Mechanical and Physical Risk Prevention S Studies and Research Projects REPORT R-833 Safety of Workers Behind Heavy Vehicles Assessment of Three Types of Reverse Alarm Véronique Vaillancourt Hugues Nélisse Chantal Laroche Christian Giguère Jérôme Boutin Pascal Laferrière The Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST), established in Québec since 1980, is a scientific research organization well-known for the quality of its work and the expertise of its personnel. OUR RESEARCH is working for you ! M ission To contribute, through research, to the prevention of industrial accidents and occupational diseases and to the rehabilitation of affected workers; To disseminate knowledge and serve as a scientific reference centre and expert; To provide the laboratory services and expertise required to support the public occupational health and safety network. Funded by the Commission de la santé et de la sécurité du travail, the IRSST has a board of directors made up of an equal number of employer and worker representatives. To find out more Visit our Web site for complete up-to-date information about the IRSST. All our publications can be downloaded at no charge. www.irsst.qc.ca To obtain the latest information on the research carried out or funded by the IRSST, subscribe to Prévention au travail, the free magazine published jointly by the IRSST and the CSST. Subscription: www.csst.qc.ca/AbonnementPAT Legal Deposit Bibliothèque et Archives nationales du Québec 2014 ISBN: 978-2-89631-745-5 (PDF) ISSN: 0820-8395 IRSST – Communications and Knowledge Transfer Division 505 De Maisonneuve Blvd. West Montréal, Québec H3A 3C2 Phone: 514 288-1551 Fax: 514 288-7636 [email protected] www.irsst.qc.ca © Institut de recherche Robert-Sauvé en santé et en sécurité du travail, July 2014 S Mechanical and Physical Risk Prevention Studies and Research Projects REPORT R-833 Safety of Workers Behind Heavy Vehicles Assessment of Three Types of Reverse Alarm Disclaimer Véronique Vaillancourt1, Hugues Nélisse2, Chantal Laroche1, The IRSST makes no guarantee as to Christian Giguère1, Jérôme Boutin2, Pascal Laferrière1 the accuracy, reliability or completeness of the information in this document. 1Audiology and Speech–Language Pathology Program, Under no circumstances may the IRSST be held liable for any physical or Faculty of Health Sciences, University of Ottawa psychological injury or material damage 2Mechanical and Physical Risk Prevention, IRSST resulting from the use of this information. Document content is protected by Canadian intellectual property legislation. A PDF version of this publication is available on the IRSST Web site. This study was funded by the IRSST. The conclusions and recommendations are solely those of the authors. This publication is a translation of the French original; only the original version (R-763) is authoritative. PEER REVIEW In compliance with IRSST policy, the research results published in this document have been peer-reviewed. IRSST - Safety of Workers Behind Heavy Vehicles: Assessment of Three Types of Reverse Alarm i ACKNOWLEDGMENTS The research team would like to thank the members of the follow-up committee for their assistance, support and thoughtful advice, as well as François Ouellet of the IRSST, for liaising between the committee members and the researchers. We are grateful to everyone who helped in one way or another in carrying out the on-site trials. Our thanks also go out to all the participants who took part in the subjective laboratory trials in Ottawa. Finally, we especially wish to express our gratitude to Yves Morissette of the Graymont Company and Gervais St-Pierre of the ASSIFQ-ASSPPQ for their assistance with the on-site trial logistics. ii Safety of Workers Behind Heavy Vehicles: Assessment of Three Types of Reverse Alarm - IRSST SUMMARY New technology for heavy vehicle reverse alarms has recently appeared on the market. According to the manufacturer, this technology, based on the use of a broadband signal (shh…shh…shh signal), is safer for workers and causes less noise pollution than the conventional tone alarm (beep…beep…beep signal). However, it is difficult to say whether this technology is better, because of the paucity of independent and rigorous scientific studies on the subject. This study compared the new broadband alarm technology with conventional alarms from the standpoint of worker safety. Two sets of trials were carried out to make the comparisons. In the first, a field trial, the sound propagation fields generated by the alarms behind heavy vehicles were measured to study their uniformity under conditions similar to those found in the workplace. In the second, human participants carried out psychoacoustic tests in the laboratory. They performed tasks related to alarm perception (detection thresholds, equal loudness, perceived urgency and sound localization). Through an analysis of the alarm signals, the broadband alarm was deemed compliant with the SAE J994 standard, which is the standard most commonly used to certify alarms installed on heavy vehicles. In addition, the overall results of both field and laboratory trials did not reveal any contraindications to the use of the broadband reverse alarm with respect to worker safety. This type of alarm provides a much more homogeneous sound field behind vehicles and is easier to locate spatially, particularly in the front/rear dimension. The potential advantages of a tonal alarm (better detection under noisy conditions and a slightly greater sense of urgency conveyed in certain situations), would probably not overcome the adverse effect of major spatial variations in sound levels found over short distances behind a vehicle with this alarm (on the order of 15 to 20 dB), which are noticeably more pronounced than those generated by the broadband alarm. In this report, the effect of parameters such as ambient noise, the use of hearing protection devices (HPD), and the type of protectors worn (ear muffs versus earplugs) on psychoacoustic indicators is presented for both alarm types. Finally, recommendations have been formulated to ensure the optimal use of broadband alarms, and important aspects requiring more extensive investigation are identified. IRSST - Safety of Workers Behind Heavy Vehicles: Assessment of Three Types of Reverse Alarm 1 1. INTRODUCTION Given the high number of accidents every year involving reversing vehicles, it is critical to ensure that reverse alarms are optimally designed so that people working nearby are alerted promptly, while limiting noise annoyance for people farther away who are not at risk. Auditory warning sounds have an advantage over visual warnings because they usually capture people’s attention no matter where they are looking. However, accidents can occur in the workplace when alarms are not well detected by workers, due to masking by noise, when hearing protection devices are worn, or when the auditory signal emitted is difficult to locate and thus does not prompt the worker to react adequately by moving in the right direction. Accidents also occur when alarms are ignored, particularly when they often go off without signalling any real danger or emergency, or when they are set at annoyingly loud levels such that people prefer to deactivate them. The most commonly used reverse alarms in Québec and elsewhere have traditionally consisted of either a single pure tone (most popular) or a warble tone. However, various groups of researchers and workplace stakeholders have voiced concerns about the use of these types of tonal alarm with respect to both worker safety and noise pollution. A relatively recent reverse alarm technology using a broadband signal has been developed to overcome the main problems related to conventional tonal alarms. This technology has been marketed and a range of broadband reverse alarms is now available. The advent of this new technology raises an important question for workplaces: will the use of these alarms instead of conventional alarms significantly improve the detection and localization of reversing vehicles as well as creating a sufficient sense of perceived urgency to ensure safety, while limiting the annoyance factor? Already, some workplaces in Québec have expressed an interest in and a desire to implement this technology. However, it remains difficult to draw conclusions about its superiority over other types of alarms, as there are few independent and rigorous scientific studies demonstrating improved worker safety when using the broadband alarm. This report presents the results of a two-part study comparing the new broadband alarm technology with that of conventional alarms. In the first part, the field trial, the sound propagation field generated by the alarms was measured behind heavy vehicles. In the second part, the laboratory trial, psychoacoustic measurements were carried out on human participants and targeted various aspects related to the perception of reverse alarms (detection thresholds, equal loudness, perceived degree of urgency and sound localization). The study focused primarily on workers’ health and safety rather than on noise pollution. The following sections present a review of the literature, the objectives of the study, the methodology used and the results obtained. A discussion and conclusion wind up the report. 2 Safety of Workers Behind Heavy Vehicles: Assessment of Three Types of Reverse Alarm - IRSST 2. CURRENT SITUATION 2.1 Occupational Safety and Health (OSH) issues associated with heavy vehicle reversing According to the motor vehicles subpart 1926.601(b)(4) in the US Occupational Safety and Health Administration’s Safety and Health Regulations for Construction (OSHA, 2000), heavy truck drivers whose rear view is obstructed must operate with a functional reverse signal alarm that is audible above the surrounding noise level or must only back up the vehicle when an observer signals that it is safe to do so. In Québec, two sections in the Safety Code for the construction industry (updated on October 1, 2011) deal with warning devices. Section 3.10.5 states that a signalman is required when a vehicle is driven in reverse “if such a move may create a hazard for any person” or if the view of the driver is obstructed. Paragraph 2 of section 3.10.12 provides a list of vehicles that must have automatic warning horns for the reverse gear, with “a noise intensity that is superior to the noise of the equipment on which it is installed and have a distinct sound.” In addition, “if the warning horn is electric, it must conform to SAE Standard J994b-1974 Performance, Test and Application Criteria for Electrically Operated Backup Alarm Devices.” Despite such guidelines, every year there are more accidents and mortalities involving heavy vehicles driving in reverse (Laroche et al., 1995; Murray et al, 1998; NIOSH, 2004; Blouin, 2005) in Québec and elsewhere. A recent accident (September 2, 2011), on a construction worksite for Highway 30, in which a surveyor was run over by a reversing truck, points to serious shortcomings in workplace safety. Despite clearly formulated guidelines by the Commission de la santé et de la sécurité au travail (CSST) following the accident, an investigation revealed that many reversing manoeuvres were still being carried out without a signal person and that some vehicles were not yet equipped with a reverse alarm (CSST, 2011). Almost a quarter of all deaths involving work vehicles take place when the vehicle is reversing (HSE, 2001). Moreover, from accident reports published by OSHA from 1972 to 2001, Purswell and Purswell (2001) estimated that approximately 43% of the 150 reported accidents that involved vehicles occurred despite the reverse alarm being in good working order at the time. In Québec, Laroche and colleagues (Laroche et al, 1991, 1995; Laroche and Denis, 2000), using the CSST computerized data bank (www.centredoc.csst.qc.ca), identified 25 fatal accidents caused by reversing vehicles in Québec between 1975 and 1991, of which 15 occurred on construction sites. The construction industry therefore appears to be especially affected by this problem. According to the US Bureau of Labor Statistics, 6% of all fatal accidents (397 deaths) in the construction industry in 2002 were due to vehicles backing over workers (Seattle District Safety Gram, 2009). A summary table of the 19 backover fatalities that occurred on construction sites in the United States between 1992 and 2007 is presented in Appendix A. In most cases, the alarm was functional and operating during the accident and the vehicle was moving at speeds of less than 5 mph. Accidents may occur in noisy work environments when audible warning devices do not attract attention, either because they are not heard or because they are ignored, for example, when an alarm often goes off without signalling any real danger or urgency (habituation phenomenon). In IRSST - Safety of Workers Behind Heavy Vehicles: Assessment of Three Types of Reverse Alarm 3 other cases, reverse alarms are so loud and irritating that they are deactivated. Additional factors that may contribute to the non-perception of reverse alarms include hearing loss, masking by ambient noise and inappropriate installation of the alarm on heavy vehicles (Laroche and Lefebvre, 1998), as well as the use of hearing protection devices (HPD). Serious concerns about the effectiveness of conventional reverse alarms in conveying an appropriate sense of urgency in the critical zone behind heavy vehicles can therefore be raised. A number of factors can contribute to the effectiveness of alarms, including the alarm’s frequency content, the workers’ hearing status, masking by ambient noise, the habituation phenomenon, signal recognition, reaction times, the degree of urgency conveyed by the signal, the ability to localize the signal and the signal’s sound propagation pattern (Morgan and Peppin, 2008). Alarms must convey information that will address three important questions (Catchpole et al, 2004): What is the danger? Where is the danger? When is it a danger? In the literature, three major problems associated with conventional reverse alarms are noted: a difficulty in localizing the sound, the non-uniformity of the sound propagation pattern behind the vehicle, and noise pollution. • Sound localization The "beep, beep, beep" of a vehicle in reverse gear is familiar to everyone, but people are often uncertain as to where the sound is coming from. Emergency vehicle sirens are another convincing example of signals that are difficult to localize and that often cause confusion as to whether the vehicle is approaching from the front, behind, right or left. Despite the broad range of frequencies that humans can hear (from 20 to 20,000 Hz), important localization cues are found mainly in frequencies lower than 1500 Hz and higher than 3000 Hz. For sounds below 1500 Hz, the main cue for differentiating between sounds in the left/right dimension is the interaural time difference (ITD), while for sounds in ranges higher than 3000 Hz, it is the interaural intensity difference (IID) that matters. A final category of spectral cues making use of high frequency (> 5000 Hz) information enables sources in the front to be distinguished from those behind and to determine the degree of elevation of the source (Middlebrooks and Green, 1991; Carlile and King, 1993; Blauert, 1997; Hartmann, 1999). In theory, broadband spectrum alarms are easier to localize because they provide a greater number of cues (ITD, IID, spectral cues) compared to tonal signals, where the frequency spectrum is limited, such as conventional emergency vehicle sirens and reverse alarms. Indeed, conventional reverse alarms typically have a dominant frequency between 1000 and 4000 Hz (Laroche and Lefebvre, 1998), a frequency region in which few localization cues are available.1 Furthermore, the SAE J994 (2009) standard recommends a predominant frequency between 700 Hz and 2800 Hz for reverse alarms. Confusion in identifying the position of the sound source may lead to a delayed response from workers, when a timely reaction is often critical to avoid danger. Finally, from a workplace health and safety perspective, the effects of HPD must also be 1 The spectral content of tonal and broadband alarms is presented in Figure 2. 4 Safety of Workers Behind Heavy Vehicles: Assessment of Three Types of Reverse Alarm - IRSST taken into consideration, because they can further compromise the ability to localize sound (e.g., Tran Quoc and Hétu; Bolia et al 2001; Berger, 2003; Simpson et al, 2005). • Sound propagation Difficulties in detection and confusion as to the position of the source can also be attributed to abrupt spatial variations in the sound pressure levels of the alarm over very short distances behind heavy vehicles. This issue has been well documented with tonal reverse alarms (Laroche et al, 2006). The uneven pattern of sound propagation behind heavy vehicles due to sound wave interference (reflection and diffraction) can lead workers to underestimate or overestimate the distance and direction of a vehicle that is outside their field of vision. This issue has been identified as a probable contributing factor in a fatal accident on a highway construction site near Montréal in 2003 (Laroche, 2006). Figure 1, taken from work by Laroche et al (1995), illustrates the sound pressure levels of an alarm behind an immobile vehicle as a function of distance. At distances of less than 2 m from the vehicle, enormous fluctuations in sound pressure levels can be noted, and reach up to 15 dB over just a few cm. Furthermore, sound pressure levels do not decrease steadily with increasing distance. axis #5 Figure 1: Example of sound pressure levels measured behind a heavy vehicle as a function of distance when a pure tone reverse alarm is in operation. In addition to a non-uniform propagation pattern, the sound of tonal alarms can travel over large distances, well beyond the hazard zone immediately behind a heavy vehicle. A false alarm occurs when an alarm is heard outside of the hazard zone, and can lead to a dissociation between the alarm and the danger, thus affecting its efficacy (Morgan and Peppin, 2008; Bliss et al 1995; Bliss and Dunn, 2000; Holzman, 2011). In fact, Bliss et al (1995) have shown that the response rate to an alarm by people involved in a cognitive task closely mirrors the alarm’s reliability rate. If the sound signal is associated with a high rate of false alarms, for example, 75% (the alarm directs attention to a real danger situation only 25% of the time), most people will respond to the signal only 25% of the time. Workplace safety may then become markedly jeopardized by perfectly audible signals that have lost their effectiveness in transmitting a danger warning because of the number of false alarms.