Verbal collision avoidance messages during simulated driving: perceived urgency, alerting effectiveness and annoyance

Matching the perceived urgency of an alert with the relative hazard level of the situation is critical for effective alarm response. Two experiments describe the impact of acoustic and semantic parameters on ratings of perceived urgency, annoyance and alerting effectiveness and on alarm response speed. Within a simulated driving context, participants rated and responded to collision avoidance system (CAS) messages spoken by a female or male voice (experiments 1 and 2, respectively). Results indicated greater perceived urgency and faster alarm response times as intensity increased from -2 dB signal to noise (S/N) ratio to +10 dB S/N, although annoyance ratings increased as well. CAS semantic content interacted with alarm intensity, indicating that at lower intensity levels participants paid more attention to the semantic content. Results indicate that both acoustic and semantic parameters independently and interactively impact CAS alert perceptions in divided attention conditions and this work can inform auditory alarm design for effective hazard matching. Matching the perceived urgency of an alert with the relative hazard level of the situation is critical for effective alarm response. Here, both acoustic and semantic parameters independently and interactively impacted CAS alert perceptions in divided attention conditions. This work can inform auditory alarm design for effective hazard matching. STATEMENT OF RELEVANCE: Results indicate that both acoustic parameters and semantic content can be used to design collision warnings with a range of urgency levels. Further, these results indicate that verbal warnings tailored to a specific hazard situation may improve hazard-matching capabilities without substantial trade-offs in perceived annoyance.     

Auditory alerts for in-vehicle information systems: The effects of temporal conflict and sound parameters on driver attitudes and performance

In-vehicle information systems will soon confront drivers with an increasing number of warnings and alerts for situations ranging from imminent collisions to the arrival of e-mail messages. Coordinating these alerts can ensure that they enhance rather than degrade driving safety. Two experiments examined how temporal conflict and sound parameters affect driver performance and acceptance. The temporal conflict of an e-mail alert occurring 300?ms before a collision warning interfered with the response to the collision warning, but an e-mail alert occurring 1000?ms before the collision warning had the opposite effect and enhanced the response to the collision warning. These results emphasize the need to consider how in-vehicle devices influence drivers' strategic anticipation of high-demand situations. Regarding sound parameters, results showed that highly urgent sounds tended to speed drivers' accelerator release, but the annoyance associated with highly urgent sounds increased workload. In fact, there was a strong positive association between ratings of annoyance and subjective workload. Consistent with the urgency mapping principle, there was a slight negative association between the differences in the rated urgency of collision warnings and e-mail alerts and subjective workload. The results suggest that warning and alert design should consider an annoyance trade-off in addition to urgency mapping.     

Auditory alerts for in-vehicle information systems: the effects of temporal conflict and sound parameters on driver attitudes and performance

In-vehicle information systems will soon confront drivers with an increasing number of warnings and alerts for situations ranging from imminent collisions to the arrival of e-mail messages. Coordinating these alerts can ensure that they enhance rather than degrade driving safety. Two experiments examined how temporal conflict and sound parameters affect driver performance and acceptance. The temporal conflict of an e-mail alert occurring 300 ms before a collision warning interfered with the response to the collision warning, but an e-mail alert occurring 1000 ms before the collision warning had the opposite effect and enhanced the response to the collision warning. These results emphasize the need to consider how in-vehicle devices influence drivers' strategic anticipation of high-demand situations. Regarding sound parameters, results showed that highly urgent sounds tended to speed drivers' accelerator release, but the annoyance associated with highly urgent sounds increased workload. In fact, there was a strong positive association between ratings of annoyance and subjective workload. Consistent with the urgency mapping principle, there was a slight negative association between the differences in the rated urgency of collision warnings and e-mail alerts and subjective workload. The results suggest that warning and alert design should consider an annoyance trade-off in addition to urgency mapping.     

Perceived urgency mapping across modalities within a driving context

Hazard mapping is essential to effective driver-vehicle interface (DVI) design. Determining which modality to use for situations of different criticality requires an understanding of the relative impact of signal parameters within each modality on perceptions of urgency and annoyance. Towards this goal we obtained psychometric functions for visual, auditory and tactile interpulse interval (IPI), visual color, signal word, and auditory fundamental frequency on perceptions of urgency, annoyance, and acceptability. Results indicate that manipulation of IPI in the tactile modality, relative to visual and auditory, has greater utility (greater impact on urgency than annoyance). Manipulations of color were generally rated as less annoying and more acceptable than auditory and tactile stimuli; but they were also rated as lower in urgency relative to other modality manipulations. Manipulation of auditory fundamental frequency resulted in high ratings of both urgency and annoyance. Results of the current investigation can be used to guide DVI design and evaluation.     

Mapping candidate within-vehicle auditory displays to their referents

OBJECTIVE: This study investigated speech, environmental sounds (naturally occurring sounds with arbitrary meanings), auditory icons (natural or synthetic sounds with specific meanings), and abstract synthetic warnings as candidates for within-vehicle interfaces. BACKGROUND: Auditory displays and warnings must satisfy certain criteria, such as being appropriately urgent and commanding appropriately fast response times. However, a semiotic analysis suggests that displays, as signals interpreted by users, should also be mapped successfully onto their referents. METHOD: Response times and accuracy were recorded in a computer task of identifying learned mappings of candidate displays to a range of referent driving events (such as "headway closing"); perceived urgency and pleasantness were assessed separately. RESULTS: Speech and auditory icons produced near-ceiling performance in response times and identification accuracy. Abstract sounds produced notably slower response times and less accuracy. Environmental sounds showed an intermediate pattern of performance for accuracy, but the response times were similar to those of the abstract sounds. Speech utterances were similarly and consistently rated as pleasant but also of intermediate perceived urgency. The three other sound types showed a consistent mapping of their perceived urgency to the situational urgency of their referents; for these sounds, perceived urgency and pleasantness were negatively correlated. CONCLUSION: The results point to the importance of considering the role of signal-referent relationships in designing auditory displays. APPLICATION: The results have applicability for auditory displays in the vehicle interface, whereas the theoretical framework is of value in auditory display design in a broader context.     

A psychophysiological evaluation of the perceived urgency of auditory warning signals

One significant concern that pilots have about cockpit auditory warnings is that the signals presently used lack a sense of priority. The relationship between auditory warning sound parameters and perceived urgency is, therefore, an important topic of enquiry in aviation psychology. The present investigation examined the relationship among subjective assessments of urgency, reaction time, and brainwave activity with three auditory warning signals. Subjects performed a tracking task involving automated and manual conditions, and were presented with auditory warnings having various levels of perceived and situational urgency. Subjective assessments revealed that subjects were able to rank warnings on an urgency scale, but rankings were altered after warnings were mapped to a situational urgency scale. Reaction times differed between automated and manual tracking task conditions, and physiological data showed attentional differences in response to perceived and situational warning urgency levels. This study shows that the use of physiological measures sensitive to attention and arousal, in conjunction with behavioural and subjective measures, may lead to the design of auditory warnings that produce a sense of urgency in an operator that matches the urgency of the situation.     

Perceived urgency of and response time to multi-tone and frequency-modulated warning signals in broadband noise

Several pulse parameters that were believed to affect the perceived urgency and response time to auditory warning signals were investigated in a factorial experiment. The independent variables included pulse format (sequential, simultaneous, and frequency-modulated pulses), pulse level (65 and 79dBC), and inter-pulse interval (0, 150 and 300 ms). The applications environments of interest were those having steady-state broadband noise. A probability monitoring task from the Criterion Task Set was used as an operator loading task to impose additional attentional demands during the signal detection and response task. The psychophysical methods of free-modulus magnitude estimation and paired comparison were used to measure subjective perceived urgency. An objective measure of response time to the signal was also obtained. Multivariate statistical analyses indicated that response time decreased significantly as perceived urgency increased. Perceived urgency of the signal increased and response time decreased as pulse level increased. Sequential signals took longer to detect and were rated as less urgent than the other two signal types. Shorter inter-pulse interval was associated with greater perceived signal urgency.     

On using psychophysical techniques to achieve urgency mapping in auditory warnings

It is well established that warning implementation should aim to achieve urgency mapping between the perceived urgency of the warning itself and the situational urgency of the condition that it indicates. This paper describes how Stevens Power Law [Psychological Review, 64, 153 181, 1957], which quantifies the relationship between objective parameters (such as the pitch of a warning) and subjective parameters (such as perceived urgency), can be applied to the design of auditory warnings to facilitate such urgency mapping. Studies that have quantified and predicted the effects of different warning parameters on perceived urgency using an application of Stevens Power Law are reported.     

Use of auditory icons as emergency warnings: evaluation within a vehicle collision avoidance application.

In the context of emergency warnings, auditory icons, which convey information about system events by analogy with everyday events, have the potential to be understood more quickly and easily than abstract sounds. To test this proposal, an experiment was carried out to evaluate the use of auditory icons for an in-vehicle collision avoidance application. Two icons, the sounds of a car horn and of skidding tyres, were compared with two conventional warnings, a simple tone and a voice saying 'ahead'. Participants sat in an experimental vehicle with a road scene projected ahead, and they were required to brake in response to on-screen collision situations and their accompanying warning sounds. The auditory icons produced significantly faster reaction times than the conventional warnings, but suffered from more inappropriate responses, where drivers reacted with a brake press to a non-collision situation. The findings are explained relative to the perceived urgency and inherent meaning of each sound. It is argued that optimal warnings could be achieved by adjusting certain sound attributes of auditory icons, as part of a structured, user-centred design and evaluation procedure.     

Use of auditory icons as emergency warnings: evaluation within a vehicle collision avoidance application

In the context of emergency warnings, auditory icons, which convey information about system events by analogy with everyday events, have the potential to be understood more quickly and easily than abstract sounds. To test this proposal, an experiment was carried out to evaluate the use of auditory icons for an invehicle collision avoidance application. Two icons, the sounds of a car horn and of skidding tyres, were compared with two conventional warnings, a simple tone and a voice saying ‘ahead’. Participants sat in an experimental vehicle with a road scene projected ahead, and they were required to brake in response to on-screen collision situations and their accompanying warning sounds. The auditory icons produced significantly faster reaction times than the conventional warnings, but suffered from more inappropriate responses, where drivers reacted with a brake press to a non-collision situation. The findings are explained relative to the perceived urgency and inherent meaning of each sound. It is argued that optimal warnings could be achieved by adjusting certain sound attributes of auditory icons, as part of a structured, user-centred design and evaluation procedure.     

Judging the urgency of non-verbal auditory alarms: a case study

When designed correctly, non-verbal auditory alarms can convey different levels of urgency to the aircrew, and thereby permit the operator to establish the appropriate level of priority to address the alarmed condition. The conveyed level of urgency of five non-verbal auditory alarms presently used in the Canadian Forces CH-146 Griffon helicopter was investigated. Pilots of the CH-146 Griffon helicopter and non-pilots rated the perceived urgency of the signals using a rating scale. The pilots also ranked the urgency of the alarms in a post-experiment questionnaire to reflect their assessment of the actual situation that triggers the alarms. The results of this investigation revealed that participants' ratings of perceived urgency appear to be based on the acoustic properties of the alarms which are known to affect the listener's perceived level of urgency. Although for 28% of the pilots the mapping of perceived urgency to the urgency of their perception of the triggering situation was statistically significant for three of the five alarms, the overall data suggest that the triggering situations are not adequately conveyed by the acoustic parameters inherent in the alarms. The pilots' judgement of the triggering situation was intended as a means of evaluating the reliability of the alerting system. These data will subsequently be discussed with respect to proposed enhancements in alerting systems as it relates to addressing the problem of phase of flight. These results call for more serious consideration of incorporating situational awareness in the design and assignment of auditory alarms in aircraft.     

Judging the urgency of non-verbal auditory alarms: a case study

When designed correctly, non-verbal auditory alarms can convey different levels of urgency to the aircrew, and thereby permit the operator to establish the appropriate level of priority to address the alarmed condition. The conveyed level of urgency of five non-verbal auditory alarms presently used in the Canadian Forces CH-146 Griffon helicopter was investigated. Pilots of the CH-146 Griffon helicopter and non-pilots rated the perceived urgency of the signals using a rating scale. The pilots also ranked the urgency of the alarms in a post-experiment questionnaire to reflect their assessment of the actual situation that triggers the alarms. The results of this investigation revealed that participants' ratings of perceived urgency appear to be based on the acoustic properties of the alarms which are known to affect the listener's perceived level of urgency. Although for 28% of the pilots the mapping of perceived urgency to the urgency of their perception of the triggering situation was statistically significant for three of the five alarms, the overall data suggest that the triggering situations are not adequately conveyed by the acoustic parameters inherent in the alarms. The pilots' judgement of the triggering situation was intended as a means of evaluating the reliability of the alerting system. These data will subsequently be discussed with respect to proposed enhancements in alerting systems as it relates to addressing the problem of phase of flight. These results call for more serious consideration of incorporating situational awareness in the design and assignment of auditory alarms in aircraft.     

A user-centred approach to the design and evaluation of auditory warning signals: 1. Methodology

This paper presents a method for the design and evaluation of auditory warning signals modelled on an existing internationally standardized method for evaluating public information systems (ISO/DIS 7001: 1979). The procedure is essentially user-centred, capitalizing upon users' associations between sounds and their meanings. The procedure is presented in a step-by-step manner, from the initial identification of referents for which warnings might be required, through the generation of ideas for warning sounds, an appropriateness ranking test, a learning and confusion test, an urgency mapping test, a recognition test and an operational test. Practical issues are discussed with respect to each of the stages, and suggestions are made as to courses of action that might be taken if problems are encountered. Theory behind the relationship between sound and meaning is discussed with reference to the practical issues addressed.     

Warn me now or inform me later: Drivers' acceptance of real-time and post-drive distraction mitigation systems

Vehicle crashes caused by driver distraction are of increasing concern. One approach to reduce the number of these crashes mitigates distraction by giving drivers feedback regarding their performance. For these mitigation systems to be effective, drivers must trust and accept them. The objective of this study was to evaluate real-time and post-drive mitigation systems designed to reduce driver distraction. The real-time mitigation system used visual and auditory warnings to alert the driver to distracting behavior. The post-drive mitigation system coached drivers on their performance and encouraged social conformism by comparing their performance to peers. A driving study with 36 participants between the ages of 25 and 50 years old (M=34) was conducted using a high-fidelity driving simulator. An extended Technology Acceptance Model captured drivers' acceptance of mitigation systems using four constructs: perceived ease of use, perceived usefulness, unobtrusiveness, and behavioral intention to use. Perceived ease of use was found to be the primary determinant and perceived usefulness the secondary determinant of behavioral intention to use, while the effect of unobtrusiveness on intention to use was fully mediated by perceived ease of use and perceived usefulness. The real-time system was more obtrusive and less easy to use than the post-drive system. Although this study included a relatively narrow age range (25 to 50 years old), older drivers found both systems more useful. These results suggest that informing drivers with detailed information of their driving performance after driving is more acceptable than warning drivers with auditory and visual alerts while driving.     

Cough once for danger: icons versus abstract warnings as informative alerts in civil aviation

OBJECTIVE: An experiment investigated the efficacy of auditory icons as warning signals in an aviation context. BACKGROUND: Iconic signals, such as a cough to signal dangerous levels of carbon monoxide, convey information about the nature of an incident and alert the operator that there is a problem, whereas signals that are arbitrarily associated with a critical incident provide relatively less information. Warning recognition speed and accuracy are likely to be influenced by modality of presentation (visual, auditory, auditory + visual) and task demand (low, high). METHODS: The 172 participants completed a computer-based training session and test task that involved responding to abstract or iconic auditory (1 s), visual, or auditory + visual warnings associated with seven critical incidents while performing low- and high-demand concurrent tasks. RESULTS: Significantly fewer training trials were required to learn iconic warnings than abstract warnings. An advantage for iconic warnings persisted into the test phase, evident most consistently as greater warning recognition accuracy. The effect was observed in both high- and low-demand conditions. Auditory abstract warnings, in particular, elicited slow reaction times and poor accuracy. CONCLUSION: Associations between a small number of meaningful environmental sounds and critical incidents can be learned with ease relative to more abstract associations, although training is required and response times are relatively slow. APPLICATION: Sets of distinctive auditory iconic warnings can be designed to alert and inform pilots about non-time-pressured events. Potential applications of language-neutral icons as informative warnings include civil, commercial, and defense aircraft.     

Semantic congruency of auditory warnings

Abstract

The aim of this study was to explore operator experience and performance for semantically congruent and incongruent auditory icons and abstract alarm sounds. It was expected that performance advantages for congruent sounds would be present initially but would reduce over time for both alarm types. Twenty-four participants (12M/12F) were placed into auditory icon or abstract alarm groupings. For each group both congruent and incongruent alarms were used to represent different driving task scenarios. Once sounded, participants were required to respond to each alarm by selecting a corresponding driving scenario. User performance for all sound types improved over time, however even with experience a decrement in speed of response remained for the incongruent iconic sounds and in accuracy of performance for the abstract warning sounds when compared to the congruent auditory icons. Semantic congruency was found to be of more importance for auditory icons than for abstract sounds.

Practitioner Summary: Alarms are used in many operating systems as emergency, alerting, or continuous monitoring signals for instance. This study found that the type and representativeness of an auditory warning will influence operator performance over time. Semantically congruent iconic sounds produced performance advantages over both incongruent iconic sounds and abstract warnings.     

A comparison of tactile, visual, and auditory warnings for rear-end collision prevention in simulated driving

OBJECTIVE: This study examined the effectiveness of rear-end collision warnings presented in different sensory modalities as a function of warning timing in a driving simulator. BACKGROUND: The proliferation of in-vehicle information and entertainment systems threatens driver attention and may increase the risk of rear-end collisions. Collision warning systems have been shown to improve inattentive and/or distracted driver response time (RT) in rear-end collision situations. However, most previous rear-end collision warning research has not directly compared auditory, visual, and tactile warnings. METHOD: Sixteen participants in a fixed-base driving simulator experienced four warning conditions: no warning, visual, auditory, and tactile. The warnings activated when the time-to-collision (TTC) reached a critical threshold of 3.0 or 5.0 s. Driver RT was captured from a warning below critical threshold to brake initiation. RESULTS: Drivers with a tactile warning had the shortest mean RT. Drivers with a tactile warning had significantly shorter RT than drivers without a warning and had a significant advantage over drivers with visual warnings. CONCLUSION: Tactile warnings show promise as effective rear-end collision warnings. APPLICATION: The results of this study can be applied to the future design and evaluation of automotive warnings designed to reduce rear-end collisions.     

Localizable auditory warning pulses

Three important forms of information available to the listener may be identified in any auditory warning: what (semantic), where (location) and when (perceived urgency). Each form is addressed in the present design study of auditory warning pulses. Recordings were made via a dummy head, and were presented dichotically to listeners in a left/right localization task. The first experiment identified a suitable notched noise component for providing location information in the pulse stimulus. The second experiment required participants to simultaneously identify a distinct tonal signature and the location of the sound when they were presented with one of three tonal types, or one of three compound (tone plus noise) stimuli. Response accuracy and response latency for this identification and localization task were significantly better with the compound than with tone alone stimuli. Perceived urgency of compound complex tone plus noise stimuli was investigated in the third experiment. While there may be a trade-off between localization acuity and perceived urgency, the addition of noise components to the auditory warning pulse was shown to enhance the location information available to the listener. It is suggested that some auditory warning designs will benefit from the simultaneous provision of what and where forms of information in the sounds.     

Effects of single versus multiple warnings on driver performance

OBJECTIVE: To explore how a single master alarm system affects drivers' responses when compared with multiple, distinct warnings. BACKGROUND: Advanced driver warning systems are intended to improve safety, yet inappropriate integration may increase the complexity of driving, especially in high workload situations. This study investigated the effects of auditory alarm scheme, reliability, and collision event type on driver performance. METHOD: Using a 2 x 2 x 4 mixed factorial design, we investigated the impact of two alarm schemes (master vs. individual) and two levels of alarm reliability (high and low) on distracted drivers' performance across four collision event types (frontal collision warnings, left and right lane departure warnings, and warnings for a fast-approaching following vehicle). RESULTS: Participants' reaction times and accuracy rates were significantly affected by the type of collision event and alarm reliability. The use of individual alarms, rather than a single master alarm, did not significantly affect driving performance in terms of reaction time or response accuracy. CONCLUSION: Even though a master alarm is a relatively uninformative warning, it produced statistically no different reaction times or accuracy results when compared with information-rich auditory icons, some of which were spatially located. In addition, unreliable alarms negatively impacted driver performance, regardless of event type or alarm scheme. APPLICATION: These results have important implications for the development and implementation of multiple driver warning systems.     

Quantifying and predicting the effects of basic text display variables on the perceived urgency of warning labels: tradeoffs involving font size,border weight and colour

Design variables surrounding the presentation of signal words in warnings have been previously shown to affect performance, or variables likely to be related to performance. The present study emphasized the signal word WARNING by presenting it in red, surrounded by a red border. It was at the head of an otherwise black pesticide label that used 8-point type. A study using 24 undergraduates investigated the effect of signal-word font size (8 to 32 points), white space around the signal word (2 to 32 points), and border width (1 to 8 points) on the rated urgency of the label. A measure of the aesthetic merit of each label was also obtained from nine design students and professionals. Seven levels of each variable were used, covering the practically-useful range of each variable in the circumstances. Results showed a linear relationship between each variable and perceived urgency, with text size having the greatest effect, followed by border width. It was shown that, within the variable ranges used, the perceived urgency of the emphasized signal word resulted from the sum of the urgencies associated with the separate dimensions used in the emphasis. Tradeoffs are discussed—for example an increase of 4 points in text size produced the same increase in urgency rating as produced by an increase in border width of 2-3 points. A second study, using a further 24 students, compared the red labels with labels in black only. It was found that the signal word had to be approximately twice as big in black as in red to give the same perceived urgency. Design implications of the finding that the most urgent labels are not necessarily the most aesthetically pleasing are discussed.