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Loudness interacts with semantics in auditory warnings to impact rear-end collisions
Transportation Research Part F 14 (2011) 36–42
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Transportation Research Part F journal homepage: www.elsevier.com/locate/trf
Loudness interacts with semantics in auditory warnings to impact rear-end collisions Carryl L. Baldwin a,⇑, Jennifer F. May b a b
George Mason University, 4400 University Drive, Fairfax, VA 22030, United States Old Dominion University, Norfolk, VA 23529, United States
a r t i c l e
i n f o
Article history: Received 15 January 2010 Received in revised form 16 August 2010 Accepted 6 September 2010
Keywords: Collision warning systems Auditory alarms Driver response Acoustic Semantic
a b s t r a c t This study examined the impact of semantic and acoustics parameters of in-vehicle collision warning system (CWS) alarms on driver response. Thirty participants drove a simulated vehicle through scenarios containing five different unexpected hazard events. As drivers approached the hazard event one of four CWS alarms, counter balanced with the hazard event type, or no alarm (control) was presented. Alarms consisted of the signal word ‘‘Notice” or ‘‘Danger” presented at either 70 or 85 dBA. Rear-end collision events resulted in the highest crash rate, accounting for 45.4% of all crashes. In these scenarios, CWSs significantly reduced crash rates. CWS alarms with an intermediate urgency level achieved through an interaction of semantics and acoustics (‘‘Danger” at 70 dB and ‘‘Notice” at 85 dB) resulted in significant reductions in crash probability. Providing an extremely urgent signal word, ‘‘Danger” at a high acoustically urgent presentation level – 85 dB was not effective in reducing crashes, nor was a low urgency signal word, ‘‘Notice” presented at a low acoustical urgency level – 70 dB. Implications of these results for the design and implementation of CWS systems and auditory alarms in general, are discussed. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Driver distraction and inattention are a persistent threat to automotive safety – contributing to a substantial number of collisions annually (Stutts et al., 2005). The increasing prevalence of personal portable electronic devices in conjunction with emerging in-vehicle technologies may lead to even greater driver distraction. Collision warnings systems (CWSs) that effectively draw a driver’s attention to critical roadway incidents have the potential to reduce both the severity and rate of occurrence of traffic collisions (Ben-Yaacov, Maltz, & Shinar, 2002; Brown, Lee, & McGehee, 2001; Chang, Lin, Hsu, Fung, & Hwang, 2009; Lee, McGehee, Brown, & Reyes, 2002; Maltz & Shinar, 2007). A key, of course, is that CWSs must effectively draw a drivers’ attention if they are to be useful. Most of us have probably had the experience of a passenger in the car suddenly yelling out something like, ‘‘Look Out” while we were driving. The startle response initiated by such an action is often the most dangerous aspect of the situation. When startled, a driver can be expected to tightly grab the steering wheel and initiate a braking response while concurrently scanning the environment for the source of the trouble. Unfortunately, such a response is not always the optimal one. Startle responses can be elicited from sudden acoustic stimuli of at least 85 dBA (Blumenthal, 1996). In laboratory settings the startle response is most frequently associated with the eye-blink response as measured by electromyographic (EMG) activity. However when driving, in addition to an eye-blink response, the prepotent response would more typically be a
⇑ Corresponding author. Tel.: +1 703 993 4653. E-mail address: [email protected] (C.L. Baldwin). 1369-8478/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.trf.2010.09.004
C.L. Baldwin, J.F. May / Transportation Research Part F 14 (2011) 36–42
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braking response. Any time spent recovering from the startle effect is lost for responding to a critical incident. At the same time that it is critical that a CWS alert not startle the driver, it must be detectable in the ambient background noise of the driving situation. Acoustic parameters such as the loudness, pitch, speed, pulse rate and duration have been shown to affect the perceived urgency of auditory warnings (Haas & Casali, 1995; Hass & Edworthy, 1996; Hellier, Edworthy, & Dennis, 1993; Momtahan, 1990; Wiese & Lee, 2004). The intensity of a sound in conjunction with the background noise level is the key determinant of whether or not an auditory warning will be detected (Patterson, 1982, 1990). Less attention has been systematically placed on the influence of alarm intensity, subjectively perceived as loudness, on perceptions of urgency. In support of the influence of loudness, Momtahan (1990) observed that sounds presented at 90 dB were rated as significantly more urgent, regardless of their interpulse interval, number of harmonics, spectral shape or fundamental frequency, than sounds presented at 75 dB. Though there is some evidence indicating that loudness impacts an auditory alarms’ perceived urgency and annoyance (Haas & Edworthy, 1996; Momtahan, 1990), loudness is generally examined in conjunction with other parameters rather than systematically manipulated across parameters. Words spoken in a more urgent style are louder and are easily perceived as more urgent by the listener (Edworthy, Hellier, Walters, Clift-Mathews, & Crowther, 2003; Hellier, Edworthy, Weedon, Walters, & Adams, 2002). However, words spoken in an urgent style have a number of other differences relative to words spoken in a neutral style. They also tend to have higher pitch and less distinct phonemic distinctions (Grommes & Dietrich, 2002). The semantic characteristics of verbal signal words have been shown to affect the perceived urgency of auditory warnings in numerous investigations (Edworthy et al., 2003; Hellier et al., 2002). In an early examination of the perceived urgency of verbal warnings, people were asked to listen to various signal words presented under one of three voice styles (monotone, emotional, and whisper) at one of two presentation levels – (60 dBA and 90 dBA) spoken by a male or female and to judge their connoted hazard level (Wogalter, Kalsher, Frederick, Magurno, & Brewster, 1998). The signal word, ‘‘Danger” received a higher hazard rating than ‘‘Warning” and ‘‘Caution”, which did not differ from each other. ‘‘Notice” received the lowest hazard rating. The hazard level (also termed the perceived urgency) of spoken signal words has been confirmed by subsequent research (Edworthy et al., 2003; Hellier, Wright, Edworthy, & Newstead, 2000; Hellier et al., 2002). Further, acoustic and semantic characteristics have been shown to interactively impact ratings of perceived urgency (Hellier et al., 2002). In the visual domain, colour has been shown to interact with semantics to impact urgency ratings (Braun, Sansing, Kennedy, & Silver, 1994). Braun et al. suggested that these interactions could be used to establish equal levels of perceived urgency using different combinations of colour and signal word. It is conceivable that a similar procedure could be used to determine the urgency associated with different combinations of signal word and acoustic parameters (i.e., loudness). However, it must be noted that relatively few investigations have examined these issues when listeners were engaged in a concurrent task. Rather, ratings are commonly obtained by asking listeners to make a series of multiple psychophysical judgments. Providing listeners with a contextual description for a particular alarm (i.e., collision warning versus email) has been shown to affect ratings of both urgency and annoyance (Marshall, Lee, & Austria, 2007). In one notable exception, participants responded as quickly as possible and then rated warnings while engaged in a low fidelity simulated driving task (Baldwin, in press). Participants were asked to acknowledge the warnings as quickly as possible by making a key press without disrupting their lane keeping ability. They subsequently provided ratings of perceived urgency, alert effectiveness and annoyance for warnings that varied independently in both semantic content and loudness. An interaction was observed between the semantic content of the messages and the loudness at which the messages were played. The interaction affected both response time to the initial acknowledgement key press and subjective ratings of urgency and annoyance. Regardless of the content of the CWS warning, when it was presented at least 8 dB above ambient background noise, participants responded more quickly relative to the lower presentation level conditions. At intermediate loudness levels, participants’ response time differed as a function of the semantic content. They responded significantly faster to warnings of potential collisions relative to regulatory notices regarding their speed. Providing alarms at loudness levels at least 8–10 dB above ambient background noise levels in the vehicle are essential to ensuring their audibility among older drivers (Baldwin, 2002) and drivers who may be distracted by other acoustic events (i.e., personal music or communication systems). Recent research indicates that older drivers may be receptive to new invehicle technologies aimed at reducing collision potential (Creaser, Rakauskas, Ward, Laberge, & Donath, 2007) and that older drivers may benefit even more than younger drivers from well designed CWS alarms (Baldwin & May, 2006; Maltz & Shinar, 2007). The goal of the present investigation was to examine the relative impact and potential interaction of two key characteristics of verbal warnings, an acoustic characteristic (loudness) and a semantic characteristic (signal word). Specifically, we sought to determine if the semantic characteristics of a CWS alert could be used to impact a driver’s response to a high collision-risk situation in a realistic driving simulation. Secondly, we sought to examine the relationship between semantic and acoustic alarm parameters and determine if the parameters would exert independent or interactive effects. Finally, we sought to determine if semantics could be used to effectively alert without startling drivers. 1.1. Current investigation For the current investigation, two signal words (‘‘Danger” and ‘‘Notice”) were chosen in an effort to represent semantically high and low urgency alarms, respectively. Each signal word was presented at one of two intensity levels, a low urgency level of 70 dBA or a high urgency level of 85 dBA.
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Based on previous literature (Barzegar & Wogalter, 1998; Hellier et al., 2000) it was predicted that the signal word ‘‘Danger” would have a greater alerting effectiveness than the signal word, ‘‘Notice” thus resulting in faster response time. It was also predicted that a relatively high 85 dB intensity would result in faster response time than the less urgent 70 dB intensity. Further, based on previous research (Baldwin, in press) both acoustic and semantic aspects of the CWS alarm were expected to impact driver’ response such that the high urgency alarm (Danger at the high intensity) was expected to result in the fastest reaction time (foot off the accelerator) but more moderate urgency level alarms (Notice at 85 dB and Danger at 70 dB) were expected to result in the most appropriate response, thus reducing crash probability. 2. Methods 2.1. Participants Thirty volunteers (57% females) between the ages of 18–26 years (mean age 21.1 years) participated in this investigation. All participants had self-reported normal or corrected to normal vision and hearing as well as a valid driver’s license. All participants gave written informed consent in accordance with the ethical guidelines for treatment of human subjects as approved by a University Institutional Review Board. Participants received a small amount of research participation credit for participating in the investigation. 2.2. CWS alarms The signal words, ‘‘Notice” and ‘‘Danger” spoken by a female at a normal conversational level were digitally prerecorded and saved as .wav files. Using a commercially available sound editing program, the .wav files were then amplified to achieve the desired loudness levels approximating 70 and 85 dBA. The ambient background noise from the driving simulator with the engine running and while driving at 35 mph was 55 dB on average. Thus, the 70 and 85 dB loudness levels resulted in signal-to-noise ratios (SNRs) of roughly +15 dB and +30 dB, respectively. When the participant’s vehicle passed into a pre-specified zone, the collision event was triggered. In similar fashion, the CWS alarms were triggered when the participants’ vehicle crossed a second specific point in the simulation. This method ensured that all participants’ were provided the CWS alarm at the same point in the scenario regardless of their individual speed or lane position. Alarms were presented via the driving simulator through speakers while the participant drove through the pre-designed scenario. Each participant drove through five different scenarios, each containing one potential collision event or hazard paired with one of the four CWS alarms or the control. All combinations of each CWS alarm were paired with each hazard scenario and all combinations were presented across the experiment. But, individual participants only encountered each alarm once and only drove through each hazard scenario once. 2.3. Driving simulator and driving scenarios A General Electric Capital I-Sim Patrol Sim II@ driving simulator was used to present the simulated driving task and the CWS warnings. The simulator is equipped with full operational controls including a steering wheel, brake and accelerator pedal as well as lights, turn signal indicators, etc. The side screens allow presentation of a 180° horizontal field of view. Side-view mirrors and a rear-view mirror allow the driver to monitor traffic from all directions, including vehicle approaching from behind. The driving scenario of interest here consisted of an urban roadway with low traffic density (one to two other simulated vehicles present at any given time). Participants were asked to maintain a speed of 35 mph while interacting with vehicular traffic and pedestrians. The simulator was also accommodated with a laptop that displayed rating scales for obtaining subject assessment of the alerting effectiveness, perceived urgency and annoyance level of the CWS warnings that were presented to the participants after each scenario. The five hazard scenarios included the following events. Scenario A consisted of a vehicle running a red light at an intersection and crossing the participant’s lane of travel at a 90° angle from the left. Scenario B consisted of a pedestrian crossing in front of the participant’s lane on travel at a 90° angle from the right. Scenario C consisted of a car traveling above the speed limit in the same lane and direction and faster than the participant’s car potentially striking the participant’s car from the rear. Scenario D consisted of an intersection occluded by a larger service vehicle. An oncoming car on the opposite side of the road approaches the intersection and attempts to make a left hand turn, potentially crossing the path of the participant’s car. Scenario E consisted of a car failing to yield the right-of-way at an intersection and making a left hand turn in front of the participant’s car. Driver’s were required to maintain a speed of 35 mph in all scenarios. In all but Scenario C, drivers could avoid the potential collision by swiftly apply the brakes. Scenario C differed in that the potential collision was from a car approaching quickly from behind the participant’s vehicle. Participant’s were slowing to make a left hand turn at an intersection as the ‘‘offending” car approached from the rear. For ease of discussion this scenario is referred to as the ‘‘speeder” scenario. The fast approaching vehicle was visible only in the driver’s rear-view mirror and did not reduce its speed when the participant slowed to make the left hand turn. There was no on-coming traffic present at the
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C.L. Baldwin, J.F. May / Transportation Research Part F 14 (2011) 36–42 Table 1 Numerical and verbal descriptor of each CWS alert subjective rating dimension. Likert rating
Alerting effectiveness
Perceived urgency
Likeability/annoyance
1 2 3 4 5
Very effective Somewhat effective Neutral Somewhat ineffective Very ineffective
Very urgent Somewhat urgent Neutral Somewhat insignificant Very insignificant
Like very much Like somewhat Neutral Dislike somewhat Dislike very much
intersection, so the participant had the right-of-way to make the left hand turn. In order to avoid the collision, the driver had to accelerate and continue to make the left hand turn. Participants drove through each hazard scenario in a counterbalanced and were subsequently asked to rate each CWS alarm in terms of its perceived urgency, alerting effectiveness, and annoyance. 2.4. Procedure The nature of the experiment was explained to each participant and written informed consent was obtained. Participants were provided the opportunity to ask questions of the experimenter or have additional information explained. Participants then completed a demographic questionnaire designed to obtain the participants’ age, sex, licensure status, driving history, general health, native language, self-reported hearing and visual ability, and experience with video games. The participant was then seated in the driving simulator, adjusted the seat to a comfortable driving position and buckled his or her seatbelt. The experimenter then demonstrated to the participant the locations of the following simulator components: three large screens, side-view mirrors, the rear-view mirror, the gearshift, the speedometer, and the ignition. The speed limit in all of the scenarios was 35 mph and participants were instructed to obey all traffic laws (i.e., stop at red lights and stop signs). Participants then completed an initial orientation drive to familiarize them with the handling characteristics of the simulated vehicle. The orientation drive included a CWS alarm consisting of the signal word ‘‘Caution” at a high presentation level in order to familiarize participants with the possibility that alarms might be provided. After driving through the orientation scenario, participants were given an introduction to the 5-point Likert type rating scale for alerting effectiveness, perceived urgency, and annoyance. Table 1 provides the scale anchor points for each rating dimension. During the experimental trials, following the presentation of the messages, participants were instructed to pull the simulated vehicle over to safe location, stop, and then rate the CWS warning that was presented during that task. Participants were given rest periods between each driving scenario and were debriefed after completing the experiment. All simulated driving tasks (orientation and experimental) were approximately 3 min long and the entire experimental paradigm lasted approximately 45 min. 3. Results 3.1. Hazard scenario Of the 150 drives (30 participants 5 drives each), a total of 27 crashes (18%) were observed. Of these 27 crashes, 12 (44.4%) occurred in Scenario C, referred to as the ‘‘speeder scenario.” Scenario A resulted in 4 of 27 (14.8%) of the crashes with Scenario B resulting in only 2 (7.4%), Scenario D resulted in 6 (22.2%) and Scenario E resulted in the remaining 3 (11.1%) of the crashes. Due to the low number of crashes in the other scenarios, the highest crash risk scenario (speeder Scenario C) was chosen for further analysis. 3.2. CWS alarm type In the high risk speeder scenario, 40% of the drivers crashed. The type of CWS alarm provided significantly predicted crash probability, v2 (4, N = 30) = 10.32, p = .035. The majority of those who crashed received either no warning (33%) or the most urgent signal word ‘‘Danger” at the highest – 85 dB intensity (33%). The next highest crash rate was observed in the lowest urgency CWS alarm condition; 25% of those who crashed received the signal word ‘‘Notice” at the low or 70 dB intensity. Crash rates were low in the two moderate CWS alarm conditions (8.3% were in the Danger low intensity condition and 0% were in the Notice high intensity condition). Examined with a focus on the type of CWS alarm received, the percentage of people who crashed after exposure to each CWS alarm is illustrated in Fig. 1. As indicated, the highest urgency alarm – Danger at 85 dB, was as hazardous as not providing any alarm at all (66.7% of participants crashed when exposed to either of these conditions). The next highest crash rate was with the low urgency alarm – ‘‘Notice” presented at 70 dB – where 60% exposed to this alarm crashed. The lowest crash rate was in the moderate urgency alarm condition – Notice played at 85 dB. Everyone was able to successfully avoid crashing if they received this alarm. The other moderate urgency level alarm – ‘‘Danger” presented at 70 dB also resulted in a relatively low crash rate of 14.3%.
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Crash Rate as a Function of CWS Alert 70
% Crashed
60 50 40 30 20 10 0 Notice 70 dB
Notice 85 dB
Danger 70 dB
Danger 85 dB
No Alert
CWS Alert Fig. 1. Percentage of people who crashed as a function of the type of CWS alert they received.
3.3. Subjective ratings Subjective ratings of alerting effectiveness, perceived urgency, and annoyance were analyzed in 2 (signal word: Notice and Danger) by 2 (intensity: low and high) repeated measures MANOVA. The multivariate main effect for signal word was marginally significant, F(2, 27) = 2.88, p = .054, using Wilks’ Lambda. Univariate analyses revealed that ratings of perceived urgency and annoyance primarily contributed to this marginally significant multivariate effect. A significant main effect for intensity was not found, nor was a significant interaction between signal word and intensity observed for the subjective ratings.
3.1 3.05 3
Rating
2.95 2.9 2.85 2.8 2.75 2.7 2.65 2.6 Notice
Danger
CWS Signal Word Fig. 2. Subjective ratings of perceived urgency for each signal word. N = 30. Note: Lower numbers indicated greater urgency.
2.75 2.7 2.65
Rating
2.6 2.55 2.5 2.45 2.4 2.35 2.3 Danger
Notice
CWS Signal Word Fig. 3. Subjective ratings of annoyance for each CWS signal word. N = 30. Note: higher numbers indicate greater annoyance.
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3.3.1. Perceived urgency Though not statistically significant, examination of ratings of perceived urgency collapsed across intensity indicated that the signal word Danger (X = 2.76, std = 1.25) was rated as slightly more urgent than the signal word Notice (X = 3.07, std = 1.35). Keeping in mind that lower numbers indicate greater perceived urgency, Fig. 2 illustrates this relationship graphically. 3.3.2. Annoyance Ratings of annoyance were also not statistically significantly different, though the means collapsed across intensity were in the expected direction. Participants tended to rate the word Danger (X = 2.72, std = .94) as slightly more annoying than the word Notice (X = 2.45, std = .90). Fig. 3 illustrates this relationship graphically, with higher numbers indicating higher perceived annoyance. 4. Discussion Previous research has shown that both acoustic (i.e., loudness level) and semantic (i.e., signal word or message) parameters affect perceptions of the perceived urgency of verbal alarms in situations involving simply listening to and rating the verbal stimuli (Hellier et al., 2002) and listening to the stimuli while engaged in a contextually appropriate task (Baldwin, in press). Results of the current investigation provide converging evidence that the semantics of an auditory alarm interact with its acoustic properties to influence crash probability in a simulated rear-end collision scenario. Both the signal word and the loudness at which it was presented impacted the driver’s ability to respond appropriately to this collision situation. An extremely high percentage of the drivers crashed in this scenario (66%) when they were not provided with a CWS alarm. This observation confirms that the scenario of interest had an extremely high crash risk. Recall that the driver could only observe the impending collision event by looking in the rear-view mirror. Thus, no matter how careful the driver was while scanning for forward traffic, in order to avoid this collision drivers had to maintain or be alerted to an event taking place behind them. Based on previous research (Blumenthal, Noto, Fox, & Franklin, 2006) it was predicted that when CWS alarms were presented at a potentially startling loudness level of 85 dB, drivers would react quickly, but not necessarily appropriately. This prediction was partially supported by the observation that presenting the signal word ‘‘Danger” at 85 dB resulted in the same collision rate (66%) as providing no warning at all. Clearly this extremely urgent alarm was not effective in reducing crashes and it is likely that this was due to its startling effect on the driver (Blumenthal, 1996). However likely this explanation is, it unfortunately cannot be conclusively made due to the lack of sensitive indices of startle in the current investigation. Of interest is the observation that the 85 dB loudness level did not appear to startle drivers when used to present the low urgency signal word, ‘‘Notice”. This CWS alarm combination was in fact the most effective – being associated with no crashes. That is, when drivers were presented with the CWS alarm – Notice at 85 dB – they apparently had time to assess the situation and react quickly enough to make the appropriate collision avoidance maneuver. This finding indicates great promise for improved design of CWS alarms and warrants further attention in future research. The most appropriate collision avoidance strategy resulted from use of either the less urgent signal word ‘‘Notice” presented at the potentially startling loudness level of 85 dB or the high urgency signal word ‘‘Danger” presented at the least urgent loudness level of 70 dB, relative to the other CWS warning combinations and the control condition in which no CWS warning was given. Previous research (Hellier et al., 2000; Wogalter & Silver, 1995) has consistently found that people rate the word Danger and more urgent than the word Notice. The current investigation predicted that this effect would also be observed when these words were presented as CWS alarms while listeners were engaged in the task of driving. The choice of signal words in this investigation showed a trend toward affecting both the perceived urgency and the perceived annoyance used of the CWS alarm. Though not statistically significant, drivers tended to rate the signal word Danger as both more perceptually urgent but also more annoying. It is somewhat surprising that intensity alone did not significantly impact ratings of perceived urgency, since previous investigations have found significant effects for manipulations of intensity (Haas & Edworthy, 1996; Momtahan, 1990). It is possible that the effects of the manipulation of intensity were subsumed by the interaction with changes in signal word. It is also possible that the hazard context within the driving simulation obscured a direct effect of intensity as providing even verbal descriptions of different contexts have been shown to alter ratings of urgency (Marshall et al., 2007). The current results were conducted in a driving simulator and though they can be expected to represent a more ecologically valid context for investigating collision warnings than presentation of sounds alone, their application to actual driving situations may be challenged. However, they contribute to an existing body of literature indicating that auditory alarms can reduce rear-end collisions, especially in distracted drivers (Baldwin, May, & Reagan, 2006; Lee et al., 2002). Results to date suggest that semantics may be used to reduce startle effects in CWS alarms. For example, some populations may benefit from the increased acoustic salience of a louder alarm (e.g. older drivers experiencing mild hearing loss or young drivers listening to MP3 players). Using a relatively low urgency level signal word at an acoustically salient loudness level may effectively alert without startling these drivers. Clearly, additional work is needed to examine both additional signal words and additional signal-to-noise ratios; but, results of this investigation provide an important step toward
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understanding the interaction of acoustic and semantic characteristics of verbal alarms in applied settings. The current results also have implications for improving the design of future CWSs and underscore the importance of urgency mapping as a potential tool for examining existing CWS alarms. Acknowledgements Gratitude is extended to all the research assistants in the CNAD lab at Old Dominion University and in particular to Katrina Lewis and Jeff Lawrence – whose assistance with data collection was invaluable. This research was funded in part by the National Institute of Aging #5 R03 AG 23881-01. References Baldwin, C. L. (2002). Designing in-vehicle technologies for older drivers: Application of sensory–cognitive interaction theory. Theoretical Issues in Ergonomics Science, 3(4), 307–329. Baldwin, C. L. (in press). Verbal collision avoidance messages during simulated driving: Perceived urgency, annoyance, and alerting effectiveness. Ergonomics. Baldwin, C. L., May, J. F. (2006). 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Contents lists available at ScienceDirect
Transportation Research Part F journal homepage: www.elsevier.com/locate/trf
Loudness interacts with semantics in auditory warnings to impact rear-end collisions Carryl L. Baldwin a,⇑, Jennifer F. May b a b
George Mason University, 4400 University Drive, Fairfax, VA 22030, United States Old Dominion University, Norfolk, VA 23529, United States
a r t i c l e
i n f o
Article history: Received 15 January 2010 Received in revised form 16 August 2010 Accepted 6 September 2010
Keywords: Collision warning systems Auditory alarms Driver response Acoustic Semantic
a b s t r a c t This study examined the impact of semantic and acoustics parameters of in-vehicle collision warning system (CWS) alarms on driver response. Thirty participants drove a simulated vehicle through scenarios containing five different unexpected hazard events. As drivers approached the hazard event one of four CWS alarms, counter balanced with the hazard event type, or no alarm (control) was presented. Alarms consisted of the signal word ‘‘Notice” or ‘‘Danger” presented at either 70 or 85 dBA. Rear-end collision events resulted in the highest crash rate, accounting for 45.4% of all crashes. In these scenarios, CWSs significantly reduced crash rates. CWS alarms with an intermediate urgency level achieved through an interaction of semantics and acoustics (‘‘Danger” at 70 dB and ‘‘Notice” at 85 dB) resulted in significant reductions in crash probability. Providing an extremely urgent signal word, ‘‘Danger” at a high acoustically urgent presentation level – 85 dB was not effective in reducing crashes, nor was a low urgency signal word, ‘‘Notice” presented at a low acoustical urgency level – 70 dB. Implications of these results for the design and implementation of CWS systems and auditory alarms in general, are discussed. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Driver distraction and inattention are a persistent threat to automotive safety – contributing to a substantial number of collisions annually (Stutts et al., 2005). The increasing prevalence of personal portable electronic devices in conjunction with emerging in-vehicle technologies may lead to even greater driver distraction. Collision warnings systems (CWSs) that effectively draw a driver’s attention to critical roadway incidents have the potential to reduce both the severity and rate of occurrence of traffic collisions (Ben-Yaacov, Maltz, & Shinar, 2002; Brown, Lee, & McGehee, 2001; Chang, Lin, Hsu, Fung, & Hwang, 2009; Lee, McGehee, Brown, & Reyes, 2002; Maltz & Shinar, 2007). A key, of course, is that CWSs must effectively draw a drivers’ attention if they are to be useful. Most of us have probably had the experience of a passenger in the car suddenly yelling out something like, ‘‘Look Out” while we were driving. The startle response initiated by such an action is often the most dangerous aspect of the situation. When startled, a driver can be expected to tightly grab the steering wheel and initiate a braking response while concurrently scanning the environment for the source of the trouble. Unfortunately, such a response is not always the optimal one. Startle responses can be elicited from sudden acoustic stimuli of at least 85 dBA (Blumenthal, 1996). In laboratory settings the startle response is most frequently associated with the eye-blink response as measured by electromyographic (EMG) activity. However when driving, in addition to an eye-blink response, the prepotent response would more typically be a
⇑ Corresponding author. Tel.: +1 703 993 4653. E-mail address: [email protected] (C.L. Baldwin). 1369-8478/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.trf.2010.09.004
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braking response. Any time spent recovering from the startle effect is lost for responding to a critical incident. At the same time that it is critical that a CWS alert not startle the driver, it must be detectable in the ambient background noise of the driving situation. Acoustic parameters such as the loudness, pitch, speed, pulse rate and duration have been shown to affect the perceived urgency of auditory warnings (Haas & Casali, 1995; Hass & Edworthy, 1996; Hellier, Edworthy, & Dennis, 1993; Momtahan, 1990; Wiese & Lee, 2004). The intensity of a sound in conjunction with the background noise level is the key determinant of whether or not an auditory warning will be detected (Patterson, 1982, 1990). Less attention has been systematically placed on the influence of alarm intensity, subjectively perceived as loudness, on perceptions of urgency. In support of the influence of loudness, Momtahan (1990) observed that sounds presented at 90 dB were rated as significantly more urgent, regardless of their interpulse interval, number of harmonics, spectral shape or fundamental frequency, than sounds presented at 75 dB. Though there is some evidence indicating that loudness impacts an auditory alarms’ perceived urgency and annoyance (Haas & Edworthy, 1996; Momtahan, 1990), loudness is generally examined in conjunction with other parameters rather than systematically manipulated across parameters. Words spoken in a more urgent style are louder and are easily perceived as more urgent by the listener (Edworthy, Hellier, Walters, Clift-Mathews, & Crowther, 2003; Hellier, Edworthy, Weedon, Walters, & Adams, 2002). However, words spoken in an urgent style have a number of other differences relative to words spoken in a neutral style. They also tend to have higher pitch and less distinct phonemic distinctions (Grommes & Dietrich, 2002). The semantic characteristics of verbal signal words have been shown to affect the perceived urgency of auditory warnings in numerous investigations (Edworthy et al., 2003; Hellier et al., 2002). In an early examination of the perceived urgency of verbal warnings, people were asked to listen to various signal words presented under one of three voice styles (monotone, emotional, and whisper) at one of two presentation levels – (60 dBA and 90 dBA) spoken by a male or female and to judge their connoted hazard level (Wogalter, Kalsher, Frederick, Magurno, & Brewster, 1998). The signal word, ‘‘Danger” received a higher hazard rating than ‘‘Warning” and ‘‘Caution”, which did not differ from each other. ‘‘Notice” received the lowest hazard rating. The hazard level (also termed the perceived urgency) of spoken signal words has been confirmed by subsequent research (Edworthy et al., 2003; Hellier, Wright, Edworthy, & Newstead, 2000; Hellier et al., 2002). Further, acoustic and semantic characteristics have been shown to interactively impact ratings of perceived urgency (Hellier et al., 2002). In the visual domain, colour has been shown to interact with semantics to impact urgency ratings (Braun, Sansing, Kennedy, & Silver, 1994). Braun et al. suggested that these interactions could be used to establish equal levels of perceived urgency using different combinations of colour and signal word. It is conceivable that a similar procedure could be used to determine the urgency associated with different combinations of signal word and acoustic parameters (i.e., loudness). However, it must be noted that relatively few investigations have examined these issues when listeners were engaged in a concurrent task. Rather, ratings are commonly obtained by asking listeners to make a series of multiple psychophysical judgments. Providing listeners with a contextual description for a particular alarm (i.e., collision warning versus email) has been shown to affect ratings of both urgency and annoyance (Marshall, Lee, & Austria, 2007). In one notable exception, participants responded as quickly as possible and then rated warnings while engaged in a low fidelity simulated driving task (Baldwin, in press). Participants were asked to acknowledge the warnings as quickly as possible by making a key press without disrupting their lane keeping ability. They subsequently provided ratings of perceived urgency, alert effectiveness and annoyance for warnings that varied independently in both semantic content and loudness. An interaction was observed between the semantic content of the messages and the loudness at which the messages were played. The interaction affected both response time to the initial acknowledgement key press and subjective ratings of urgency and annoyance. Regardless of the content of the CWS warning, when it was presented at least 8 dB above ambient background noise, participants responded more quickly relative to the lower presentation level conditions. At intermediate loudness levels, participants’ response time differed as a function of the semantic content. They responded significantly faster to warnings of potential collisions relative to regulatory notices regarding their speed. Providing alarms at loudness levels at least 8–10 dB above ambient background noise levels in the vehicle are essential to ensuring their audibility among older drivers (Baldwin, 2002) and drivers who may be distracted by other acoustic events (i.e., personal music or communication systems). Recent research indicates that older drivers may be receptive to new invehicle technologies aimed at reducing collision potential (Creaser, Rakauskas, Ward, Laberge, & Donath, 2007) and that older drivers may benefit even more than younger drivers from well designed CWS alarms (Baldwin & May, 2006; Maltz & Shinar, 2007). The goal of the present investigation was to examine the relative impact and potential interaction of two key characteristics of verbal warnings, an acoustic characteristic (loudness) and a semantic characteristic (signal word). Specifically, we sought to determine if the semantic characteristics of a CWS alert could be used to impact a driver’s response to a high collision-risk situation in a realistic driving simulation. Secondly, we sought to examine the relationship between semantic and acoustic alarm parameters and determine if the parameters would exert independent or interactive effects. Finally, we sought to determine if semantics could be used to effectively alert without startling drivers. 1.1. Current investigation For the current investigation, two signal words (‘‘Danger” and ‘‘Notice”) were chosen in an effort to represent semantically high and low urgency alarms, respectively. Each signal word was presented at one of two intensity levels, a low urgency level of 70 dBA or a high urgency level of 85 dBA.
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Based on previous literature (Barzegar & Wogalter, 1998; Hellier et al., 2000) it was predicted that the signal word ‘‘Danger” would have a greater alerting effectiveness than the signal word, ‘‘Notice” thus resulting in faster response time. It was also predicted that a relatively high 85 dB intensity would result in faster response time than the less urgent 70 dB intensity. Further, based on previous research (Baldwin, in press) both acoustic and semantic aspects of the CWS alarm were expected to impact driver’ response such that the high urgency alarm (Danger at the high intensity) was expected to result in the fastest reaction time (foot off the accelerator) but more moderate urgency level alarms (Notice at 85 dB and Danger at 70 dB) were expected to result in the most appropriate response, thus reducing crash probability. 2. Methods 2.1. Participants Thirty volunteers (57% females) between the ages of 18–26 years (mean age 21.1 years) participated in this investigation. All participants had self-reported normal or corrected to normal vision and hearing as well as a valid driver’s license. All participants gave written informed consent in accordance with the ethical guidelines for treatment of human subjects as approved by a University Institutional Review Board. Participants received a small amount of research participation credit for participating in the investigation. 2.2. CWS alarms The signal words, ‘‘Notice” and ‘‘Danger” spoken by a female at a normal conversational level were digitally prerecorded and saved as .wav files. Using a commercially available sound editing program, the .wav files were then amplified to achieve the desired loudness levels approximating 70 and 85 dBA. The ambient background noise from the driving simulator with the engine running and while driving at 35 mph was 55 dB on average. Thus, the 70 and 85 dB loudness levels resulted in signal-to-noise ratios (SNRs) of roughly +15 dB and +30 dB, respectively. When the participant’s vehicle passed into a pre-specified zone, the collision event was triggered. In similar fashion, the CWS alarms were triggered when the participants’ vehicle crossed a second specific point in the simulation. This method ensured that all participants’ were provided the CWS alarm at the same point in the scenario regardless of their individual speed or lane position. Alarms were presented via the driving simulator through speakers while the participant drove through the pre-designed scenario. Each participant drove through five different scenarios, each containing one potential collision event or hazard paired with one of the four CWS alarms or the control. All combinations of each CWS alarm were paired with each hazard scenario and all combinations were presented across the experiment. But, individual participants only encountered each alarm once and only drove through each hazard scenario once. 2.3. Driving simulator and driving scenarios A General Electric Capital I-Sim Patrol Sim II@ driving simulator was used to present the simulated driving task and the CWS warnings. The simulator is equipped with full operational controls including a steering wheel, brake and accelerator pedal as well as lights, turn signal indicators, etc. The side screens allow presentation of a 180° horizontal field of view. Side-view mirrors and a rear-view mirror allow the driver to monitor traffic from all directions, including vehicle approaching from behind. The driving scenario of interest here consisted of an urban roadway with low traffic density (one to two other simulated vehicles present at any given time). Participants were asked to maintain a speed of 35 mph while interacting with vehicular traffic and pedestrians. The simulator was also accommodated with a laptop that displayed rating scales for obtaining subject assessment of the alerting effectiveness, perceived urgency and annoyance level of the CWS warnings that were presented to the participants after each scenario. The five hazard scenarios included the following events. Scenario A consisted of a vehicle running a red light at an intersection and crossing the participant’s lane of travel at a 90° angle from the left. Scenario B consisted of a pedestrian crossing in front of the participant’s lane on travel at a 90° angle from the right. Scenario C consisted of a car traveling above the speed limit in the same lane and direction and faster than the participant’s car potentially striking the participant’s car from the rear. Scenario D consisted of an intersection occluded by a larger service vehicle. An oncoming car on the opposite side of the road approaches the intersection and attempts to make a left hand turn, potentially crossing the path of the participant’s car. Scenario E consisted of a car failing to yield the right-of-way at an intersection and making a left hand turn in front of the participant’s car. Driver’s were required to maintain a speed of 35 mph in all scenarios. In all but Scenario C, drivers could avoid the potential collision by swiftly apply the brakes. Scenario C differed in that the potential collision was from a car approaching quickly from behind the participant’s vehicle. Participant’s were slowing to make a left hand turn at an intersection as the ‘‘offending” car approached from the rear. For ease of discussion this scenario is referred to as the ‘‘speeder” scenario. The fast approaching vehicle was visible only in the driver’s rear-view mirror and did not reduce its speed when the participant slowed to make the left hand turn. There was no on-coming traffic present at the
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C.L. Baldwin, J.F. May / Transportation Research Part F 14 (2011) 36–42 Table 1 Numerical and verbal descriptor of each CWS alert subjective rating dimension. Likert rating
Alerting effectiveness
Perceived urgency
Likeability/annoyance
1 2 3 4 5
Very effective Somewhat effective Neutral Somewhat ineffective Very ineffective
Very urgent Somewhat urgent Neutral Somewhat insignificant Very insignificant
Like very much Like somewhat Neutral Dislike somewhat Dislike very much
intersection, so the participant had the right-of-way to make the left hand turn. In order to avoid the collision, the driver had to accelerate and continue to make the left hand turn. Participants drove through each hazard scenario in a counterbalanced and were subsequently asked to rate each CWS alarm in terms of its perceived urgency, alerting effectiveness, and annoyance. 2.4. Procedure The nature of the experiment was explained to each participant and written informed consent was obtained. Participants were provided the opportunity to ask questions of the experimenter or have additional information explained. Participants then completed a demographic questionnaire designed to obtain the participants’ age, sex, licensure status, driving history, general health, native language, self-reported hearing and visual ability, and experience with video games. The participant was then seated in the driving simulator, adjusted the seat to a comfortable driving position and buckled his or her seatbelt. The experimenter then demonstrated to the participant the locations of the following simulator components: three large screens, side-view mirrors, the rear-view mirror, the gearshift, the speedometer, and the ignition. The speed limit in all of the scenarios was 35 mph and participants were instructed to obey all traffic laws (i.e., stop at red lights and stop signs). Participants then completed an initial orientation drive to familiarize them with the handling characteristics of the simulated vehicle. The orientation drive included a CWS alarm consisting of the signal word ‘‘Caution” at a high presentation level in order to familiarize participants with the possibility that alarms might be provided. After driving through the orientation scenario, participants were given an introduction to the 5-point Likert type rating scale for alerting effectiveness, perceived urgency, and annoyance. Table 1 provides the scale anchor points for each rating dimension. During the experimental trials, following the presentation of the messages, participants were instructed to pull the simulated vehicle over to safe location, stop, and then rate the CWS warning that was presented during that task. Participants were given rest periods between each driving scenario and were debriefed after completing the experiment. All simulated driving tasks (orientation and experimental) were approximately 3 min long and the entire experimental paradigm lasted approximately 45 min. 3. Results 3.1. Hazard scenario Of the 150 drives (30 participants 5 drives each), a total of 27 crashes (18%) were observed. Of these 27 crashes, 12 (44.4%) occurred in Scenario C, referred to as the ‘‘speeder scenario.” Scenario A resulted in 4 of 27 (14.8%) of the crashes with Scenario B resulting in only 2 (7.4%), Scenario D resulted in 6 (22.2%) and Scenario E resulted in the remaining 3 (11.1%) of the crashes. Due to the low number of crashes in the other scenarios, the highest crash risk scenario (speeder Scenario C) was chosen for further analysis. 3.2. CWS alarm type In the high risk speeder scenario, 40% of the drivers crashed. The type of CWS alarm provided significantly predicted crash probability, v2 (4, N = 30) = 10.32, p = .035. The majority of those who crashed received either no warning (33%) or the most urgent signal word ‘‘Danger” at the highest – 85 dB intensity (33%). The next highest crash rate was observed in the lowest urgency CWS alarm condition; 25% of those who crashed received the signal word ‘‘Notice” at the low or 70 dB intensity. Crash rates were low in the two moderate CWS alarm conditions (8.3% were in the Danger low intensity condition and 0% were in the Notice high intensity condition). Examined with a focus on the type of CWS alarm received, the percentage of people who crashed after exposure to each CWS alarm is illustrated in Fig. 1. As indicated, the highest urgency alarm – Danger at 85 dB, was as hazardous as not providing any alarm at all (66.7% of participants crashed when exposed to either of these conditions). The next highest crash rate was with the low urgency alarm – ‘‘Notice” presented at 70 dB – where 60% exposed to this alarm crashed. The lowest crash rate was in the moderate urgency alarm condition – Notice played at 85 dB. Everyone was able to successfully avoid crashing if they received this alarm. The other moderate urgency level alarm – ‘‘Danger” presented at 70 dB also resulted in a relatively low crash rate of 14.3%.
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Crash Rate as a Function of CWS Alert 70
% Crashed
60 50 40 30 20 10 0 Notice 70 dB
Notice 85 dB
Danger 70 dB
Danger 85 dB
No Alert
CWS Alert Fig. 1. Percentage of people who crashed as a function of the type of CWS alert they received.
3.3. Subjective ratings Subjective ratings of alerting effectiveness, perceived urgency, and annoyance were analyzed in 2 (signal word: Notice and Danger) by 2 (intensity: low and high) repeated measures MANOVA. The multivariate main effect for signal word was marginally significant, F(2, 27) = 2.88, p = .054, using Wilks’ Lambda. Univariate analyses revealed that ratings of perceived urgency and annoyance primarily contributed to this marginally significant multivariate effect. A significant main effect for intensity was not found, nor was a significant interaction between signal word and intensity observed for the subjective ratings.
3.1 3.05 3
Rating
2.95 2.9 2.85 2.8 2.75 2.7 2.65 2.6 Notice
Danger
CWS Signal Word Fig. 2. Subjective ratings of perceived urgency for each signal word. N = 30. Note: Lower numbers indicated greater urgency.
2.75 2.7 2.65
Rating
2.6 2.55 2.5 2.45 2.4 2.35 2.3 Danger
Notice
CWS Signal Word Fig. 3. Subjective ratings of annoyance for each CWS signal word. N = 30. Note: higher numbers indicate greater annoyance.
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3.3.1. Perceived urgency Though not statistically significant, examination of ratings of perceived urgency collapsed across intensity indicated that the signal word Danger (X = 2.76, std = 1.25) was rated as slightly more urgent than the signal word Notice (X = 3.07, std = 1.35). Keeping in mind that lower numbers indicate greater perceived urgency, Fig. 2 illustrates this relationship graphically. 3.3.2. Annoyance Ratings of annoyance were also not statistically significantly different, though the means collapsed across intensity were in the expected direction. Participants tended to rate the word Danger (X = 2.72, std = .94) as slightly more annoying than the word Notice (X = 2.45, std = .90). Fig. 3 illustrates this relationship graphically, with higher numbers indicating higher perceived annoyance. 4. Discussion Previous research has shown that both acoustic (i.e., loudness level) and semantic (i.e., signal word or message) parameters affect perceptions of the perceived urgency of verbal alarms in situations involving simply listening to and rating the verbal stimuli (Hellier et al., 2002) and listening to the stimuli while engaged in a contextually appropriate task (Baldwin, in press). Results of the current investigation provide converging evidence that the semantics of an auditory alarm interact with its acoustic properties to influence crash probability in a simulated rear-end collision scenario. Both the signal word and the loudness at which it was presented impacted the driver’s ability to respond appropriately to this collision situation. An extremely high percentage of the drivers crashed in this scenario (66%) when they were not provided with a CWS alarm. This observation confirms that the scenario of interest had an extremely high crash risk. Recall that the driver could only observe the impending collision event by looking in the rear-view mirror. Thus, no matter how careful the driver was while scanning for forward traffic, in order to avoid this collision drivers had to maintain or be alerted to an event taking place behind them. Based on previous research (Blumenthal, Noto, Fox, & Franklin, 2006) it was predicted that when CWS alarms were presented at a potentially startling loudness level of 85 dB, drivers would react quickly, but not necessarily appropriately. This prediction was partially supported by the observation that presenting the signal word ‘‘Danger” at 85 dB resulted in the same collision rate (66%) as providing no warning at all. Clearly this extremely urgent alarm was not effective in reducing crashes and it is likely that this was due to its startling effect on the driver (Blumenthal, 1996). However likely this explanation is, it unfortunately cannot be conclusively made due to the lack of sensitive indices of startle in the current investigation. Of interest is the observation that the 85 dB loudness level did not appear to startle drivers when used to present the low urgency signal word, ‘‘Notice”. This CWS alarm combination was in fact the most effective – being associated with no crashes. That is, when drivers were presented with the CWS alarm – Notice at 85 dB – they apparently had time to assess the situation and react quickly enough to make the appropriate collision avoidance maneuver. This finding indicates great promise for improved design of CWS alarms and warrants further attention in future research. The most appropriate collision avoidance strategy resulted from use of either the less urgent signal word ‘‘Notice” presented at the potentially startling loudness level of 85 dB or the high urgency signal word ‘‘Danger” presented at the least urgent loudness level of 70 dB, relative to the other CWS warning combinations and the control condition in which no CWS warning was given. Previous research (Hellier et al., 2000; Wogalter & Silver, 1995) has consistently found that people rate the word Danger and more urgent than the word Notice. The current investigation predicted that this effect would also be observed when these words were presented as CWS alarms while listeners were engaged in the task of driving. The choice of signal words in this investigation showed a trend toward affecting both the perceived urgency and the perceived annoyance used of the CWS alarm. Though not statistically significant, drivers tended to rate the signal word Danger as both more perceptually urgent but also more annoying. It is somewhat surprising that intensity alone did not significantly impact ratings of perceived urgency, since previous investigations have found significant effects for manipulations of intensity (Haas & Edworthy, 1996; Momtahan, 1990). It is possible that the effects of the manipulation of intensity were subsumed by the interaction with changes in signal word. It is also possible that the hazard context within the driving simulation obscured a direct effect of intensity as providing even verbal descriptions of different contexts have been shown to alter ratings of urgency (Marshall et al., 2007). The current results were conducted in a driving simulator and though they can be expected to represent a more ecologically valid context for investigating collision warnings than presentation of sounds alone, their application to actual driving situations may be challenged. However, they contribute to an existing body of literature indicating that auditory alarms can reduce rear-end collisions, especially in distracted drivers (Baldwin, May, & Reagan, 2006; Lee et al., 2002). Results to date suggest that semantics may be used to reduce startle effects in CWS alarms. For example, some populations may benefit from the increased acoustic salience of a louder alarm (e.g. older drivers experiencing mild hearing loss or young drivers listening to MP3 players). Using a relatively low urgency level signal word at an acoustically salient loudness level may effectively alert without startling these drivers. Clearly, additional work is needed to examine both additional signal words and additional signal-to-noise ratios; but, results of this investigation provide an important step toward
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understanding the interaction of acoustic and semantic characteristics of verbal alarms in applied settings. The current results also have implications for improving the design of future CWSs and underscore the importance of urgency mapping as a potential tool for examining existing CWS alarms. Acknowledgements Gratitude is extended to all the research assistants in the CNAD lab at Old Dominion University and in particular to Katrina Lewis and Jeff Lawrence – whose assistance with data collection was invaluable. This research was funded in part by the National Institute of Aging #5 R03 AG 23881-01. References Baldwin, C. L. (2002). Designing in-vehicle technologies for older drivers: Application of sensory–cognitive interaction theory. Theoretical Issues in Ergonomics Science, 3(4), 307–329. Baldwin, C. L. (in press). Verbal collision avoidance messages during simulated driving: Perceived urgency, annoyance, and alerting effectiveness. Ergonomics. Baldwin, C. L., May, J. F. (2006). 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