for another movement. Therefore, the approach with the highest traffic volume receives the highest green time. I know of a traffic signal that has indications out or appears to be malfunctioning. Who do I contact? If the signal is on a state route, Call _______, and be prepared to give them the following information: The _________ that you are in, The name of the two intersecting roads, Is the signal flashing or blank? What do I do if all of the signal indications are blank? When a traffic signal has gone dark due to power failure, it is considered a multi-way stop. Each driver must stop and yield before entering the intersection. What do I do if a signal is flashing? If the signal is flashing yellow in your direction, proceed through the intersection with caution. If the signal is flashing red in your direction, you are required to stop. How is the yellow time of a traffic signal determined? Typical yellow time is approximately 3 to 6 seconds. The largest determining factor in yellow time is the posted speed of the roadway. The slower the posted speed limit, the shorter the yellow time and the faster the posted speed limit, the longer the yellow time. How is the all red time of a signal determined? The purpose of the all red time (the time when all traffic signal lights are red) is to allow vehicles that entered the intersection before the red to clear the intersection before conflicting traffic receives a green. When is a left-turn arrow needed at a signalized intersection? Left-turn arrows, or a left-turn phase, are implemented based on traffic volumes and/or crash history. If the left turning volume and the conflicting through movement volume are not high enough, the traffic signal typically will not need a left turn arrow or phase unless there is an established crash problem. What are other alternatives to traffic signals? Investigations of a new traffic signal should include evaluation of several improvement scenarios, including improvements that do not include a traffic signal. New traffic signals tend to be permanent and may have adverse impacts to arterial networks even if warranted at a particular location. Other alternative to signalization may include: Roundabouts: At certain low to moderate volume conditions, a roundabout may improve intersection capacity and safety. Roundabouts are particularly effective for intersections with skewed geometry and intersections with balanced turning movements. Multi way stops: Intersections that do not meet warrants for a traffic signal may meet warrants for multi-way stop control, particularly at intersections with balanced approach volumes and/or high crash rates as a two-way stop condition. Geometric Improvements such as left turn lanes, right turn lanes, medians, islands, or channelization may also improve intersection operations and/or safety without the need for a traffic signal. |