Traffic Signal Controllers and APS Technologies


Control of vehicle and pedestrian movements in the U.S. via traffic signals has been in place for over 100 years. The earliest traffic signals were manually operated, requiring a police officer or city employee to manually “switch” traffic flow by changing red and green panels or lights in a switch stand. As traffic continued to grow, it became obvious that mechanical/electrical signal controllers were required to relieve police officers of their traffic control duties.

Electrically operated traffic signals have been in operation in the U.S. since 1914. Electric traffic lights were first installed in Cleveland, Ohio, at Euclid Ave. and 105th Street (4). The traffic signal consisted of two long cross-arms, red and green lights, and buzzers. Two long buzzes signaled Euclid Avenue traffic to proceed, one long buzz meant it was 105th Street's turn to go. Note that this signal had both a visual and audio component.

Since then, traffic signal control equipment has undergone continual improvement. The earliest signal controllers used motors and gears to time the durations of the signal indications; some of these controllers can still be found in use. Technology and the computer era have brought microprocessor-based signal control equipment to what we have today, a complex yet functional system of automated control located at more than 300,000 intersections.

Current Controller Standards

Two standards are used today for traffic signal controllers (5, 6). The National Electrical Manufacturers Association (NEMA) developed one controller standard. Two generations of the NEMA standard are used today, namely the TS 1 (1976) and the TS 2 (1992). The TS 2 is further divided into Type 1 and Type 2 controllers. A second standard was developed by the California Department of Transportation (Caltrans) and the New York Department of Transportation (NYDOT). The original standard was developed under the Type 170 platform in 1979. Updated platforms have included the Type 179, Type 170E, and Type 2070.

Detailed information about these standards and controller features is provided in Appendix A of this document.



Accessible pedestrian signals (APS) are enhancements to the traffic signal system to provide signal phase information in audio, tactile, and/or vibrotactile formats for the pedestrian. APS devices available today are of four general types: pedhead mounted, pushbutton integrated, vibrotactile only, and receiver-base

Pedhead-mounted APS are the only type that have been commonly installed in the U.S. for the past 25 years. This type has a speaker on top of or inside the pedhead with a bell, buzzer, cheep, cuckoo, speech message, or some other tone during the walk phase of the signal only. Some models respond to ambient sound, becoming louder when the traffic noises are louder and quieter when the traffic is quiet. They are usually intended to be heard across the street and act as a beacon, and are relatively loud as a consequence. Manufacturers include Mallory, Novax, US Traffic, and Wilcox. Prisma and Campbell also have an optional additional pedhead mounted speaker that can be used in conjunction with their pushbutton integrated device

Pushbutton-integrated APS have a speaker and a vibrating surface or arrow at the pedestrian button. The sound comes from the pedestrian pushbutton housing, rather than the pedhead. This type has been common in Europe and Australia for years and can be used at both actuated and fixed-time signal timing locations. A constant quiet locator tone, repeating once per second, provides information to the blind individual about the presence of a pedestrian pushbutton and its location. The locator tone is intended to be audible only 2 to 4 meters (6 to 12 feet) from the pole or from the building line, whichever is less.

The walk interval may be indicated by the same tone at a faster repetition rate (Panich, Prisma), by a speech message (Polara, Campbell, Novax, Prisma), or by other tones (Campbell, Polara, Novax). All devices of this type respond to ambient sound levels. These signals are intended to be loud enough to be heard only at the beginning of the crosswalk, although volume can be increased by special activation (Polara, Prisma, Campbell). Manufacturers include Campbell, Georgetown Electric (locator tone not ambient sound responsive), Novax (locator tone and vibrotactile arrow combined with pedhead speaker), Panich, Polara, and Prisma.

Vibrotactile-only APS provide only vibration at the pedestrian pushbutton. The arrow or button vibrates when the WALK signal is on. It must be installed very precisely next to the crosswalk to be of value, and the pedestrian must know where to look for it. Manufacturers include Campbell and Georgetown Electric.

Receiver-based APS provide a message transmitted by infrared or LED technology from the pedhead to a personal receiver. The pedestrian scans the intersection with the receiver to receive the message emitted on the pedhead. These devices may also give other types of information, including information about the name of the streets or the shape of the intersection. Manufacturers include Relume and Talking Signs.

The section on APS devices provides details of each of the APS devices available at the time of this report. Information was obtained from phone, e-mail, Internet, and mail contacts with manufacturers and from the draft report, Accessible Pedestrian Signals: Synthesis and Guide to Best Practice by Accessible Design for the Blind [], being prepared as part of NCHRP Project 3-62: Guidelines for Accessible Pedestrians Signals (7). A matrix summarizing features provided by each device is also included.

Information about APS devices produced by the following manufacturers is included in this publication:

  • Campbell Company
  • Georgetown Electric, Ltd.
  • Mallory Sonalert
  • Novax Industries Corporation
  • Bob Panich Consultancy
  • Polara Engineering
  • Prisma Teknik
  • Relume
  • Talking Signs, Inc.
  • U.S. Traffic Corporation
  • Wilcox Sales

For more information on intersection design to accommodate APS, consult the references and associated web sites.



APS devices work with existing traffic signal controller logic to provide the desired information to the pedestrian. Most APS devices require no additions or activity within the traffic signal controller cabinet for installation and operation. APS devices are designed to work with either the green, red, WALK, or DON’T WALK indications, and to coordinate calls with the pedestrian pushbutton. Therefore, existing wiring from the traffic signal controller to the pedestrian signal heads and pushbuttons is maintained and unaltered. There is nothing done to the traffic signal controller during the installation of an APS device. In most cases, the only way a traffic controller system knows that an APS device is present is if voltage problems in the APS unit are detected by the controller’s conflict monitor.

In most cases, the APS device is driven by the pedestrian signal head (pedhead) indications. When electrical power is sent to the pedhead to illuminate the WALK indication, the APS device is also activated. All units currently available are wired in some way to the pedestrian signal wiring.

APS devices that are pedhead-mounted provide sound only during the walk interval. These devices are typically wired directly to the WALK indication, without other features or issues.

APS devices that are pushbutton integrated have additional control units that may be installed (depending on the manufacturer and the device) in the pedestrian signal head (16" clamshell style, typical) or contained completely within the pushbutton unit. At least one manufacturer also provides control units that can be installed in the traffic controller cabinet in lieu of the pedhead. Some provide a separate housing that must be mounted on the signal pole. Control units control operation of the locator tone, pushbutton messages and other features available on the pushbutton integrated devices. Pushbutton-integrated devices can also function at pretimed intersections, without pushbutton operation.

Receiver-based devices vary depending on the type of signal. Some include a control unit mounted in the pedhead will others include a replacement pedhead that is installed and wired as a regular pedhead.

Wiring and Power Requirements

The control units for integrated APS devices require additional wiring, most often from the pedhead to the pushbutton. Wiring requirements are quite similar with each mounting strategy. Power requirements are consistent with the traffic signal controller, pedhead, and pushbutton specifications directed by NEMA. APS control units in the pedhead usually convert the 120 VAC to a lower voltage (24 VDC) to the pushbutton. Specific wiring and power requirements are described with each APS device later in this report.

Interaction with MMUs and Conflict Monitors

The MMU, or conflict monitor, performs several functions, the most important of which is to prevent two conflicting green indications from being illuminated. The conflict monitor is really a voltage monitor, looking for inappropriate voltages in inappropriate locations or voltages that are above or below desired levels. The conflict monitor will track multiple components, including the cabinet field wiring terminals for voltage on conflicting signal indications and cabinet voltage to assure that the proper operating range is maintained.

APS devices do not directly interact with the conflict monitor, outside of the fact that the voltage requirements and outputs from the APS units will be monitored within the system specifications. Problems with APS devices can develop when the voltage outputs exceed current conflict monitor limits. When this happens, the conflict monitor overrides the signal control system and places the signal into flash mode. Other problems exist when additional wiring is added from the pedhead to the pushbutton that creates a separate circuit that is no longer detectable by the conflict monitor. Some discussion of this problem is included in the troubleshooting described in the next section, “Lessons Learned from Existing Installations.”