Adjustments
Introduction
Devices should be carefully adjusted in the field and evaluated after installation to be sure they are working properly from an engineering perspective and from the perspective of pedestrians who are visually impaired.
- If the APS has been added in response to a request from a pedestrian who is blind or visually impaired, that individual should also be involved in evaluation after installation.
- Because installers may be unfamiliar with new types of APS devices, extra supervision and attention will be required during the first few installations by any crew or contractor.
- Even when carefully specified, installations sometimes do not match the specifications because installers do not understand that failure to exactly follow specifications may lead to an installation that cannot be accessed by pedestrians who use wheelchairs, or that could cause a pedestrian who is blind to push the wrong pushbutton, to veer into the center of the intersection, or mistake which crosswalk has the WALK interval and start crossing at an unsafe time.
The sound level of the speakers must be carefully set and evaluated at the time of installation, and then checked at a time with different traffic volumes to assure that settings are correct. It is better to install APS with volumes that may be too low and adjust upwards as needed. If volumes are set too high initially, problems can arise with neighboring residents and businesses that can lead to the removal of the APS and difficulties for future installations.
Setting and evaluating sound levels
Figure 7-9. Resetting APS settings with handheld device supplied by manufacturer
How to adjust APS sound levels
The controls to adjust the settings of the APS vary among the different manufacturers. The settings may be adjusted by:
- Screws on control board, which may be mounted in pedhead or in the APS speaker unit.
- Remote handheld device, using infrared communication, that must be held in close range to the APS
Settings
Most devices require setting:
- Microphone sensitivity or automatic gain control (AGC) sensitivity,
- Volume of the pushbutton locator tone, and
- Volume of the WALK indication.
The microphone sensitivity or AGC controls how the other tones/message volumes respond to ambient noise levels. The setting must be adjusted by the installer to provide output at one of a number of ranges between the maximum and minimum. The number and width of ranges varies by manufacturer.
How loud should the sound be?
In general, installers have a tendency to set volume levels of devices too loud. Loud overhead audible signals have been problematic to neighbors of APS installations.
In addition, the loud sound of the signal may prevent pedestrians who are visually impaired from:
- Hearing critical traffic sounds used for alignment
- Determining that cars have stopped
- Hearing cars that may be turning across their path
- Localizing on the signal source
Installers should set volume levels according to the following guidance:
- The WALK indication must be audible from the beginning of the associated crosswalk (MUTCD Section 4E.11, P4 Standard).
- The MUTCD (Section 4E.12, P6 Standard ) requires that pushbutton locator tones be responsive to ambient sound levels and audible 6 to 12 feet from the pushbutton, or to the building line, whichever is less.
- Manufacturers typically set a default maximum and minimum output level on APS devices. The settings should be checked .
- At no time should sound be more than 5 dB above ambient sound (except by special actuation for audible beaconing) (MUTCD Section 4E.11, P9).
Figure 7-10. Illustration of pedestrian signals at an intersection with split phasing
Figure 7-11. Channelized turn lane illustration
Volume level considerations
The correct setting will vary depending on whether there are buildings close to the APS, split phasing, or slip lanes.
- When buildings are close to the APS, the sound reflected from the buildings will make the sound seem louder. The reflected sound may also influence the microphone and automatic gain control such that the APS will sound louder for the same setting than if the APS was in an open area.
- At intersections having split phasing, APS at parallel crosswalks must not be audible across the street (at the other parallel crosswalk), or users may begin crossing with the wrong WALK signal. Check this at times of low ambient sound as well as at times with normal sound. In Figure 7-10, the APS giving the WALK indication on corner B should not be heard by someone standing on corner A.
- APS at intersections having channelized turn lanes must not be audible from the corners of the intersection. If an APS on the splitter island is too loud, pedestrians who are blind may believe the turn lane is signalized, or that the intersection crosswalk extends all the way to the corner. If the volume is too loud, pedestrians might assume that they have a WALK indication to begin crossing, when, in fact, they may be entering an uncontrolled slip lane, yield or stop sign controlled slip lane, or a separately signalized turn lane. In Figure 7-11, a pedestrian standing on the corner at location A should not be able to easily hear an APS that is located on the splitter island.
Automatic volume adjustment
Pre-set automatic volume adjustment or automatic gain controls cannot assure that the volume meets the criterion for distance at which the APS should be audible. Similar automatic volume adjustment settings on APS by different manufacturers may seem to provide quite different loudness, as judged by listeners.
- Automatic volume adjustment technology used by different manufacturers varies in the rate of sampling of ambient sound and in the speed with which output adjusts to changes in ambient sound.
- Some APS and some installations will be more subject to responding to their own noise than others. For example, as the WALK signal continues throughout the WALK interval, the signal may get louder and louder in response to its own noise.
- Different tones or speech will seem louder or quieter depending on their frequency content, although they may measure the same on the dBA scale.
Measuring the sound level
Due to the short duration of pushbutton locator tone and WALK tone pulses, conventional analog or digital sound level meters are not able, in the crosswalk environment, to accurately measure the absolute sound level (dBA) of APS tones, or the sound level of APS tones relative to ambient sound.
At present, setting and evaluation of APS sound level is typically done by ear. There can be errors in making the sound too quiet or too loud. It is critical for the WALK indication to be loud enough to be audible at the crosswalk waiting location. Both the locator tone and WALK indication should be audible within 6 to 12 feet of the device, but not more than that. It has been the authors' experience that many APS installations have been set louder than was optimal either for blind pedestrians or APS neighbors.
Measuring sound where audible beaconing is needed
At crosswalks where audible beaconing is needed, sound level should be evaluated from the middle of the street, when the loud WALK indication has been called, to be sure beaconing will be provided throughout the crossing. However, current MUTCD language limits the maximum output of APS to 100 dB, and most manufacturers pre-set this maximum. Therefore, at exceptionally wide crossings, and when and where there is high ambient sound, there may be a distance in the middle of the crosswalk where the beaconing is not readily heard.
Microphone location and effect on perceived loudness
APS devices that respond to ambient sound have microphones to pick up the ambient sound.
- Microphones for pedhead type devices are typically in or on the pedhead, incorporated into the APS.
- Pushbutton integrated devices may have microphones at the pedhead or the microphones may be incorporated into the pushbutton housing
An APS microphone should be mounted as close as possible to the position of the pedestrian who is waiting to cross the associated crosswalk, because sound pressure is halved for each doubling of the distance from the sound source in a free field.
The farther from that ideal position the microphone is for a given APS device, the greater will be the following problems.
- The ambient traffic sound when a microphone is located on a signal pole 10 feet from the curb line will be quieter at the microphone than at the street; therefore the resulting output will be quieter than it would be if the ambient sound were measured near the street.
- The WALK indication, if it comes from the same pole location, may already be too quiet because the ambient sound level has been measured too far from waiting pedestrians, and it will be quieter still when its sound reaches the ears of pedestrians waiting at the crosswalk because the sound has traveled farther to reach them.
- If the microphone is closer to the intersecting street than to the street the pedestrian is waiting to cross, or the microphone is oriented toward the intersecting street, it will respond to the traffic sound on that street instead of the street the pedestrian is waiting to cross. This may result in WALK signals that are too loud or too quiet as perceived by pedestrians waiting to cross.
Ideal microphone location
The best location for the microphone is as close as possible to the position of pedestrians who are waiting to cross the associated street. This results in pedestrians being clearly able to hear APS signals with no need to set the automatic volume adjustment so high that sound levels will be too high or that APS neighbors will be annoyed.
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