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The City of Sacramento Traffic Signal Operations Group deploys advanced technology on the City’s roadway system in a strategic manner. The technology is networked into the Traffic Operations Center so traffic signals can be coordinated. The City’s operations staff strives to be a leader in using such technology in our region.
FAQs
What makes a traffic signal work?
A traffic signal is a complex system made of many components, the poles that hold the traffic signal heads are only a small part of this system. There are a multitude of sensors and electronic components that comprise the majority of a signal system. The brains behind the operation are enclosed in a traffic signal controller cabinet.
Each of the City’s 700 traffic signals has a signal controller. This controller is basically a micro computer that interprets data received from roadway sensors and assigns the appropriate amount of time for a given direction. In cases where intersections are within a close proximity, like downtown, the signal controllers run a fixed amount of time for each direction, to facilitate coordination between the intersections. At other locations, sensors are in place that are actuated by vehicles and help the controller know where the vehicles are on the road. Operations staff program specific parameters into both fixed time and actuated signals that are then interpreted by the controller. Many of these controllers allow these parameters to be remotely programmed through communication with the City’s Traffic Operation Center.
Traffic signals interact with more than just cars. Parameters for these other variables also need to be programmed. Pedestrians, bicyclists, fire trucks and light rail trains all need to be serviced at a signalized intersection. All these variables are integrated into the operation of the signal.
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What happens when a traffic signal malfunctions?
The traffic signal controller is monitored by a device called the Malfunction Management Unit (MMU). This device monitors specific problems that would result in an unsafe condition. For example, if the controller tries to display green lights in conflicting directions at the same time, the MMU would place the intersection into flashing mode. The MMU monitors many other things as well, such as the power voltage and the length of time that a yellow light is displayed.
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Are traffic signals coordinated from one to another?
In most cases, traffic signals are typically coordinated from one to the next. It is easy to see such coordination on downtown streets. 16th street is a good example of this coordination. Other streets are also coordinated, even though it may not be as obvious to the observer.
Many factors affect coordination: the spacing of signals, the time needed for pedestrians to cross, light rail trains, and volume of cars in all directions have an impact on when a light can turn green for you. Our downtown grid is mostly one-way streets that are evenly spaced, which makes coordination easier, but the majority of the City’s streets that are outside of downtown are two-way and the signals are not evenly spaced. This causes vehicle arrival times to the signal from both directions to vary. There are still a few of the City’s corridors that are not coordinated and the department is working to close the gap in the system.
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What do pedestrian crossing indications mean at a signal?
- Walking person means you can legally enter the street to cross.
- Flashing red hand means do not start to cross, but if you are already in the street, you may legally finish crossing.
- Solid red hand means do not enter the street as cross traffic has the right-of-way.
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How do traffic signals detect cars?
Many of the City’s traffic signals have a way to detect vehicles. Some locations have a loop of wire placed in the roadway that can determine when a vehicle is present. The installation of loops in the pavement can cause the pavement to fail and can be difficult to maintain.
To minimize the pavement problem, the City has been using video detection. The video detection system works by providing an image to a small computer that can detect a change in the image and determine the presence of a vehicle.
Video detection can sometimes be compromised by interactions with the environment other than vehicles. Fog, dust, shadows, trees, headlights and the reflection of the sun on the asphalt can all have adverse affects on the accuracy of detection. The City continually tests new technologies that come to market to find the best solutions.
Why are there cameras at some intersections?
Cameras can serve many purposes at an intersection. Several years ago, the City made the move to reduce the amount of hardware installed in pavement. To achieve this, the City has deployed video detection camera systems that use a machine vision processor and have the ability to detect vehicles in the camera's field of view. This system sends the signal to the traffic signal controller that a vehicle is there and needs service.
CCTV cameras are also located at some intersection. These cameras are used to provide information to the City’s Traffic Operation Center that operators use to verify traffic conditions and traffic signal timing. The City of Sacramento DOT does not record the video.
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Do trains or light rail affect the signals?
Yes and no. There are many locations that detect light rail and heavy rail trains through an intersection. To improve safety, many of these locations have gate arms that stop traffic across the tracks when trains are present. The traffic signals must provide a clearance interval for any vehicles that may be on the tracks. This is done by providing a green light in the direction needed to clear the tracks. This can hold up traffic at the intersection but is necessary to help prevent accidents with trains.
On several streets in the downtown area, the light rail trains share lanes with vehicular traffic. Typically, some advantage is given to the light rail to maintain efficiency sometimes interrupting traffic signal timing.
For rail crossing safety information please go to Operation Life Saver
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Can Emergency vehicles cause the signal to change?
Yes, but not at all intersections. Emergency vehicles carry a device that provides a signal to the traffic signal controller. As an emergency vehicle approaches the intersection with emergency lights on, the signal controller will finish serving the current pedestrian and vehicle phase, and then provide a green light for the oncoming emergency vehicle. This helps to reduce emergency vehicle response times and increases intersection safety.
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What does a traffic signal cost?
The cost to install a traffic signal varies depending on the size of intersection and if roadway modifications are needed. Typically, the cost can range between $200,000 and $300,000.
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Where do I get red light camera information?
The red light running cameras are not operated by the City of Sacramento, but are managed by the County of Sacramento. For information on the program please use the following links. .
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