Since January 2006, traffic signals, traffic lights, and exit signs have been required to meet Energy Star efficiency standards for inputs of 5W and above. Although traffic and pedestrian signals were estimated to be the primary application where halogen lamps were still in use in 2007, this standard created a transition to, and a demand for, LED lighting in these applications. To further support energy savings in LED lighting applications, as well as to comply with European regulations for traffic signals and remain competitive in the market, greater intelligence is needed. This can be achieved through closed-loop sensors, communication technologies, and efficient LED control.
Traffic signal suppliers tend to provide energy-efficient and sophisticated traffic signals that can also combine good optical quality with an attractive design.
There are various traffic signal systems available on the market today, including traffic signs, stop signs, traffic lights, traffic displays, and streetlights. Most basic systems are powered from a distribution block located in a central control unit. The control block provides DC power to the traffic signal via conventional power cables using a simple ON/OFF switch. The distributed power is available in 12V, 24V, 40V, and 48VDC. Since different countries have different regulations, the supply voltage to traffic signals depends on the country and its standard brightness requirements. For example, to meet UK market requirements for traffic lights, each unit must be supplied with 48VDC.
Some traffic lights are powered directly from the 230VAC mains. Most of these systems also include a countdown timer, in addition to the three standard red, green, and amber lights. The countdown timer is displayed using a pair of LED strips within the signal, showing the countdown value in 7 seconds in either red or green, as appropriate. This countdown timer indicates the time in seconds remaining until the system advances to the next phase, contributing to better orientation and reduced risky behavior by pedestrians and motorists at intersections. This provides drivers with helpful information, allowing them to prepare to start or stop. The signal's countdown timer typically has its own integrated LED driver, which also controls the countdown timer and the 7-second algorithm. For the other three signal colors, the LED driver is usually housed within a separate control unit. Many manufacturers build the 7s with 42 LEDs, where 6 LEDs are used in each segment of the 7s display.
It is well known that basic traffic signals are equipped with the three colors red, green, and amber. However, in many cases, the red light contains some orange in its color, and the green light has a certain blue tint. This is to provide some assistance to people who are red-green colorblind. This type of system requires additional intelligence and multi-color support, where the light intensity is appropriately calibrated and then dimmed to a certain level to produce the correct shade.
Modern traffic lights are also equipped with smart sensors for detecting and counting passing vehicles. These sensors are used to improve traffic light performance and adjust the timing between different light colors. They are also used to detect pedestrians or cyclists crossing the street, allowing the traffic lights to change accordingly. Another important use of sensors is managing traffic jams. If a traffic jam occurs, the system can trigger the traffic lights to change earlier than usual to help disperse the congestion.
There are also systems with sensors that can be overridden and prioritize special traffic, such as ambulances or police vehicles. This type of system can detect an approaching emergency vehicle with its sirens activated and change the lights accordingly. The sensors used for priority vehicle detection are generally based on wireless, infrared, or optical transmitters that send a request to the traffic light controller to change the lights when an approaching emergency vehicle is detected. Some systems also use sound sensors that can read a specific type of siren and are used to detect approaching ambulances.
To optimize traffic flow, traffic light manufacturers also integrate sensors to detect vehicles and bicycles with metal rims approaching a pedestrian crossing or intersection. They then change the lights accordingly. These sensors are generally built with inductive sensors and operate by detecting changes in the electromagnetic field.
When a vehicle enters the field of the magnetic sensors, it causes the light to change. The fundamental weakness of this type of detection is that a sensor failure can cause the system to fail to detect cars waiting in a turning or crossing lane. This can cause considerable, even indefinite, traffic delays, as well as resulting in non-compliance with safety signals. Another disadvantage of this technique is the limitation on the type of vehicle the sensor can detect. Larger vehicles containing metals like steel can affect the sensors' magnetic field and trigger the system. However, this doesn't always work for small vehicles, such as motorcycles, without proper sensitivity adjustment. Detecting a small car, motorcycle, or even a bicycle requires the detection system to be adjustable to be more sensitive than normal for the levels required to detect a standard vehicle. Modern detectors, such as rack-mount and shelf-mounted detectors, offer multiple sensitivity levels for this purpose. Although inductive sensors are the most common technique, alternative solutions include laser-based sensors or air-filled rubber hoses.
Changing message signals that integrate proximity sensors to warn of nearby events, such as driver speed indicators or flashing warning messages when cars approach, are very popular today. These systems are inexpensive to install, and many are solar-powered, making them ideal for accident reduction in areas without access to the electrical grid. Sensors are interconnected to the system controller in various ways. The output of analog sensors can be read by an analog-to-digital converter (ADC). Digital sensors, which are more common these days, offer serial interfaces, such as an I²C (Inner-integrated Circuit) bus or an SPI (Serial Peripheral Interface) bus, to communicate with microcontrollers. The advantage of the I²C bus is its networkability, allowing multiple I²C-compatible sensors or other devices to be connected in a network. On the other hand, SPI offers faster communication speeds.
Some traffic lights are equipped with speed cameras, so an additional signal is used to display the speed of an approaching vehicle in 7 seconds. There are different types of traffic lights available today from various manufacturers. The combination of intelligence and high luminance in these systems helps to increase traffic safety at intersections. The variety of options allows customers to select the appropriate systems to suit their application requirements and comply with regulations in their country.
Different manufacturers use various methods to apply LEDs in traffic light systems, reducing energy consumption by up to 20% compared to halogen lamps. Some use only two to five LEDs per signal to produce light, saving on system costs and energy. Many modern traffic lights also incorporate symbols within the signal. These include a green arrow, indicating that forward movement is permitted when the signal is active, and a pedestrian symbol at a crossing. Non-motorized traffic, such as pedestrians and cyclists, are also illuminated with LED backlighting. In some northern and eastern European countries, this additional signal is also dedicated to public transport, such as trolleybuses and trams. This signal is generally built using a multi-channel LED approach, which allows for the integration of special traffic symbols for public transport. The actual image of an arrow, pedestrian, or other symbol is etched onto the optical material during manufacturing and used as an optical envelope for each signal. When the LEDs on the back of the optic are illuminated, the light is only visible within the arrow, while the rest of the area within the sign does not conduct light. Again, since fewer LEDs are needed to produce the image, the advantage of this approach is the cost savings on the LEDs and the resulting lower energy consumption. Other manufacturers use dozens of LEDs in each sign and use individual LEDs to create images and shapes within the sign. The advantage of this approach is the flexibility and programmability of the images. It also allows for the insertion of moving objects into the sign, where the LED matrix within the sign changes the appropriate LEDs to create the required image. To provide the brightness level that meets local traffic authority regulations, LED-based traffic signs must be dimmable. This allows installers to set the required brightness level.
As mentioned earlier, control systems are the central control units used to provide power management and configuration services to traffic signal systems. Most control systems can provide up to 900 W, some even more. These control units operate and manage all the traffic signals at a location. So, if there are multiple traffic lights at an intersection or traffic signals mounted on a bridge, a single control system manages all the traffic lights and signals at that particular location.
Knowing that different voltage levels must be compatible in different countries, these control systems include intelligent and configurable power supplies to provide a wide range of DC voltage to suit varying voltage requirements.
Control system manufacturers implement a variety of HMI options for configuring and programming traffic lights and signals. Many companies offer touchscreens with a reliable operating system, such as Linux, as well as various serial interfaces, such as USB, Ethernet, and RS485. These allow technicians to perform configurations and upload bitmap data.
Traffic signs and variable message signs are used to display information on roads and are gaining wider adoption, thereby improving driver information and safety, while also alleviating congestion, as part of an Intelligent Transportation System (ITS). Sign types range from simple signs, based on a few one- to three-channel LEDs, used primarily in cities and other built-up areas. These include flashing speed limit, parking space, and entrance signs, as well as other warning signs. These have low throughput and minimal LED lighting requirements, but offer significantly greater market growth potential.
Large variable message signs on highways, such as highway information, downtown congestion, and toll lane signs, are constructed with an RGB LED pixel matrix to provide color variation and the effect of moving objects, depending on the requirements. Each manufacturer has a different approach to designing large LED displays for traffic, but the most common method involves dividing the display area into pixel segments and placing a series of identical small controllers in each segment to control a number of pixels within it. These segment controllers are centrally managed by a control system within the main unit, which is programmed with the message or bitmap being displayed. RS-485 is a common interface among most manufacturers. It is used as a communication protocol between the main controller and the segment controllers, which are pre-programmed with unique addresses.
One of the fastest-growing markets for LED replacement is street lighting. In the early stages of this developing market, a single-color white LED was considered sufficient to meet the main requirements. However, it's worth noting that this isn't actually the best option for human visual perception, which varies depending on lighting conditions and even location. Interestingly, photopic vision in a well-lit environment and scotopic vision at night overlap somewhere in the green/blue color spectrum. At this point, the human eye has the best visual perception in low-light conditions. Therefore, it's more efficient to produce colored light in the correct spectrum, allowing for better visibility and energy savings. For example, human visual perception differs between rural and urban environments. In the countryside, the eye requires more of a green tone in lights, while in the city, where the environment is brighter, it requires more red. Different developers have different approaches to addressing this, but it's common to develop a multi-channel LED driver to meet color mixing requirements. To further contribute to energy savings, producing the right color combination allows for a higher level of brightness with lower power consumption. Many street lighting manufacturers today also integrate proximity sensors that automatically dim or brighten the lights depending on whether someone is detected within a certain radius. There are also lights with ambient light sensors that can automatically adjust the brightness based on the ambient light level. All these features add intelligence to the systems and, above all, contribute to significant energy savings.
Although LEDs offer significant advantages such as their lifespan, luminous efficacy, and low operating voltage, two unavoidable drawbacks of current LED technology are light degradation, which typically begins over time, and color shift with temperature changes. Unfortunately, all manufactured LEDs have different characteristics that LED driver designers must consider when developing LED power supplies for traffic lights and other industrial LED lighting products. In traffic light systems, streetlights, and other LED lighting products, the ability to detect the onset of any light intensity degradation is essential. This is to ensure that the light output level complies with European standards required for the minimum brightness of traffic signals, such as ECE65. The system can be configured to automatically compensate for the light output level by increasing the current through the LEDs once degradation is detected. This can be achieved through continuous monitoring of the current through each LED channel. Renesas lighting microcontrollers integrate up to nine 10-bit ADC channels and up to three channel comparators, which can be used to control currents and adjust PWM outputs accordingly in single-channel or multi-channel LED systems. Another important aspect to consider is LED fault detection. This can be crucial in LED-based traffic signals, as the system must be protected from accidentally illuminating the wrong colors. Furthermore, one of the main advantages of adopting LEDs for streetlights is their longevity, which reduces maintenance costs. The system must be able to detect LED faults and act accordingly. Therefore, it is important to be able to monitor the system for LED faults, as well as for excessive current and temperature. Renesas lighting microcontrollers integrate dedicated peripherals to enable the required protection procedures within the system. Many manufacturers also use multi-channel LED controllers to support traffic light and street light signals with additional LEDs if one of the channels fails.
Some lighting-specific microcontrollers, such as those from Renesas, integrate dedicated LED driver peripherals to facilitate integration and reduce costs. They provide a single-chip solution for both power stage control and controlling multiple LED channels without involving the CPU.
Author: Alex Zaretsky, Renesas Electronics Europe.
