DEGREE PROJECT, IN , SECOND LEVELCOMPUTER SCIENCESTOCKHOLM, SWEDEN 2015

A Study of the Effect of LoomingIntensity Rumble Strip Warnings inLane Departure Scenarios

DAVID SANDBERG

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF COMPUTER SCIENCE AND COMMUNICATION (CSC)

A Study of the E�ect of Looming IntensityRumble Strip Warnings in Lane Departure

Scenarios

DAVID SANDBERGdsandb@kth.se

Master’s Thesis in Computer ScienceSchool of Computer Science and Communication (CSC)

Royal Institute of Technology, StockholmSupervisor at CSC: Sten Ternström

Supervisor at Osaka University: Takao OnoyeExaminer: Jens Lagergren

November 12, 2015

Abstract

In lane departure warning systems (LDWS) it is impor-tant that the auditory warning triggers a fast and appro-priate reaction from the driver. The rumble strip noise isa suitable warning to alert the driver of an imminent lanedeparture. A short reaction time is important in lane de-parture scenarios, where a late response may have fatal con-sequences. For abstract sounds an increase in intensity caninfluence the perceived urgency level of the warning, whichmay also trigger a faster reaction from the listener. In thisthesis, the e�ect of a rumble strip warning with looming(increasing) intensity was analyzed by letting test personsdrive a driving simulator and measuring how quickly theyreacted to the auditory warning. These results were com-pared with those for a rumble strip warning with a constantintensity, and two versions of an abstract warning; constantintensity and looming intensity. A survey regarding the per-ceived urgency, annoyance and acceptance of the warningswas also carried out.

The results show no di�erences in reaction time be-tween the four warning signals. This may be because thetest persons expected the warnings, or because of their lim-ited experience. The survey suggests that adding a loomingintensity to the rumble strip warning results in a higher ur-gency, while keeping the annoyance low, which could be ofimportance to avoid unwanted reactions from the driver.

Referat

En studie av e�ekten av bullerrä�eljud med

ökande intensitet vid ofrivilligt lämnande av

körfältet

I varningssystem för personbilar används ofta ett systemsom signalerar ett stundande ofrivilligt lämnande av körfäl-tet, s.k. lane departure warning systems (LDWS), genomatt en varningssignal ljuder. Det är viktigt att en sådanakustisk varningssignal frammanar en snabb och lämpligreaktion från föraren. Ljudet av en bullerrä�a är en lämp-lig varningssignal för detta ändamål. En kort reaktionstidär viktig när fordon är på väg att ofrivilligt lämna körfäl-tet, då en långsam reaktion kan ha förödande konsekven-ser. Studier på abstrakta akustiska varningssignaler har vi-sat att en ökande intensitet kan få en varning att verkamer brådskande, vilket i sin tur kan leda till att lyssna-ren reagerar snabbare. I denna rapport analyseras hur ettbullerrä�eljuds ökande intensitet påverkar förarens reak-tionstid. Analysen gjordes genom att mäta reaktionstidenhos testpersoner som körde en bilsimulator med fyra oli-ka varningssignaler; en bullerrä�eljudsvarning och en ab-strakt varning, båda med konstant intensitet och ökandeintensitet. Reaktionstiderna för de olika signalerna jämför-des, varpå en enkät utfärdades där testpersonerna uppgavhur brådskande och irriterande de uppfattade varningarna,samt till vilken grad de skulle acceptera varningarna i ettverkligt körscenario.

Resultaten visar inga skillnader i reaktionstid mellanvarningarna, vilket kan bero på att testpersonerna förut-såg när varningarna skulle komma, eller på grund av derasbegränsade erfarenhet av bullerrä�eljud. Enkätens utfallantyder att bullerrä�eljudsvarningen med ökande intensi-tet är mer brådskande än versionen med konstant intensi-tet, men att irritationsnivån inte påverkas när intensitetenökar, vilket kan vara viktigt för att inte framkalla oönskadereaktioner hos föraren.

AcknowledgementsThank you to my supervisor at KTH, Prof. Sten Ternström, for continuous supportand feedback throughout the research process.Thank you to my supervisor at Osaka University, Prof. Takao Onoye, for giving methe opportunity to do the research in his lab and providing the necessary equipmentand guidance.Thank you Dr. Wataru Kobayashi of Arnis Sound Technologies for suggesting thefield of this research and providing suggestions for the warning signal designs.Thank you Prof. Jens Lagergren for examining the thesis.Thank you to fellow students Andreas Wedenborn, Dennis Johansson, Christo�erCarlsson and Jens Eriksson for reading and commenting on the thesis throughoutthe research process.Thank you Ann Bengtsson for helping me find a supervisor and examiner on shortnotice.Thank you Dr. Johan Fagerlönn for answering my questions and providing feedbackregarding the auditory warning signal design.Thank you to all the test persons who participated in the evaluation process.Thank you to the Scandinavia-Japan Sasakawa Foundation for providing fundingto help make this research possible.Thank you to my family for always supporting me.

Contents

1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 What makes an auditory warning? . . . . . . . . . . . . . . . . . . . 21.3 Non-speech auditory warnings . . . . . . . . . . . . . . . . . . . . . . 3

1.3.1 Auditory icons . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3.2 Earcons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 Speech warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.5 Urgency and annoyance . . . . . . . . . . . . . . . . . . . . . . . . . 71.6 Related work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.6.1 Ziegler et al. (1995) . . . . . . . . . . . . . . . . . . . . . . . 71.6.2 Haas & Edworthy (1996) . . . . . . . . . . . . . . . . . . . . 71.6.3 Suzuki & Jansson (2003) . . . . . . . . . . . . . . . . . . . . . 81.6.4 Gray (2011) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.7 This thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Method 11

2.1 Driving simulator design . . . . . . . . . . . . . . . . . . . . . . . . . 112.1.1 Game engine . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.1.2 Car model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.1.3 Road and surroundings . . . . . . . . . . . . . . . . . . . . . 122.1.4 Triggers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 Warning signal design . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.1 Rumble strip warning . . . . . . . . . . . . . . . . . . . . . . 152.2.2 Abstract warning . . . . . . . . . . . . . . . . . . . . . . . . . 162.2.3 Choosing intensity levels . . . . . . . . . . . . . . . . . . . . . 172.2.4 Implementing the looming . . . . . . . . . . . . . . . . . . . 18

2.3 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.3.1 Test persons . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.3.2 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.3.3 Secondary task . . . . . . . . . . . . . . . . . . . . . . . . . . 202.3.4 Driving test . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3 Results 23

3.1 Driving test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.1.1 Reaction times . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.3 Test persons’ opinions . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4 Conclusion 27

4.1 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.1.1 Reaction times . . . . . . . . . . . . . . . . . . . . . . . . . . 274.1.2 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Bibliography 31

Chapter 1

Introduction

In the following section the background to this thesis will be presented and impor-tant theories in the field of auditory warnings will be explained. Lastly, related workthat has been the foundation for, or in other ways inspired, the current researchwill be discussed.

1.1 BackgroundIn 2009, 34,500 people were killed in road tra�c accidents in the EU. Although thenumber of road casualties was twice as high ten years earlier, it was still an indicationthat a greater e�ort was needed in order to increase road safety. Therefore, in 2011the European Commission set a goal to reduce the number of casualties by half by2020, and to have zero fatalities by 2050 (European Commission, 2011). Measures ofreaching this goal include educating drivers on the importance of using the providedsafety equipment, for example seat-belts, as well as developing a safer infrastructure.Furthermore, it is also important to make use of current road safety technology tobattle this problem. Implementing Advanced Driver Assistance Systems (ADAS)in cars is one way of making use of such technology. ADASs are systems thatassist the driver in various ways, and can involve systems handling features such ascruise-control, collision detection and lane departure warnings. At present, thereare no strict guidelines as to what should be included in an ADAS, and whetheror not to implement an ADAS in a vehicle is up to the manufacturer to decide.Consequently, the warning signals for collision detection and lane departure canalso di�er between di�erent ADASs. However, it is common to use some sort ofauditory warning signals to catch the driver’s attention. The question is what kindof auditory warning signal is the most appropriate for a certain situation.

According to a report published by The American Association of State Highwayand Transportation O�cials (2008), close to 60% of all fatal car accidents in theU.S. were due to the car leaving its lane (i.e. a lane departure). Another analysisof data from the Swedish Tra�c Accident Data Acquisition (STRADA) system onheavy trucks in Sweden between 2003 and 2008 revealed that lane departure was

1

CHAPTER 1. INTRODUCTION

the cause in 40% of the cases where the driver was killed or seriously injured (VolvoTrucks, 2013). These figures suggest that a large amount of lives can be saved if theamount of lane departure accidents is reduced. One common way of trying to solvethis problem is through implementing Lane Departure Warning Systems (LDWS)into cars. The LDWS will assist the driver by triggering a warning signal if thecar gets too close to the lane boundary, or crosses it. Most research has focused onauditory warning signals, but there are also studies comparing auditory and hapticwarning signals (e.g. Ziegler et al., 1995; Stanley, 2006). One reason that auditorywarning signals are widely used is that they catch our attention immediately even ifwe are not looking at the source of the sound, as compared with visual warnings thatrequire us to already be focusing on the source in order to alert us (Patterson, 1989).One commonly used auditory warning signal is the “rumble strip noise”, whichimitates the sound that is produced as the wheels of a car hits the rumble strip thatis often present at the edge of a road. Research has shown that the rumble strip noisecan be e�ective in alerting a driver of an imminent lane departure (Ziegler et al.,1995; Fagerlönn, 2011a). These studies were performed on professional truck driverswith years of driving experience. However, the perceived urgency of a sound doesnot only depend on its acoustical characteristics, but also on the listener’s previousexperience of that sound (Västfjäll et al., 2006). It can therefore be interesting tostudy if less experienced drivers perceive the rumble strip noise in the same wayas experienced drivers. Furthermore, most research evaluating auditory warningsignals in cars have used signals that have constant acoustical parameters. In otherwords, parameters such as pitch and intensity do not change over time. However, astudy on auditory warning signals for frontal collision detection showed that a signalwith a looming (increasing) intensity can be more e�ective in reducing a driver’sreaction time (Gray, 2011). Applying this idea to lane departure warnings couldtherefore be of interest.

This thesis presents an implementation of a looming intensity rumble strip warn-ing signal. The signal will be evaluated in lane departure situations by letting testpersons, with limited driving experience, drive a driving simulator and measuringtheir reaction time (RT) once a warning signal has been triggered. A survey willalso be conducted to find out how urgent, annoying and accepted the signal is in alane departure scenario.

1.2 What makes an auditory warning?

When designing auditory warnings there are many things to keep in mind. If thewarning system is to be used in an environment which requires multiple warnings,such as hospitals or flight decks, one of the challenges lies in designing signals thathave di�erent acoustic characteristics, in order to avoid confusion. Another problemis masking. Masking happens when one sound interferes with another sound of thesame, or a very similar, frequency (Ulfvengren, 2003b). For example, if you aretalking to a friend at a sound level of 65 dB and someone turns on the vacuum

2

1.3. NON-SPEECH AUDITORY WARNINGS

cleaner at 75 dB, then the speech will be masked by the vacuum cleaner, and youwould have to raise your voices above 75 dB to hear each other. The level di�erencebetween the two signals is known as the masked threshold. If the sound levels arethe same, the signals will merge and be di�cult to tell apart. Furthermore, if thewarning signal is too loud it may cause a startle reaction which can result in theconcentration level dropping. It may also make any necessary communication moredi�cult (Edworthy et al., 1994). Another problem that is often found in warningsystems is that the perceived (psychoacoustic) urgency of a warning is not properlymatched with the urgency of the situation (Edworthy et al., 1991). Consider twowarnings with di�erent perceived urgency; high and low. Let us assume that thereis a mismatch between the two warnings’ urgency levels and the situations theyportray; the “high” warning is mapped to a low urgency situation, and vice versa.If the two warnings are being sounded at the same time, the less urgent situationmay be attended to first, which may have unwanted consequences.

Edworthy et al. (1994) observed two main requirements that need to be fulfilledin order to create an auditory warning signal. Firstly, the warning has to havean appropriate intensity level; not too loud, but still loud enough to catch thelisteners attention. If the intensity of a warning rises too quickly, it will startle thelistener who in turn might respond instantaneously, and out of reflex. These types ofresponses are often not the most appropriate in a given situation (Patterson, 1989).Secondly, the warning should in some way be properly related to the situation thatit is portraying. For example, the psychoacoustic urgency of a warning shouldmatch the urgency of the situational urgency, or have a natural connection to thedesired action (e.g. screeching tires to tell a driver to hit the brakes). Furthermore,a warning must also invoke a quick and accurate response (Graham, 1999).

Västfjäll et al. (2006) claimed that the bond between a sound and a person’sprevious experiences of that sound is important. When we first hear a sound, theacoustical characteristics (pitch, intensity etc.) are analyzed to see if it should beinterpreted as a warning. If a certain arousal threshold is exceeded, then we respondto it. However, if the “arousal potential” is too low we try to find a connection to thesound in our bank of memories. If a match is found that has a dangerous or negativeconnotation, then we respond. Otherwise, the sound will not be be regarded as awarning, and no response is necessary. This suggests that warnings may be moree�ective if they not only trigger an arousal, but that they also have a connection toa person’s past experiences.

1.3 Non-speech auditory warnings

A method to solve the problems mentioned in the previous section was found inPatterson’s guidelines to designing auditory warnings in 1982, and consisted of fourmain steps; deciding the appropriate loudness level of the warning (15-25 dB overthe masked threshold), creating a short sound pulse (100-300 ms long), creatinga burst consisting of several pulses of various frequencies and pauses, and finally

3

CHAPTER 1. INTRODUCTION

adding several bursts together with a silence in between to form the final warning.It was also suggested that each pulse should have an onset and o�set of 20 ms toavoid startling the listener. (Patterson (1982) cited in Edworthy et al., 1991). Theseguidelines proposed a new way of designing auditory warnings, where the perceivedurgency level could be altered to better fit the situational urgency. The maskingproblem has also been addressed by Patterson (1989), who claims that if the soundconsists of four or more harmonics it is less likely to be masked than one that hasall the energy focused on just one harmonic.

Using Patterson’s proposed guidelines, Edworthy et al. (1991) showed in moredetail how pulse and burst parameters should be altered to convey di�erent levels ofurgency. The acoustic parameters studied were fundamental frequency, harmonicregularity, amplitude envelope and delayed harmonics. These were part of thedesign of the pulse. In a regular harmonic series, every harmonic is an even integermultiple of the fundamental frequency. A delayed harmonic is a harmonic whoseonset is delayed compared with the fundamental frequency. It was discovered thatfundamental frequency had a smaller impact on urgency than expected, and thatirregular harmonics and a standard onset (20 ms) were the two acoustic parametersthat were most important to convey a high level of urgency. Among the burstparameters, it was found that the single most e�ective parameter was speed; thefaster the burst, the more urgent the warning. By altering the pulse and burstparameters accordingly, it was possible to create auditory warnings with the desiredurgency levels.

A few years later, research showed that the signals yielding the highest perceivedurgency were those with a high speed, a high frequency and a high level of loudness(Haas and Edworthy, 1996). Furthermore, increasing pitch and loudness showed adecrease in reaction time.

The research mentioned above has focused only on variations of tones, which ispart of the abstract sounds family. Abstract sounds have no obvious connection towhat they are portraying, and consist of sounds such as bells, buzzers and sirens.Even though they can be learnt over time, “they may not be well suited to emergencywarning situations, which in certain applications occur only very rarely” (Graham,1999, p. 1234). It has also been shown that abstract sounds may produce slowerreaction times than other types of sound (McKeown, 2005). A di�erent type ofsounds that has a more natural connection to the situation it is portraying, isknown as auditory icons.

1.3.1 Auditory iconsThe concept of auditory icons was introduced by Gaver in 1989 who defined them as“everyday sounds meant to convey information about computer events by analogywith everyday events” (p. 67). The main idea of auditory icons is to take everydaysounds and apply them to related events within another field, for example by usingthe sound of a piece of paper being crumpled when emptying the trashcan in acomputer. By using sounds that have connections with our everyday lives, it is

4

1.3. NON-SPEECH AUDITORY WARNINGS

easier to convey the meaning of the sounds in the new context. This may bebecause we tend to hear the source of the sound rather than the sound itself. Forexample, as a door shuts, we hear its size and with what force it shuts, and not thepitch, loudness or other acoustical parameters of the sound.

Ulfvengren (2003a, cited in Ulfvengren, 2003b) introduced the concept of “asso-ciability” as “the required e�ort to associate sounds to their assigned alert functionmeaning” (p. 53). Furthermore, she states that the more “associable” a sound is,the fewer cognitive resources are needed, which would make it appropriate for au-ditory warnings. Other benefits with sounds with a high associability are that theyare easy to remember and discern from other sounds. Ulfvengren carried out testswith various types of auditory warnings to find out which sounds were the easiestto remember. It was shown that auditory icons were better than all other sounds,including animal sounds and abstract sounds. These results suggests that audi-tory icons has the highest associability, and may therefore be suitable for auditorywarnings.

Graham (1999) compared two auditory icons (tyre-skid sound and car-hornsound), a verbal warning (’ahead!’) and a beep sound in a car driving environ-ment. The warnings were triggered when a collision was imminent. Examining thereaction times of the drivers showed that the auditory icons produced significantlyfaster reaction times than the other warnings. However, it was also shown that theauditory icons produced a higher rate of inappropriate responses, such as makingthe driver hit the brakes even in a non-collision situation (false-positives). A sur-vey carried out among the test participants revealed that the car-horn sound wasconsidered to be very appropriate for all collision situations (more than the beepsound), while the tyre-skid sound was much less appropriate. This suggests thatalthough auditory icons are related to everyday events, they may have several possi-ble interpretations, some being less appropriate for a certain situation than others.Consequently, they may also be misinterpreted.

Similar to Graham’s results, research by McKeown (2005) showed that auditoryicons resulted in faster reaction times than abstract sounds when used in within-vehicle scenarios. This strengthens the claim that auditory icons are suitable aswarnings when a quick response is needed. In the tests, warnings were mappedto driving scenarios of various urgency. It was observed that all warnings exceptspeech were perceived as less pleasant the more urgent the scenario was, revealinga connection between annoyance and urgency. This suggests that the annoyancelevel of a warning may depend on its learnt meaning, more than its acoustic charac-teristics. Furthermore, McKeown points out that it may be better to use auditoryicons with di�erent learned urgency, than to manipulate one sound acoustically tomake it sound more or less urgent. This is because such manipulation may a�ectthe recognizability of the auditory icon.

5

CHAPTER 1. INTRODUCTION

1.3.2 Earcons

The concept of earcons was introduced by Blattner, Sumikawa and Greenberg(1989), who described them as “nonverbal audio messages used in the user-computerinterface to provide information to the user about some computer object, operation,or interaction” (p. 13). The earcons were divided into two types; representationalearcons and abstract earcons. The former is just another name for the auditoryicons described in the previous section, while abstract earcons are “abstract, syn-thetic tones that can be used in structured combinations to create sound messagesto represent parts of an interface” (Brewster, 1994, p. 7). Parameters such as pitch,timbre, and dynamics may be altered to form a specific earcon. While auditoryicons sound like what they represent, the meaning of an abstract earcon is not in-tuitive, and has to be learned. From here on, the word earcon will be used forabstract earcons. An example of a usage of an earcon may be to indicate whereon the computer screen a message is displayed. An earcon with a low pitch couldindicate a message appearing at the bottom of the screen, while an earcon with ahigher pitch could indicate a message appearing at the top of the screen.

Lucas (1995) studied how well auditory icons and earcons described actions andobjects in a human-computer interface. Test persons were asked to connect eachsound to the action or object that they thought the sound best represented. It wasshown that it was more di�cult to connect accurately the earcons to the correctaction or object, than for the auditory icons. However, Lucas points out that thelimited number of sounds used in the test may have impacted the result to a higherdegree than the audio cue design method.

1.4 Speech warnings

In the research by McKeown (2005) mentioned in section 1.3.1, it was shown thatspeech sounds could accurately be mapped to the situations they were supposed toportray. However, the urgency of the speech sounds were constantly rated as inter-mediate, regardless of the situational urgency. On the other hand, earlier researchhad shown that speech warnings could in fact produce various levels of urgency,although the range of potential urgencies were shown to be wider for nonspeechwarnings (Edworthy, et al., 2000).

Furthermore, other research suggests that the reaction times for speech can beslower than those for both abstract sounds and auditory icons (Graham, 1999).Graham mentions that the reason is that even a short spoken warning will takea longer time to interpret, and that it is not likely that the reaction time wouldbe improved by changing the warning’s parameters. However, recent research hasshown that the accuracy and reaction time for short speech warnings and auditoryicons may be similar (Fagerlönn & Alm, 2010). Still, it is mentioned that speech canbe a�ected by background noise or other people talking, which may cause problemsin a real driving scenario.

6

1.5. URGENCY AND ANNOYANCE

1.5 Urgency and annoyanceAlthough the urgency of a warning signal is an important parameter when it comesto catching a person’s attention, the perceived annoyance level must also be con-sidered. The urgency may have an e�ect on a driver’s ability to quickly recognizeand react to a warning, but the annoyance can have a negative influence on thedriver, causing the warning to be disabled or ignored (Marshall et al., 2007). Inother words, if the warning is too annoying it may be turned o�, which defeats thewhole purpose of using a warning system. Furthermore, if a driver gets angry, itmay a�ect the driving performance and result in more tra�c violations (King andParker, 2008). Tan and Lerner (1995) showed that there is a correlation between awarning’s perceived urgency level and its annoyance level, while other research hassuggested that certain acoustical parameters have a stronger e�ect on urgency thanothers (Marshall et al., 2007). It is therefore of importance to evaluate both theurgency and annoyance when designing auditory warnings.

1.6 Related workSeveral important sources have already been described in the previous sections.However, below is some of the research that has been most significant to the decisionsmade regarding the evaluation methods and warning signal designs in this thesis.

1.6.1 Ziegler et al. (1995)Ziegler et al. let 18 professional truck drivers drive a driving simulator for 30minutes. Upon an imminent lane departure a warning signal was triggered. Thewarning signals consisted of one haptic signal with a slightly oscillating steeringwheel, one haptic warning with a torque that turned the steering wheel towards themiddle of the lane, and one auditory icon (rumble strip noise). After the test drive,interviews showed that the highest acceptance was achieved by the rumble stripwarning, and that the majority of drivers said that it was well suited for the lanedeparture scenario. Further tests measuring reaction times showed that the rumblestrip warning had a mean reaction time of 0.5 seconds, which was considered asbeing “very fast”.

1.6.2 Haas & Edworthy (1996)Haas and Edworthy carried out research to find out what acoustic parameters makea sound more urgent. They also investigated how the perceived urgency was relatedto reaction time. The fundamental frequency of a pulse, the inter-pulse interval(time between pulses), and the pulse level (intensity) were the independent variables.The evaluation was made by letting 30 college students listen to 27 auditory signals.They were asked to subjectively rate each signal’s urgency, and were then asked to

7

CHAPTER 1. INTRODUCTION

press a button as soon as they heared the signal, in order to measure the reactiontime for each signal. It was concluded that the most urgent signals also had theshortest reaction times. These signals shared the same characteristics; they had ahigh frequency, a fast speed, and were loud.

Haas and Edworthy suggested that an auditory signal with a fundamental fre-quency of at least 500 Hz, a loudness of between 15 and 30 dB above ambient,and with a 0 ms inter-pulse interval was a suitable signal for designers who wishto achieve the highest level of perceived urgency and the shortest reaction times.In this thesis, these guidelines were followed when designing the abstract warningsignal.

1.6.3 Suzuki & Jansson (2003)Suzuki and Jansson investigated the e�ect of four di�erent warning methods ondrivers’ reaction times in lane departure scenarios. The warning methods used weremonaural beeps (sound played from both speakers), stereo beeps (sound playedfrom the speaker on the side of the lane departure), steering vibration, and pulse-like steering torque. The tests were carried out with 24 experienced drivers, reportedto drive at least 5000 km/year. In the tests where the subjects were aware that thewarning signified a lane departure, both the monaural and stereo beeps reduced thereaction times. However, there were no pairwise di�erences between the monauraland stereo beeps, and it was observed that the drivers looked at the road beforereacting in both cases. In the tests where the drivers were unaware of the meaning ofthe warning, the steering vibration showed a significant decrease in reaction times.

In order to trigger a lane departure, the subjects were asked to perform a sec-ondary task shown on a separate display mounted on the passenger seat. On theseparate display, five random numbers were shown, and the subjects had to readthe numbers out loud. While the subjects were engaged in the secondary task, theyaw angle of the vehicle was changed randomly to the left or right by two degrees.

In this thesis, a similar secondary task and yaw angle change were implemented,as described in more detail in Chapter 2: Method. The subjects were asked ifthey drove more than 5000 km/year, which is the same threshold that Suzuki andJansson used. The road and shoulder widths used in this thesis also correspond tothose used by Suzuki and Jansson, at 3.5 m and 1.0 m respectively.

1.6.4 Gray (2011)Gray investigated what auditory warning signals produced the fastest reaction timesin a collision scenario. 20 test persons drove behind a lead car in a driving simulatorand were asked to react to an auditory warning signal when the lead car sloweddown. In total seven signals were tested; four nonlooming signals and three loomingsignals. The nonlooming signals were constant intensity, ramped, pulsed and a carhorn sound. The looming signals, which had rising intensities to simulate that thelead car was getting closer, were veridical (actual), early or late. The early signal

8

1.6. RELATED WORK

had a steep time-intensity curve to suggest that the time to collision (TTC) withthe lead car was shorter than it actually was, while the late signal had a flatter time-intensity curve which suggested that the lead car was further away than it was. Theveridical signal had a time-intensity curve that matched the TTC, meaning that themaximum intensity would be reached at the same time as a collision with the leadcar would occur. All signals except the car horn had a frequency of 2000 Hz. Theconstant intensity signal had an intensity of 75 dB, the ramped signal increasedlinearly from 60 dB to 85 dB, while the pulsed signal started at 0 dB and reacheda maximum of 75 dB for each pulse. The intensity of the looming warnings variedas shown in Figure 1.1.

Figure 1.1: Time-intensity profiles for veridical looming warnings triggered at di�erentspeeds (from Gray, 2011, used with permission).

The tests showed that the veridical looming warning signal produced, on aver-age, 77 to 115 ms faster brake reaction times (BRT) than the abstract warnings.Although the car horn had BRTs similar to the veridical looming warning, it alsoproduced more brake responses in the situations when the warning was triggered asa false alarm. It was also observed that the BRT was faster for the early loomingwarning than for the late looming warning. The results suggest that it is possibleto influence how fast a driver reacts in a collision situation by altering the TTC ofa looming warning.

The time-intensity curve used for the looming warnings in this thesis was createdto be an approximation of the curves used by Gray, but with di�erent onset ando�set intensities, and with a shorter duration. Furthermore, Gray’s idea of tellingthe driver how far away a potential danger is by altering the intensity has been an

9

CHAPTER 1. INTRODUCTION

inspiration for this thesis.

1.7 This thesisThe primary goal of this thesis is to see if a looming rumble strip warning willproduce faster reaction times than warning signals with constant intensity in a lanedeparture scenario. This is done by first building a simple driving simulator usedin the driving tests.

Two types of auditory warning signals will be used; one rumble strip warningand one abstract warning. Each warning will have two variations; one version withconstant intensity, and one version with looming intensity. The warnings will bedesigned in Matlab and automatically triggered from the driving simulator.

Test persons will participate in the evaluation which involves driving the caralong the road. At certain points where the car is driving on a straight section, asecondary task will be triggered in order to force a lane departure. While the testpersons are busy with the secondary task, a slight shift in direction of heading willbe introduced. This will prevent the drivers from knowing on which side of the roadthe lane departure will occur. As a lane departure occurs, a warning signal willbe triggered, and the test persons have to respond to the warning by steering backonto the road. The elapsed time from the warning onset to the time of the reaction(when the driver turns the steering wheel) will be recorded. This experiment willbe performed for each of the four di�erent warning signals, and the results will thenbe evaluated for statistical significance.

A second goal is to determine the perceived urgency, annoyance and acceptancefor each warning signal. This will be done through a survey where the test personsrate the warnings on a scale from 1 to 5 for each category. It is important to knowif a driver would accept a certain warning in the lane departure scenario, since thewarning may otherwise be ignored or have unwanted e�ects on the driver.

The hypothesis is that both variations of the rumble strip warning (constantintensity and looming) will result in faster reaction times than their abstract warningcounterparts. Furthermore, the looming rumble strip warning is expected to showfaster reaction times than the constant intensity variant, due to the increasingintensity. Since previous research has shown a high acceptance for the rumblestrip warning, similar results can be expected from the survey in this thesis.

10

Chapter 2

Method

The method can be divided into four main parts; designing the driving simulator,designing the auditory warning signals, evaluating the reaction times, and evaluat-ing the perceived urgency, annoyance and acceptance. Each part will be describedin detail in the following sections.

2.1 Driving simulator design

In order to get the best results from tests in the field of collision and lane departurewarnings, it is important to perform the tests in an environment that is as close toa real driving experience as possible. Since performing the tests in a real drivingscenario would be dangerous and unethical, most research has used top of the linedriving simulators consisting of a vehicle cabinet that the test person enters, whereseveral displays present the driving environment to the driver. The driving physicsare also designed to properly simulate the behavior of a real vehicle. Although asimilar simulator environment would have been the most suitable for the currentresearch, due to time and budget constraints it was not a feasible solution.

The modification of an existing open-source driving game such as Speed Dreams(2014) was also considered. However, this idea was abandoned since it was notcertain that triggering warning signals, saving reaction time data to file, and othernecessary functionality would be available without extensive modifications. Instead,in order to fit the purpose of this research and to have total control over the test en-vironment, it was decided to build the the driving simulator from scratch. Althoughthe main goal of this research was to evaluate the various warning signals, since alarge portion of the research was spent on developing the driving simulator, somedetails of the development process and how the driving simulator works should alsobe mentioned.

11

CHAPTER 2. METHOD

2.1.1 Game engineThe driving simulator was built in the game engine Unity (Unity Technologies,2014), which is one of the most popular game engines used by game developerstoday. The decision to use Unity was mainly due to the author’s previous experiencewith the game engine, and also because Unity has a large community of userswhich has provided assets such as road building tools. Furthermore, the size of thecommunity suggested that many of the potential problems involved in building adriving simulator had most likely already been faced and discussed by other users inthe community online forums. The API is well documented and most functionalityrequired for designing a simple driving simulator exists. The programming in Unityis done through writing scripts in either Javascript or C# and attaching them tothe di�erent game objects. In the current driving simulator the choice was to useC#.

2.1.2 Car modelThe car model used in the driving simulator was called “Peugeot” and was includedin the o�cial Unity Car Tutorial package available from the Unity Asset Store.The package included scripts for the car’s behavior. The scripts included variablessuch as damping, grip, friction and braking torque, all of whose values were kept asdefaults. The size of the car was altered to fit the dimensions of a Peugeot 205 asfollows; 3.705 m (length) x 1.572 m (width) x 1.4 m (height). The script controllingthe car only supported digital steering, acceleration and braking. Therefore, inorder to fit the steering wheel input that was going to be used during the tests, thescript was edited to support analog input as well. The speed of the steering alsohad to be altered in order to make the car controllable. Although these alterationswere made to the best of the author’s abilities, and thoroughly tested throughoutthe development process, it should be noted that they may not perfectly representthe properties of a real vehicle. However, it was estimated that the settings wereappropriate enough to give the drivers good control over the vehicle, and also tomeasure their reaction times when a warning was triggered. Handling the car onlyinvolved acceleration and braking - no gear shift was implemented.

2.1.3 Road and surroundingsThe road was built using the road building tool EasyRoads3D Free (AndaSoft,2015), available from the Unity Asset Store. This tool provides the user with agraphical interface and the ability to create roads by placing markers on the desiredpoints on the map. Once all the markers have been placed, the road will be drawnfollowing the path of the markers. The road used in the current research was 9.85 kmlong, connected, and included five longer straights (see Figure 2.1). The straightswould be suitable places to introduce the secondary task to the driver and therebyforcing a lane departure. The road consisted of a single 3.5 meter wide lane, withone 1.0 meter wide shoulder on each side. The 3.5 meter width of the lane is the

12

2.1. DRIVING SIMULATOR DESIGN

Figure 2.1: Overhead view of the course used in the driving tests

standard for Swedish motorways. Each side of the lane was marked with white lanemarkings. An overhead view of the road can be seen in Figure 2.2. The choice ofusing only a one-lane road was due to limitations in the EasyRoads3D Free tool.At first, the plan was to use the car’s position relative to the road in order todetermine when a lane departure had occurred. However, these coordinates couldnot be acquired since the position of a game object is relative to the whole gameworld. Consequently, the idea to trigger a lane departure warning based on the carcoordinates was not feasible. However, the road building tool had a function toalter the width of the road. Therefore, once the road (R1) had been drawn (with awidth of 5.5 m), an invisible copy of the road (R2) with a narrower width (3.5 m)was added on top of R1. Changing the width to 3.5 m meant that R2 became 1.0m narrower on each side, compared with R1. A lane departure could therefore beregistered as soon as the car had started to leave R2 on either side. This was doneby using triggers, as described in the next subsection.

The surroundings were kept simple, without any buildings, trees, vehicles orother objects. This was mainly due to the limited time available, but also in orderto prevent any latency that may appear due to heavy rendering. The first-personview that the test persons experienced while driving can be seen in Figure 2.3.

13

CHAPTER 2. METHOD

Figure 2.2: Close up of the car and the road

Figure 2.3: First-person view of the road and surroundings

14

2.2. WARNING SIGNAL DESIGN

2.1.4 TriggersIn Unity, a game object (x) can be set to be a trigger. This means that if anothergame object (y) enters the space where x is currently residing, instead of the twoobjects colliding with each other, y can enter x. When this happens, the OnTrig-gerEnter method in the script assigned to x is called. Any code that should beexecuted when another game object enters x can be put in this method. A similarmethod, OnTriggerExit, is called when an object leaves a trigger.

In the driving simulator, one trigger was attached to each side of the car, closeto the front wheels. Both triggers were in contact with the road. Whenever one ofthe triggers left the R2 road, the OnTriggerExit method was called, in which thecode to playback the warning signal was executed.

2.2 Warning signal designThe main purpose of this research was to study the e�ect of a looming intensityrumble strip warning on a driver’s reaction time. In order to evaluate the e�ect ofthe looming intensity, it was necessary to compare it with a rumble strip warningwith constant intensity. Furthermore, it could be interesting to examine whetherthe looming intensity had an e�ect also on abstract warning signals. Therefore, twoabstract warning signals were also developed; one with looming intensity and onewith constant intensity. All the warning signals were developed in Matlab, and thelooming was introduced using the sound editor software Audacity (Audacity Team,2015). In the following sections, the warning signal design will be discussed in moredetail.

2.2.1 Rumble strip warningIn order to create a synthetic warning signal resembling a real rumble strip noise asmuch as possible, a recording of a rumble strip noise from inside a car was used as areference (U.S. Department of Transportation, 2011). Since the rumble strip noiseis a low-frequency sound, the fundamental frequency of the warning signal was setto 90 Hz. The fundamental frequency is the lowest frequency present in a soundwave. Four additional harmonics were used to complete the sine wave in the rumblestrip noise. Each harmonic is an integer multiple of the fundamental frequency.The harmonics chosen were 180, 270, 360 and 450 Hz, and the amplitude ratiosrelative to the fundamental frequency were 1.14, 1.43, 1.14 and 0.71 respectively.In order to create a rumbling sound, the Matlab Audio Toolkit (Smith, 2012) wasdownloaded, and the noisefilter function was used to create a white noise in thefrequency range 150 to 500 Hz, with an amplitude ratio of 2.86 relative to thefundamental frequency. The frequency of a rumble strip noise is not constant.Instead, the sound displays acoustic properties similar to that of a vibrato or abeating. A vibrato is a pulsating pitch change in a sound wave, revolving aroundthe fundamental frequency. To simulate a vibrato, the vibrato function in the Audio

15

CHAPTER 2. METHOD

Figure 2.4: Frequency spectrum of the rumble strip warning signal

Toolkit was used. The sine wave was assigned a vibrato frequency of 10 Hz, and adepth of 0.01. The noise was assigned a vibrato frequency of 15 Hz, and a depthof 0.05. The depth is the size of the frequency deviation around the fundamentalfrequency and it determines between which frequencies the vibrato should vary. Adepth of 0.01 means that the vibrato will vary with +/- 1% from the fundamentalfrequency. The vibrato frequency is the speed at which the vibrato should move.Combining the sine wave and the noise completed the design of the rumble stripwarning signal. The frequency spectrum can be seen in Figure 2.4. In order toconfirm that the created signal was similar to a real rumble strip noise, Dr. WataruKobayashi of Arnis Sound Technologies was consulted because of his expertise inthe field of sound engineering. He confirmed that the acoustic characteristics of thesignal were similar to those of a real rumble strip noise.

2.2.2 Abstract warningA signal with a fundamental frequency of 500 Hz or higher has been found to bee�ective when a fast reaction time and a high perceived urgency is desired (Haas& Edworthy, 1996). An abstract warning with a 500 Hz fundamental frequencywas therefore created. In order to avoid any masking, three additional harmonicswere added to the signal. These were 1000, 1500 and 2000 Hz, with amplituderatios relative to the fundamental frequency of 0.5, 0.2 and 0.1 respectively. Thefrequency spectrum can be seen in Figure 2.5.

16

2.2. WARNING SIGNAL DESIGN

Figure 2.5: Frequency spectrum of the abstract 500 Hz warning signal

2.2.3 Choosing intensity levels

As mentioned in the introduction, an appropriate level of intensity for an auditorywarning signal is around 15-25 dB above the ambient noise (Patterson (1982) citedin Edworthy et al., 1991). The ambient and engine noise in this research was arecording from inside a driving car, downloaded from the internet (SoundE�ects-Factory, 2014). A part of this recording was looped and its intensity level wasadjusted to approximately 55 dB. The intensity level was measured with a ThankoRama 11O08 sound level meter, at the ears of a head and torso manikin seatedin the driver’s seat. The same method was used to measure the intensity of thewarning signals.

The looming warning signals had an initial intensity of approximately 66 dB,which was considered audible even with the ambient noise present. The intensityincreased by 12 dB to reach a peak intensity of 78 dB (23 dB above ambient), whichwas within the suggested intensity range for warning signals as mentioned above.A small onset and o�set (less than 20 ms) was added to the signal in order to avoidany clipping.

The constant intensity warning signals were given an intensity of 72 dB (17 dBabove ambient), which was the median value for the looming warning signal, andan onset and o�set of 20 ms to avoid any startling reactions from the drivers.

17

CHAPTER 2. METHOD

2.2.4 Implementing the loomingThe looming intensity was added to the rumble strip noise and the 500 Hz abstractwarnings by using the Adjustable Fade function in Audacity. While a standard fadefunction creates a linear fade, the Adjustable Fade function can be used to shapethe curve of the fade, by pushing the middle of the fade up or down.

In order to create a looming intensity that rises slowly at first, and then graduallyrises more rapidly, the fade type was set to Fade Up, and the Mid-Fade Adjust to-70% (the lower the value, the more curved the fade will be). The intensity was setto increase by 12 dB. The time-intensity curve can be seen in Figure 2.6.

Figure 2.6: Time-intensity curve for the looming warning signals

2.3 EvaluationThere are many parameters that can be evaluated in order to decide whether anauditory warning signal is appropriate in a lane departure scenario. Parameters suchas lateral deviation, time to return to the lane and correct response percentage (isthe steering wheel turned in the correct direction?) are often considered. However,one of the most studied parameters is the reaction time since a fast response in adangerous tra�c situation can be the di�erence between life and death. Due to thelimited scope of this thesis, the focus has therefore been to examine if there is adi�erence in reaction time between the four warning signals. The reaction time willbe evaluated through a driving test session.

Although the reaction time is an important factor to consider when designinga warning signal, how the drivers perceive the signal is also of interest, since anannoying warning may cause unwanted behavior (Marshall et al., 2007). It has also

18

2.3. EVALUATION

been shown that angry drivers are more prone to violate tra�c rules (King & Parker,2008), which is why it is important that the drivers accept the warning signal. Inorder to assess how the test persons perceived the warning signals, a survey wasconducted after the driving tests had been completed. The survey was designed toevaluate the perceived urgency, annoyance and acceptance of the warning signals inthe lane departure scenario. The test persons were asked to listen to each warningsignal as many times as they wanted and then rate each warning signal from 1 to5. For the urgency and annoyance, 1 meant that the signal was “not urgent” or“not annoying”, while 5 meant that it was “very urgent” or “very annoying”. Toevaluate the acceptance, the test persons were asked; “Would you accept the soundas a warning signal if it were used in a lane departure scenario?”. They would thenrate each signal from 1 to 5, 1 meaning “would not accept it at all”, and 5 meaning“would accept it very much”.

One test session including instructions, driving test and completing the survey,took about 1 hour and 20 minutes to complete.

2.3.1 Test persons14 test persons participated in the evaluation. All the test persons were students atOsaka University in Japan, between 22 and 26 years of age (M = 23.43, SD = 1.29),and of varying nationalities. The majority were Japanese (8 persons). All of theparticipants were licensed drivers with between 1 to 7 years of experience (M = 4.25,SD = 1.35). However, only three test persons reported an annual driving distanceof 5000 km or more, which is the lower threshold for experienced drivers (Suzuki& Jansson, 2003). Several participants stated that they had not been driving sincethey received their driver’s licenses. One person reported a lot of driving gameexperience, but only using a standard game controller. No test persons had anysignificant experience playing a driving game using steering wheel and pedals. Alltest persons reported normal hearing and normal or corrected-to-normal vision. Fivetest persons were members of the laboratory where this research was conducted, andhad some prior knowledge of the purpose of the research. However, none of the testpersons knew any details regarding the driving test procedures before participating.The rest of the test persons had no knowledge of the purpose of the research. Alltest persons were told that the driving tests main purpose was to measure theirability to concentrate on a secondary task while driving.

2.3.2 SetupThe driving tests were carried out in a regular o�ce room. The driver’s seat wasa swivel chair with freely adjustable height. The driving simulator ran on a 2,7GHz Intel Core i5 Macbook Pro with 8GB of RAM and an Intel Iris Graphics6100 1536 MB graphics card. The Macbook was connected to a 37 inch ToshibaRegza 37Z2000 TV via HDMI with a resolution of 1920x1080 pixels. The ambientnoise was played back from a music device connected to two JBL Creature 3 cm

19

CHAPTER 2. METHOD

diameter loudspeakers, placed on each side of the TV, directed towards the driverat approximately 1.20 m from the driver’s ears.. The car was controlled using aLogicool (Logitech’s brand name in Japan) G27 steering wheel and pedals. Thedriving simulator warning signals were played back through a Panasonic RX-ED57portable stereo system, with two 8 cm diameter loudspeakers. The stereo systemwas aligned with the center of the steering wheel, facing the driver at a distanceof approximately 0.80 m from the driver’s ears. The initial idea was to put loud-speakers on each side of the driver, and play back the warning signal from only oneloudspeaker, depending on the side of the lane departure. However, since research(Suzuki & Jansson, 2003) suggests that the reaction times are the same regardless ifthe driver is exposed to the warning signal from both speakers at the same time, oronly from one side, this idea was abandoned. The secondary task (described below)was triggered on a Samsung Galaxy S2 smartphone.

2.3.3 Secondary task

In real driving scenarios, a lane departure may occur if the driver is occupied witha secondary task while driving, such as using a smartphone, and not focusing onthe driving. A lane departure can also be the result of the driver nodding o� whiledriving. When testing warning signals in a driving simulator, it is common tosimulate one of these two scenarios. The response to a warning signal collectedfrom drowsy drivers may be a more natural response than if the warning signal isexpected, as in a secondary task scenario. However, more e�ort may be requiredto make the test persons drowsy, and more equipment such as eye-tracking devicesand physiological sensors is needed. The research by Ziegler et al. (1995), as wellas Rimini-Doering et al. (2005) used this method. Forcing lane departures by usinga secondary task has the advantage that a similar number of lane departures canbe collected for each test person, and that no external equipment is needed. Thishas been the method of choice in other research (Suzuki & Jansson, 2003; Stanley,2006), and is also the method used in this thesis.

An Android smartphone app was developed and used as a secondary task. Whentriggered from the driving simulator, the app sounded a short alarm signal. Aftera brief pause (0.9 s) to let the driver react to the alarm, the display showed fivearrows in succession, pointing either to the left or the right of the screen. Each arrowwas displayed for 0.5 s. As soon as an arrow was displayed, the driver was told tokeep the steering wheel fixed in a neutral position while pressing the correspondingbutton on the steering wheel; either on the right-hand side or the left-hand side. Alltest persons were told to focus on the secondary task unless they heard a warningsignal, in which case they were told to immediately react and steer back into thelane. The smartphone was placed on a chair to the left of the driver’s seat, in linewith the test person’s head. This forced the test person to look down to the left inorder to view the smartphone display, thereby removing the focus from the road,as can be seen in Figure 2.7.

20

2.3. EVALUATION

Figure 2.7: The secondary task triggered while driving

2.3.4 Driving test

The test persons started by driving one lap around the 9.85 km long course as apractice run, in order to become familiar with the car handling and the road. Theywere asked to drive at the max speed of 80 km/h as often as possible, and to performlane departures on purpose in order to get a feel for the width of the road. The lanedeparture warning signal during the practice run was a short musical sequence (G4,H4, G4, H4) designed in Logic Pro X (Apple Inc., 2015), played with the 80s FMPiano instrument for a duration of 1.0 s. The secondary task was triggered manuallyby the author several times during the practice run to teach the test persons howthey were supposed to behave. All test persons were told that the warning signalssignified a lane departure.

When the test persons had familiarized themselves with the controls, the roadand the secondary task, four test runs were conducted; one for each warning signal.Each test run lasted approximately eight minutes (one lap around the course), andbetween each run there was a short five minute break. The order of the warningsignals was randomized in order to avoid any bias. The course had five longerstraight sections. For each straight, the secondary task was triggered at randomplaces, resulting in a total of approximately five secondary tasks per run (on a fewoccasions the test persons were asked to drive longer if enough lane departure datahad not been collected by the completion of one lap). In order to force a lanedeparture while the test persons were busy performing the secondary task, the yawangle of the car was changed (+/- 4 deg) manually by the author as soon as thetest persons started looking at the smartphone display. The direction of the yaw

21

CHAPTER 2. METHOD

angle change was randomized. This yaw angle change resulted in a lane departurealmost every time. Once a lane departure was committed, the warning signal wasplayed back and the current angle of the steering wheel and the timestamp (t1)was recorded. As soon as the test person turned the steering wheel more than thethreshold (2 deg), the new timestamp (t2) was recorded. The time di�erence t2 -t1 was saved as the reaction time. The two degree threshold was added becauseit was considered to be too di�cult to hold the steering wheel completely still.Without this threshold the slightest movement of the steering wheel would be falselyrecorded as a response to the warning signal. The data was recorded at a rate ofapproximately 70 Hz.

22

Chapter 3

Results

3.1 Driving test

All test persons tried the secondary task at least five times per warning signal.Although the secondary task almost always forced a lane departure, some lanedepartures were judged as being invalid, and were consequently excluded from thetest data. The main reasons were that the test persons had been looking at theroad, or that the reaction time was too fast. All reaction times below 0.16 s wereremoved because this is considered to be the minimum reaction time threshold forhuman beings reacting to sounds (Triggs & Harris, 1982). The total number ofexcluded lane departures was 37.

In total, 248 valid lane departures were registered and the reaction times recorded.The abstract warning with constant intensity (AC) accounted for 63 lane departures,the abstract warning with looming intensity (AL) for 59, the rumble strip warningwith constant intensity (RC) for 62, and the rumble strip warning with loomingintensity (RL) for 64 lane departures.

On average, each test person left the lane 18 times (SD = 1.62). Except for onetest person who only had two valid lane departures for the RC warning, all testpersons had at least three valid lane departures per warning.

3.1.1 Reaction times

Contrary to the expected results, the mean reaction times showed little variancebetween the warning signals, as can be seen in Table 3.1. In order to decide whatmethod to use for determining the statistical significance of the data, a normal prob-ability test was performed. This was done by creating a normal probability plot(see Figure 3.1), which showed that the reaction times were positively skewed (skew-ness = 2.07), and consequently not normally distributed. Since the data was notnormally distributed, a non-parametric Kruskal-Wallis test was conducted, whichshowed that there was no significant di�erence in reaction time between the fourwarning signals (H = 2.397).

23

CHAPTER 3. RESULTS

Table 3.1: Mean reaction time per warning signal

Warning signal Count Mean reaction time (s) Standard deviation (s)AC 63 0.58 0.28AL 59 0.66 0.33RC 62 0.64 0.33RL 64 0.60 0.23

Figure 3.1: Normal probability plot for distribution of reaction times

3.2 SurveyAll 14 test persons participated in the survey after completing the driving test. Themean ratings and standard deviations are found in Table 3.2 below. The data wastested for normal distribution in the same way as for the reaction times. The normalprobability plots showed that the data was normally distributed, which allowed fora one-way Analysis of Variance (ANOVA) to be conducted. The ANOVA is amethod to determine whether any changes between various means are statisticallysignificant. The p-value is evaluated to determine the significance of the results.In this case, a p-value lower than .05 would indicate that any di�erences betweenthe groups (the warnings) were most likely significant. The ANOVA showed thatthere were significant di�erences at the p<.05 level between the four warning signalsfor each of the three categories (urgency [F(3, 52) = 13.76, p < .001], annoyance[F(3, 52) = 3.62, p = .02] and acceptance [F(3, 52) = 4.28, p = .01]). However,the ANOVA can only indicate that there are di�erences among all the warnings,but not between which warnings. Therefore, two-sided paired-samples t-tests were

24

3.2. SURVEY

conducted for each category to evaluate pairwise di�erences between the warningsignals.

Analyzing the perceived urgency, it was found that the rumble strip warningswere significantly less urgent than their abstract warning counterparts (Table 3.2);RL was less urgent than AL; t(13)=5.83, p<.001, and RC was less urgent than AC;t(13)=4.22, p=.001. It can be seen that AL displays a high level of urgency, whileRC is perceived as not very urgent. Furthermore, for the rumble strip warnings,the looming warning was perceived as significantly more urgent than the constantwarning; t(13)=2.86, p=0.01. No similar di�erence was found among the abstractwarnings; t(13)=2.09, p=.06.

Looking at the Annoyance column in Table 3.2, it is found that the rumble stripwarnings were significantly less annoying than their abstract warning counterparts;RL was less annoying than AL; t(13)=2.33, p=.04, and RC was less annoying thanAC; t(13)=2.31, p=.04. Both versions of the rumble strip noise were equally an-noying; t(13)=1.33, p=.21, and the same relation was found between the abstractwarnings; t(13)=0.74, p=.47. No significant di�erence in perceived annoyance wasfound between RL and AC; t(13)=1.13, p=.28. It can be noted that no warningwas rated as being not annoying; all of the warnings have a medium high or higherperceived annoyance.

Lastly, the degrees to which the test persons would accept the warning signals ina lane departure scenario are shown in the Acceptance column (Table 3.2). It can beobserved that the two rumble strip warnings have the same means, and were henceequally accepted. Comparing the two abstract warnings with each other, it can beseen that these two warnings were also accepted to the same degree; t(13)=0.18,p=.86. Looking at the rumble strip warnings compared with the abstract warnings,the paired-samples t-tests found that RC was significantly less accepted than AC;t=(13)=2.83, p=.01, but that there was no significant di�erence compared with AL;t(13)=2.11, p=.05. However, RL was found to be significantly less accepted thanboth AC; t(13)=2.45, p=.03, and AL; t(13)=2.38, p=.03.

Table 3.2: Mean perceived urgency, annoyance and acceptance per warning signal

Warning signal Urgency Annoyance Acceptancemean sd mean sd mean sd

AC 3.71 0.96 3.64 1.23 3.79 0.86AL 4.36 0.81 3.93 0.70 3.71 1.22RC 2.43 0.73 2.64 1.11 2.64 1.04RL 3.07 0.70 3.00 1.31 2.64 1.29

*Ratings are on a scale from 1 (lowest) to 5 (highest)

25

CHAPTER 3. RESULTS

3.3 Test persons’ opinionsDuring the driving test session and while answering the survey, some test personsvoiced their opinions on the warning signals. The first person said that the rumblestrip warning sounded too much like an engine sound, and that he had never heard arumble strip noise before. He also stated that a warning was supposed to be urgentand annoying to catch a driver’s attention.

The second person claimed that the rumble strip warning sounded too much likea real rumble strip noise. He then explained that he had become used to the noisewhen driving in real life, and sometimes drove on the rumble strips just for fun. Inother words, the warning signal did not convey any sense of urgency to him.

The third person mentioned that the rumble strip warning sounded “more real-istic” than the practice warning signal, and that it sounded urgent and “felt like inreality”. It was also mentioned that the abstract warnings conveyed the feeling of acar approaching.

26

Chapter 4

Conclusion

4.1 Discussion

4.1.1 Reaction times

Contrary to the hypothesis, all warning signals displayed similar reaction times;neither the rumble strip noise or the looming intensity reduced the reaction times.

Auditory icons, with a sound related to the real world, have displayed faster reac-tion times than other sounds when used in driving scenarios (Graham, 1999; McKe-own 2005). Although they are easier to remember than abstract sounds (Ulfvengren,2003a, cited in Ulfvengren, 2003b), they nevertheless have to be learnt through pre-vious experience. Since 11 of the 14 test persons were inexperienced drivers, likelywith limited experience of the rumble strip noise, they may not have related therumble strip noise to a dangerous situation, which in turn could have caused areaction similar to when responding to an abstract warning signal.

On the other hand, since the test persons knew that all warning signals signifieda lane departure, they understood how to react regardless of what signal they heard,which may have caused the similar reaction times. This may also explain why thewarning signals with a looming intensity showed no faster reactions than the signalswith constant intensity.

Another reason for the lack of di�erence between the warning signals may havebeen that the test persons learnt that almost each time the secondary task wastriggered, there would be a following lane departure. They may therefore haveexpected the warning signal, and were ready to react once they heard it. Thismay have been avoided if, for some secondary tasks, the yaw angle of the car hadnot been altered. By altering the yaw angle less frequently, there would have beenseveral occasions where the secondary task was triggered without a following lanedeparture.

27

CHAPTER 4. CONCLUSION

4.1.2 SurveyIt was not surprising that the abstract warnings were perceived as more urgentthan the rumble strip warnings, since their frequency was higher and followed theguidelines suggested by Haas and Edworthy (1996) for creating a high urgencywarning. That the high urgency abstract warnings displayed a higher perceivedannoyance than the low urgency rumble strip warnings, was also in line with previousresearch that suggests that as the urgency increases, so does the annoyance (Tan& Lerner, 1995; McKeown, 2005). It may also be argued that because of theparticipants’ limited driving experience, they may not relate the rumble strip noiseto a dangerous situation, and thereby perceiving the warning as less urgent. ThatRL showed a significantly higher level of urgency than RC suggests that a loomingintensity may be used to portray how urgent a certain situation is. This couldfor example be used to tell the driver how close the car is to the lane edge; a lowintensity meaning that there is still plenty of space left, while increasing the intensitymeans that the lane edge is getting closer.

Interestingly enough, although RL is perceived as more urgent than RC, there isno di�erence between them when it comes to the perceived annoyance. This resultsuggests that a rumble strip warning with a looming intensity could e�ectively beused to increase the urgency of a warning while maintaining a low level of annoyance,which is important in a driving scenario in order to avoid any unwanted responsesfrom the driver.

That the rumble strip warnings were not accepted to a high degree was contraryto what previous research has found (Fagerlönn, 2011b). However, these resultswere based on a survey among professional truck drivers, with much more experiencethan the participants in this research. The lack of experience may be a reason whythe acceptance rate was not higher. It could also be because this research solelyfocused on lane departure warnings; no other warnings, such as collision warnings,were present, which minimized the confusion regarding how to react. In this casethe urgency of a signal may be more important than how easy it is to interpret themeaning of the signal. However, in a real driving scenario the ability to discernthe meaning of a warning from another is important, which is why the rumble stripnoise may be preferred to an abstract sound.

4.2 LimitationsIt should be noted once again that the driving simulator was built from scratch,with no time to properly measure any latency issues, which may have a�ected thereaction times to some degree. Furthermore, since the road did not have any othertra�c, and there was no risk of collision, the perceived danger of a lane departuremay have been low, which would result in few incentives for staying on the road.However, the lack of danger is apparent even in standard driving simulators (Blana,1996). Naturally, since the driving tests were conducted in a regular o�ce room,and not in a car cabinet, the feeling of being in a real world driving situation was

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4.3. FUTURE WORK

limited. No studies have been found comparing how research results from using ahigh-cost driving simulator di�er from those of a low-cost driving simulator.

4.3 Future workIf the same driving simulator was to be used again, it would be interesting to carryout longer driving tests, where only some of the secondary tasks are followed bya lane departure, in order to make the drivers less anticipating of the warningsignals. Since this research only focused on lane departure warnings, it would alsobe interesting to study how the rumble strip warning is perceived if other warningsystems, such as collision warnings, are present as well. In these situations it willbe important to tell the various warnings apart, potentially making the abstractwarning less accepted, and reducing the reaction time for the rumble strip warning.Furthermore, it could be relevant to study what e�ect altering the intensity curvefor the looming would have. Using the looming intensity to alter the urgency of awarning, and thereby telling the driver how close the vehicle is to the lane boundary,could also be interesting. Such an implementation could potentially be used to guidethe driver when the weather is bad or when other factors prevents the driver fromaccurately determining how much space is left until the lane boundary has beenreached.

29

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