Effects of WALK signal characteristics
Introduction
Access Board Draft Guidelines for Accessible Public Rights-of-Way (revised 2005) and this APS Guide recommend using a rapid tick WALK indication. Research on WALK signal characteristics, described in this section, is the basis for that recommendation.
It is important that any audible WALK signal be detectable and localizable, and, if it is a speech message, it must also be intelligible. Two factors make it difficult to satisfy these requirements. First, both detection and localization are influenced by the ambient sound, especially the sounds of vehicles. Second, any degree of hearing impairment makes it more difficult to detect and localize sounds and to understand speech. One might think that simply making the WALK signal loud enough would overcome both problems. However, the problems are much more complex.
Not only volume, but the nature of the sound, that is, the spectral and temporal characteristics must also be taken into account. A body of research, reported below, describes research to identify the characteristics of sounds or messages that make them detectable, localizable, and intelligible. Some of this research particularly addresses detectability, localizability and intelligibility for people with hearing impairments.
There are also important limits to the amount of WALK signal noise that will be tolerated in neighborhoods, and limits to signal volume that are based on OSHA requirements. Furthermore, if the WALK signal is too loud, it may make it difficult for pedestrians who are blind to hear the sounds of vehicles that are about to cross their path.
APS available today respond to ambient sound, providing WALK signals that are louder when the ambient sound is high, such as when a large truck accelerates, and quieter when the ambient sound is low, such as at night. This greatly increases the public acceptance of APS.
APS users must not only be able to detect the WALK signal and localize it, they must be able to quickly and unerringly determine which crosswalk the signal is for. The nature of the signal and its location are critical in the performance of this important part of the street-crossing task.
Results of research
Hearing and blind pedestrians
A majority of persons who are severely visually impaired are age 65 or older, and typically have some amount of upper frequency, age-related, hearing loss. In addition, the incidence of hearing loss in people with visual impairments is higher than for the general population because a number of causes of blindness also result in hearing loss.
Upper frequency hearing loss results in a decrease in the ability to localize sound and to understand speech, particularly in noisy environments (Wiener & Lawson, 1997).
Characteristics of traffic sound
The sound produced by vehicular traffic is concentrated in the low frequencies, especially for vehicles that are accelerating from a stop. The noise produced by accelerating vehicles is approximately 10 dB louder than that of vehicles traveling at a constant rate of speed. The mean intensity of accelerating traffic, measured from the position of a pedestrian waiting to cross streets in residential and small business areas, was found by Wiener and Lawson (1997) to be 89 dB, equal to the maximum APS volume currently permitted by the MUTCD. (The 89 dB maximum in the MUTCD was based on OSHA 8 hour exposure limits). This means that signals at their maximum permitted volume will sometimes be difficult to hear.
Detectability
Hulscher (1976) found that, because of the masking of high frequency signals by predominantly low frequency traffic noise, and because a majority of blind pedestrians have some upper frequency hearing loss, the optimal fundamental frequency of the WALK tone should be between 300 Hz and 1000 Hz, and the tone should be comprised of multiple short bursts of sound to aid localization.
Staffeldt (1968), in research cited by Hulscher (1976), conducted extensive testing of APS at crossings where they were mounted on the pedestrian signal head, and found that an 880 Hz signal was most detectable in a background of traffic noise.
Hulscher's recommendation and Staffeldt's result was supported by Poulsen (1982) who compared the noise spectrum of traffic as attenuated by windows to arrive at a recommended signal frequency (880Hz) that would not be largely masked by traffic noise, but would also not transmit through windows and become a public annoyance.
In San Diego research, laboratory measurements of "birdcall" signals from Nagoya Electric Works of Japan found that neither signal was highly directional. However, the cheep was more detectable than the cuckoo. The cheep was produced by a continuous frequency variation with a fundamental frequency base of 2800 Hz and the cuckoo consisted of two frequencies with a combined frequency base of 1100 Hz. (currently available "cuckoo/cheep" signals may vary from this manufacturer's standard.)
Hall, Rabelle & Zabihaylo (1996) worked with audiologists to develop a signal that provided the most localizable melody for an accessible signal and recommended a melody that was composed of fundamental frequencies between 300 Hz and 1000 Hz, but including harmonics extending to 7000 Hz.
In NEI research (unpublished data), Wall, Ashmead, Barlow, and Bentzen carried out a series of experiments on detectability of WALK signal indications in a laboratory setting. Experiment 1 evaluated the detectability of signals in white noise, experiment 2 evaluated the detectability in traffic noise, and experiment 3 evaluated detectability in traffic noise for subjects with age related hearing loss. Signals evaluated were an 880Hz square wave, a bird chirp, a cuckoo, two click trains, two percussive signals ("bink" and "tok"), a 4-tone melody, and female and male voice signals. Results indicated that audible signals of a more percussive nature with predominantly lower frequencies were best heard in background traffic noise. In addition to one of the percussive signals, participants with age related hearing loss were better able to detect male voice signals. Signals with a simple percussive nature tended to need less gain to be heard in noise, relative to the levels necessary to be heard in quiet. In other words, percussive signals were more detectable at lower volumes. When asked their preference, most participants liked voice signals best, but all of the voice signals needed more gain to be heard. Note that the measurement in these studies was the point at which the signal was detected, not the point at which the message was easily understandable. Intelligibility of voice messages was not evaluated or assessed by participants. The cuckoo and chirp, most commonly used in the U.S. as an audible signal at the time of the study, required the most gain to be detectable amidst background noise.
Localizability of WALK signal
An additional factor, if audible pedestrian signals are to be used as beacons to guide pedestrian with visual impairments across a street, is how well signals can provide directional information. Laroche, Giguère and Poirier (1999) compared localization of cuckoo and cheep signals to localization of four four-note melodies varying in fundamental frequencies, harmonics, note duration, and temporal separation between notes. In combined objective and subjective testing, the cheep and a melody with minimal harmonics were found to be less localizable than the cuckoo and the other three melodies. In a follow-up study, Laroche, Giguère and Leroux (2000) compared the typical cuckoo-cheep sounds used in Canada with a cuckoo having a lower fundamental frequency, and the melody that was recommended as a result of their 1999 research. In both studies, the signal was 36 seconds long (much longer than typical U.S. WALK indications) and the measurements were in a simulated pedestrian corridor in a quiet environment. In situations with actual traffic sounds, the cheep was found to result in significantly greater veering and longer crossing time than any of the other signals, which did not differ from each other.
In NEI research, Wall, Ashmead, Bentzen and Barlow (2004) found no significant differences in localizability among several disparate signals including cuckoo, chirp, "tok" and voice messages when tested in research that involved multiple crossings of a simulated street, in the presence of recorded vehicular sound. The five signals used were representative of signals in wide use, or which showed promise for directional beaconing. None of the analyses indicated any systematic differences between the five signals. Further experiments focused on presentation mode and signal location, rather than signal sound characteristics.
Speech WALK indicatons
Several APS systems in the U.S. are capable of producing directly audible speech messages, either from a speaker that is integrated into the pushbutton housing, or from a speaker at the pedestrian signal head. APS with speech messages are considered by many people to be especially user-friendly, when demonstrations are given indoors, to an audience for whom English is the predominant first language. However if messages are not correctly understood by users, APS with speech messages, especially speech WALK indications, can lead to catastrophe.
Intelligibility of speech messages is influenced not only by the relationship between signal volume and ambient sound, but also by the nature of the message, how familiar the hearer is with the English language, and any kind of hearing impairment that the user may have.
Understanding speech in noise
Listeners with normal hearing require that speech be 15 dB louder than background noise for intelligibility to reach 90% (Killion, 1999). This means that, in order to be intelligible, speech messages should be louder than tone indications. The effect of that louder sound level on the ability of blind pedestrians to hear other sounds in the intersection, or on near neighbors, may limit the acceptability of speech messages. At this time, MUTCD and Draft PROWAG limit the output of APS to 5 dB above ambient sound, except when special actuation requests a louder beaconing signal for a single pedestrian phase. As noted in the section on detectability, speech messages can be detected in traffic sounds, but that does not guarantee that they can be understood.
Speech WALK message structure and wording
Bentzen, Barlow and Franck (2002) conducted research to obtain information from stakeholders regarding the structure and content of speech messages for APS WALK messages and for "pushbutton information messages" that are available during flashing and steady DONT WALK only. WALK messages convey that the WALK signal is on, and provide the name of the street being crossed. Pushbutton messages provide intersection and crosswalk identification information, and may also provide information about unusual intersection signalization and geometry.
The research utilized an expert panel of stakeholders, who prepared a survey comprised of sample messages to rate, and items to determine respondent understanding of messages. The survey was administered to people who are visually impaired, O&M specialists, transportation engineers, and APS manufacturers.
Speech WALK messages should provide information to pedestrians who are blind that is similar to the information being provided to pedestrians who are sighted. The message should not be worded in a way that seems to provide a 'command' to the pedestrian. For example, 'Cross Howard Street now' would not be an appropriate message.
Messages also should not tell users that it is "Safe to cross."
Research resulted in recommended messages for the WALK interval, which include begining with the name of the street being crossed, for example, "Broadway, Walk sign is on to cross Broadway". Recommended messages begin on page 4–8 of this APS Guide.
Effect of Speech Messages on all pedestrians
Van Houten, Malenfant, Van Houten and Retting (1997) found that redundant information conveyed by audible pedestrian signals increases the attention of all pedestrians to turning traffic and may contribute to a reduction in pedestrian-vehicular conflicts and crashes at signalized intersections. Their research in Clearwater, Florida used prototype speech message technology in which speech messages were broadcast from the pedestrian signal head. When the pedestrian push button was pressed, the message was "Please wait for WALK signal." The message "Look for turning vehicles while crossing [street name]" began 200 msec before WALK signals were illuminated.
The signal also gave participants who were blind precise information about the onset of the WALK interval and which street had the WALK interval.
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