Influence of Ambient Sound Fluctuations on the Crossing Decisions of Pedestrians Who Are Visually Impaired: Implications for Setting a Minimum Sound Level for Quiet Vehicles

Travelers who are visually impaired (that is, those who are blind or have low vision) use traffic sounds in many critical orientation and mobility tasks. For example, they often use a dip in traffic sound, which generally corresponds to the time of lower ambient sound, to identify an appropriate time to begin crossing at unsignalized intersections where no traffic lights are present (Hill & Ponder, 1976). The acoustic characteristics of the ambient sound as well as those of the vehicles seem to affect an individual's ability to perform orientation and mobility tasks; this is because signals (for instance, target vehicle sound) are always detected against a background of activity (for instance, ambient noise) (Macmillan & Creelman, 2004). Yet the focus of most previous studies on the impact of quiet cars such as hybrid electric vehicles has been on examining acoustic characteristics of those vehicles and those of artificial alert sounds added to them. Findings from a series of field studies (in proving grounds, parking lots, and on streets) that tested the effect of adding artificial alert sounds to hybrid vehicles indicated that the vehicles with added alert sounds could be detected at a significantly farther distance or with a larger safety margin (Kim, Wall Emerson, Naghshineh, Pliskow, & Myers, 2012a), but the benefit of adding an alert sound was less clear for identifying right-turning vehicles from the ones that were proceeding in a straight line (Kim, Wall Emerson, Naghshineh, & Myers, 2012b; Wall Emerson, Kim, Naghshineh, Pliskow, & Myers, 2011). Although examination of ambient sounds was not the primary focus, of a study conducted on a very quiet vehicle testing track, Wall Emerson, Kim, Naghshineh, Pliskow, and Myers (2011) reported that ambient sound level was one of the significant independent predictors of the distance at which the approaching vehicles could be detected by pedestrians who are visually impaired. In response to the concerns expressed by blind consumer organizations, Congress passed a law-the Pedestrian Safety Enhancement Act of 2010 (S. 841)-that requires new hybrid and battery electric vehicles to be equipped with an alert sound that would be emitted in certain low-speed conditions beginning in 2015, and mandated the National Highway Traffic Safety Administration (NHTSA) to establish performance requirements for an alert sound that allows pedestrians, including those who are visually impaired, to reasonably detect a nearby electric or hybrid vehicle operating below a crossover speed. The Pedestrian Safety Enhancement Act spurred a flurry of discussions between the auto manufacturers and NHTSA on how to set a minimum vehicle sound level for hybrid and battery electric vehicles. However, these discussions have rarely included environmental factors such as ambient sound level as one of the key factors in determining a desired minimum vehicle sound level. Continued neglect of including the effect of ambient sound in these discussions stems partly from the paucity of information on ambient sound patterns in the environment blind pedestrians often travel. In fact, we have not found any published studies that investigated ambient sound patterns at different types of intersections as they affect the ability of pedestrians who are visually impaired to negotiate these intersections. [FIGURE 1 OMITTED] The purpose of the first study presented here was to systematically investigate and describe the patterns of ambient sound level fluctuations at selected intersections. The aim of the second study was to examine the timing and performance of critical street-crossing decisions by blind pedestrians at these intersections. Study one (ambient sound patterns) METHODS Apparatus A sound level meter (Larson-Davis System 824A) connected to a microphone was used to continuously record ambient sound at selected intersections. …