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APS home go to front of Accessible Pedestrian Signals
Go to Front page Background section Travel by Blind
Rules & Regulations
Technologies and Features section Types
Walk Indications
Other Features
Choosing and Installing section Where to Install
Designing Installations
New Construction or Reconstruction
Retrofitting an Intersection with an APS
Installation Specifications
Field Adjustments
State of Practices section Case Studies
International Practice
Devices section Manufacturers
Selection Tool
Product Matrix
Downloads section Full Guide
Rating Scales
Field Adjustments

Major considerations

Four attributes
The WALK indicator should be: Readily Detectable
High detectability essential for usability
  • Pedestrians who are visually impaired must be able to hear the WALK indication clearly, over varying types and intensities of traffic sound.
  • While the signal needs to be detectable, it is desirable to have a sound that is not irritating to individuals in the area of the signal.
Detection of WALK signals in ambient traffic sound
  • Vehicular sounds are concentrated in the lower frequencies.
  • The most detectable signals are those that are concentrated in frequencies different from those of traffic sound
  • Factors that aid signal detection in ambient traffic sound

    • • Multiple, sharp onsets
      • Large frequency component at about 880Hz
Less detectable signals
  • Both the commonly used cuckoo and chirp are less detectable than more rapidly repeating tones in 880Hz range, and speech messages
  • Pedestrians with visual impairments who have age-related upper frequency hearing loss may have difficulty hearing signals having a fundamental frequency above 1kHz.
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Highly Localizable
High localizability helps users:
  • Determine which signal is sounding
  • Use the signal for alignment information (in locations with audible beaconing)
  • Travel more directly toward the signal during their crossings.
Characteristics of highly localizable tones
  • Not pure tones
  • Multiple harmonics or frequencies required-high and low
  • Large frequency component at about 880Hz
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Uniquely recognizable
Where the Walk interval is conveyed by tones, the tones should not be confusable with other tones in the street environment, such as vehicle backup warning beepers.

The most common tones used currently are the bird sounds like cuckoo and chirp.
  • The 'chirp' sound is similar to the sounds made by several birds in the U.S. and is also mimicked by mockingbirds.
  • Pedestrians who are blind have crossed streets with actual bird chirps, or failed to cross with APS tones because they were perceived to be actual birds.
  • The cuckoo indication has not been reported to have been confused with birds in the U.S.
Vehicle backup beepers are not tightly specified in the U.S. but typically have a repetition rate of once every 1 to 2 seconds, and the beep is typically about half the length of the repetition rate. For APS WALK indications that consist of tones (excluding the cuckoo and chirp), the repetition rate in the U.S. is typically 8 to10 repetitions per second. The cuckoo and the chirp have a typical repetition rate of once every 1 to 2 seconds with a very short duration of the tones.

Speech messages for the Walk interval must be recognizable as a WALK message and not confused with pushbutton messages or other voices at an intersection. This can partially be accomplished by use of standardized wording for speech messages. In addition, the speech message must be understandable.

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Unambiguous information
It is critical that pedestrians recognize which street is being signaled and begin their crossings within the Walk interval. Unfortunately, the most common solution used in the US is ambiguous. Surveys of pedestrians who are blind and the authors' evaluation of typical overhead pedhead-mounted signals have revealed that they frequently provide ambiguous information about the crosswalk being signaled.

When the APS sound for both streets comes from the same general location, it is difficult to discriminate which street the tone or speech message applies to. The pedestrian who is blind waits to cross while standing approximately at the curb line, and may be 10 to 15 feet or more from the device speaker. The mounting of speakers does not provide clear indications of which street is being signaled, unless the speakers are mounted on two separate poles, at least 10' apart, and aligned with the crosswalk they signal.

The use of different tones for each direction requires pedestrians to know their direction of travel, and to know which tone is associated with which travel direction in a particular jurisdiction. (See discussion of Tones) While in many situations, traffic movements also help clarify the signal status, beginning to cross with the wrong signal can be a fatal mistake.

One factor that particularly affects ambiguity is location of the speakers for the WALK indication. This is discussed in the next section.

Installation as well as device selection can affect the ambiguity of the information provided by the WALK indication. Designing Installations provides more information on deciding what features would be necessary or appropriate at a particular location.

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