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101 A P P E N D I X A A key component of accessibility for a pedestrian who is blind is the availability of adequate audible cues to assure that a blind traveler can independently navigate the roundabout or CTL. The availability of audible cues is related to the presence of noise sources in the vicinity of the site, as well as obstacles that may interfere with the ability to clearly hear approaching vehicles. Such obstacles may include signs, poles, or landscaping, which may impact audibility in a matter similar to their impact on sight distances. However, a clear difference is that while these obstacles generally impede sight distances, they may in some cases improve audibility. For example, heavy landscaping in the splitter island may help separate audible cues from the two directions of traffic, and thus enhance audibility for a blind pedestrian waiting on the splitter island (NCHRP Report 674). In general, audibility is less understood than sight distance, which makes an audibility assess- ment more challenging due to limited available guidance. This section introduces concepts of audibility and high-level principles that should be considered in the design of a roundabout or a CTL. The analyst should identify and flag any concerns about the audible environment. The outcome is a yes/no check on whether audibility is likely to be compromised at the site. To date, no quantitative method exists to accomplish this, but some guidance is provided below. Location of Crosswalk Relative to Noise Sources The first and foremost audibility consideration is the location of the crosswalk relative to sources of noise. In the case of a CTL, the majority of traffic noise is generated at the main intersection. It is generally expected that smaller radius CTLs result in smaller channelization islands, which in turn place the pedestrian closer to that noise source. In a similar fashion, crossing from the channelization island to the curb is expected to have higher levels of interfering noise (from behind the pedestrian) than crossings from the curb to that island. For roundabouts, the separation between the crosswalk and the circulatory roadway impacts the level of noise at the crosswalk. Noise levels are further expected to be different between entry legs (quiet traffic slowing down in approach of the roundabout) and exit legs (louder traffic accelerating away from the roundabout). Similar to CTLs, the splitter island is expected to have exceptionally high levels of noise, with traffic traversing in front of and behind the waiting pedes- trian. Wider islands and landscaping on the island may help with reducing noise levels on the splitter islands, although this has not been documented in research. Landscaping further has the potential of limiting lines of sight from the driver to the pedestrian. Other noise sources that have a high impact on the ability to hear conflicting traffic may exist in the vicinity of the site; these make it difficult for a person to distinguish conflicting Discussion of Audible Environment and Noise Effects
102 Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities: A Guidebook traffic from background noise. Common examples of this include nearby freeways (especially at interchanges), work zones or construction activity, and general industrial activity. Noise levels are also oftentimes amplified in locations with a high percentage of trucks and other heavy vehicles. Considering Curvature and Directionality of Traffic A key commonality between roundabouts and CTLs is roadway curvature. Research has shown that pedestrians can have difficulties distinguishing noise generation from through traffic and turning traffic at a CTL, or exiting and circulating traffic at a roundabout exit leg (Ashmead et al., 2012). With trajectories of these movements being similar, the sound pat- terns generated are also similar. As such, a blind pedestrian waiting to cross at a CTL, or at the exit leg of a roundabout, will likely have a difficult time distinguishing between vehicles that conflict directly with the crosswalk from those that proceed through the main intersection or continue to circulate. Additional separation between the crosswalk and the point where the two trajectories separate is expected to enhance the ability to identify conflicting traffic accurately. Absolute and Relative Noise Levels One key principle in acoustics research is the difference between absolute and relative noise levels. Research on the ability of blind travelers to identify quiet hybrid vehicles, as well as internal combustion engine vehicles, was shown to be highly correlated to the level of ambi- ent noise (Emerson et al., 2015). In other words, even a âquietâ vehicle can be audible at low ambient noise levels. Similarly, even a âloudâ vehicle can be difficult to hear when the level of background noise is elevated. The notion of relative sound levels makes the audibility assessment of a new site difficult, as the designer needs to make assumptions about the level of ambient noise. For example, a very rural location is likely to have lower ambient noise levels than a busy downtown location, although unusual noise generators like agricultural equipment or industrial developments may pose an exception to that rule. Many audible traffic control devices and Audible Pedestrian Signal (APS) systems include adjustments for the level of ambient noise that increase the decibel level of the audible indication in loud environments. Impact of Grades There is some evidence that roadway grade may impact the audibility at the crosswalk. Specifically, a crosswalk located in a downhill portion may provide better acoustic informa- tion about an approaching vehicle than a crosswalk approached in an uphill section. This pattern was suggested by research performed at two CTLs on opposing approaches at a sig- nalized intersection in NCHRP Report 674. With the main roadway having a notable grade (3% to 4%), one CTL was approached by downhill traffic, while the other was approached by uphill traffic. Blind study participants and researchers noted that identical sound strip treatments installed in the CTL were more audible on the downhill section than on the uphill section. A potential explanation for this is that vehicle engine noises can propagate toward the crosswalk in a downhill approach, while the sound waves get trapped between the vehicle and the roadway on uphill approaches.
Discussion of Audible Environment and Noise Effects 103 Location and Separation of Traffic Control Devices The location of traffic control devices and the separation of two or more (audible) devices can impact audibility at the crosswalk, as well as how well the devices themselves can be heard and distinguished from each other. MUTCD provides specifications for installation of APS devices at signals, which should have a minimum separation of 10 feet between two devices or the installation of speech walk messages and additional features. This guidance applies at any location where APS are installed. For the placement of other traffic control devices like crosswalk signs, MUTCD specifies that the signs need to be placed adjacent to the crosswalk, but is silent on whether they should be placed on the upstream or downstream side. Prior research and significant feedback from blind travelers suggests that a downstream sign placement is preferable. Specifically, a downstream placement assures that the sign does not block the view or sound between the pedestrian and oncoming traffic. Impacts of Landscaping and the Built Environment As discussed above, landscaping can impact the audibility of a crosswalk in two critical ways. Landscaping can block critical audible information about an approaching and conflict vehicle and can thus have a harmful impact on audibility. However, landscaping can also block unwanted or distractive traffic noise (for example from behind the pedestrian, or from across the other side of the roundabout) and may thus have a positive impact on audibility. The built environment surrounding the crosswalk is similarly expected to impact audibility. The presence of (tall) buildings close to the crosswalk can cause traffic sounds to be reflected and amplified and thereby impact the ability to clearly distinguish directionality of conflicting traffic. Bridges or expressways nearby also affect audibility. References Ashmead, D. H., D. W. Grantham, E. S. Maloff, B. Hornsby, T. Nakamura, T. J. Davis, F. Pampel, and E. G. Rushing. 2012. âAuditory Perception of Motor Vehicle Travel Paths.â Human Factors, Vol. 54, No. 3, pp. 437â453. Emerson, R. S. W., D. S. Kim, K. Naghshineh, and K. Myers. 2015. âEffect of Artificial Alert Sound, Background Noise, and Vehicle Type on Detectability and Localizability of Quiet Cars by Blind Pedestrians.â ITE Journal. Vol. 85, No. 4, pp. 42â44.
