Research on the effect of particular APS features on the ability of pedestrians who are blind or visually impaired to make safe, efficient street crossings has been limited to a very few of the many variables that can affect crossing by blind pedestrians. Variables that have been somewhat systematically investigated are limited to:
In most experiments, the WALK indication has sounded during the entire crossing time. Yet, in the field, the WALK indicator usually sounds only for the four to seven seconds of the Walk interval.
In a typical installation, the APS sounds simultaneously from both ends of the crosswalk. The typical delay while blind pedestrians recognize the onset of the Walk interval and determine that it is safe to begin crossing, even when the intersection has an APS, means that they are seldom more than half way across a crosswalk when the APS ceases to provide the WALK indication. This means that they may never hear the sound coming from the APS on the destination corner, so it cannot be used for directional guidance. Recent and on-going research is manipulating the source of the WALK signal as a means to improve beaconing.
Much of the research that has been conducted has looked at the effect of the WALK signal on the speed and accuracy with which blind pedestrians make crossings, i.e., the effectiveness of the signal as a beacon. However at most intersections the APS WALK signal needs only to inform the user of the onset and end of the Walk interval.
In order to interpret the results of APS research, it is helpful to understand something about the hearing of blind pedestrians, the characteristics of vehicular sound, and the human ability to understand speech in noisy environments.
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 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; and 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 permitted by the MUTCD. The 89 dB maximum in the MUTCD is based on OSHA limits.
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), yet MUTCD limits the output of APS to 5 dB above ambient sound except when special actuation requests a louder beaconing signal for a single pedestrian phase.
Early research on WALK signal characteristics
Staffeldt (1968), in research cited by Hulscher (1976), conducted extensive testing of audible beacons at intersections and found that an 880 Hz signal was most detectable in a background of traffic noise.
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.
San Diego (1988) laboratory measurements of a "birdcall" signal from Nagoya Electric Works of Japan found that neither signal was highly directional. However the chirp was more detectable than the cuckoo. The chirp 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 "birdcall" signals may vary from this manufacturer's standard.)
Comprehensive research on the effect of one APS
Crandall and colleagues (Crandall et al., 1998; Crandall et al., 2001) compared the street crossing of 20 blind pedestrians at four intersections in downtown San Francisco without APS to crossings of the same intersections with the remote infrared audible sign technology, Talking Signs®. The intersections had pretimed signalization and no pushbuttons.
The following measures were made.
Recent research on WALK signal characteristics
Laroche, Giguère and Poirier (1999) compared localization of cuckoo and chirp 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 chirp and a melody with minimal harmonics were found to be less localizable than the cuckoo and the other three melodies.
Laroche, Giguère and Leroux (2000) compared the typical birdcall 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. The chirp 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.
Research on source of WALK signal
Stevens (1993) and Tauchi, Sawai, Takato, Yoshiura and Takeuchi (1998) tackled the problem of improving localization of WALK signals (beacons) by varying the source of the sound. They found that blind pedestrians could cross more quickly and with less veering when the WALK signal alternated back and forth from one end of the crosswalk to the other.
Laroche et al. (2000) confirmed the superiority and subjective preference for an alternating signal for beaconing at a simulated quiet intersection but found no advantage of the alternating signal when data were collected at an intersection with steady traffic on both streets. This was true for all tones tested (chirp, cuckoo, low cuckoo, and melody). It may have been that blind participants had such good directional information from vehicular sound that the APS were essentially irrelevant to their crossing.
Research on locator tones
Bentzen, Barlow, & Gubbé (2000), compared the speed of blind pedestrians on locating and walking to an APS with a locator tone (880 Hz square wave, with multiple harmonics, 3 ms attack time, 15 ms sustained tone) at three repetition rates and three loudness levels relative to traffic sound along an eight lane artery in Los Angeles. Best performance was with a repetition rate of 1/sec and loudness of 2-5 dBA above ambient sound measured at 1 m from the locator tone speaker.
Research on speech message structure and wording
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 pedhead.
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.
Recommended model messages are contained in WALK Indications and Other APS Features.
Effect of Speech Messages
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. Research in Clearwater, Florida, with prototype speech message technology in which speech messages were broadcast from the pedhead, indicated that voice messages can be used to increase the attention of all pedestrians to turning vehicles and to decrease pedestrian/motor vehicle conflicts at signalized intersections.
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.
Research in the U.K.
Wilson (1980) conducted a pre- and post- APS installation study of adult non-disabled pedestrian behavior at one intersection. Key results were as follows.
Experience of a New Zealand engineer
Hulscher (1976) cites a personal communication from Leith (1975) in which Leith estimated that delay in beginning crossings for all pedestrians was reduced an average of 2-3 seconds, for all pedestrians, by the use of APS.
There are a number of other variables that can affect the ability of APS users to hear, recognize, and localize WALK and locator tones, to effectively use pushbuttons, to determine which crosswalk has the Walk interval, and to complete crossings to a destination corner quickly and without undue veering.
Other variables that have had little or no research include:
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