Interaction design of automatic steering for collision

Heesen, M., Dziennus, M., Hesse, T., Schieben, A., Brunken, C., Löper, C., Kelsch, J., Baumann, M.: 'Interaction design of automatic steering for collision avoidance: challenges and potentials of driver decoupling'. IET Intell. Transp. Syst., pp. 1–10, doi: 10.1049/ietits.2013.0119, Authors Manuscript, 2015

Interaction design of automatic steering for collision avoidance: challenges and potentials of driver decoupling Matthias Heesen*, Marc Dziennus, Tobias Hesse, Anna Schieben, Claas Brunken, Christian Löper, Johann Kelsch, Martin Baumann German Aerospace Center, Lilienthalplatz 7, 38108 Brunswick, Germany *Fraunhofer Institute for Communication, Information Processing and Ergonomics, Fraunhoferstraße 20, 53343 Wachtberg, Germany E-Mail: [email protected], , [email protected], [email protected], [email protected], [email protected], [email protected], [email protected] , [email protected] Abstract Studies concerning collision avoidance show that most drivers tend to brake, even if evasive manoeuvres were better. Automatic steering for collision avoidance would help here. Studies in the EU project interactIVe observed that drivers show a tendency to hold the steering wheel and disturb the automatic steering when kept in the control loop. A strategy of driver decoupling e.g. by means of steer-by-wire systems could improve the automatic steering performance. However, the major challenge of using steer-by-wire systems is to enable the driver to compensate false system activation, e.g. evasion into oncoming traffic. A time dependent strategy of driver decoupling using steer-by-wire combined with override recognition by counter steering above a certain threshold was implemented in a research vehicle. The interaction strategy was tested with 45 participants on a test track in two different scenarios; a collision situation with justified evasion and a false alert scenario with unjustified system activation. In a between-subject design the decoupling strategy (using steerby-wire) was compared against automatic steering with fully coupled driver and force feedback on the steering wheel and against Manual Driving without automatic steering. When the driver was temporarily decoupled, the obstacle avoidance performance was better but the driver was less able to counteract a false avoidance manoeuvre. The analysis of driver behaviour revealed options to improve the interaction strategy. 1

1. Introduction Vehicle automation bears the potential to make driving more comfortable, more efficient and first of all safer. There are many situations in which automation can perform far better than the human driver. The most prominent situations are emergency situations where the automation can help to avoid or mitigate collisions. [1]. In such situations, the automation profits by very fast reaction times and the ability to control the vehicle within its physical limits. Because of these advantages emergency collision avoidance systems are among the first ones in which highly or fully automated vehicle interventions might be realized for series-production vehicles (e.g. [2], [3]). First instantiations of emergency braking assistance systems are already in series production (e.g. Volvo City Safety, Mercedes Pre-Safe Brake etc.). In addition, emergency systems that use not only braking but also steering are currently under research. These systems could help to avoid collisions in situations where evasive steering is a better reaction than braking. These situations occur at certain velocities and time to collisions. (e.g. [4], [5]). A crucial aspect in the development of automatic steering systems for collision avoidance is to find a good balance between manoeuvre effectiveness in justified system activation on the one side and controllability of unjustified system activation on the other side. The term controllability here is used as defined in the RESPONSE Code of Practice [6] and refers to the driver’s ability to override unjustified automatic interventions. The effectiveness of automatic steering manoeuvres can be disturbed by the driver when he/she is allowed to override automatic manoeuvres to maintain controllability. Some drivers tend to disturb automatic steering interventions by holding the steering wheel, possibly as an effect of some fear reaction, thus decreasing the overall manoeuvre effectiveness (e.g.[7], [8]). However controllability of automatic evasive manoeuvres still plays an important role, since problems like reliably sensing oncoming traffic are not sufficiently solved yet. This leads to a manoeuvre effectiveness – controllability dilemma, because attempts to minimize the 2

driver’s tendency to counteract automatic steering in the interaction design directly interferes with the controllability of the automatic steering. Unfortunately, studies show that one potential solution to increase the steering torque yields an unacceptable controllability before the desired level of manoeuvre effectiveness is achieved. As a consequence, the resulting effectiveness-controllability dilemma seems to be difficult to solve by varying the level of steering torque (e.g. [9], [8]). Strategies of brief driver decoupling during the automatic steering intervention with the option to recouple the driver either after a defined time or when the driver counter steers above a certain degree could improve the overall effectiveness of an automatic evasive manoeuvre, while at the same time maintaining a sufficient degree of controllability (e.g. [10]). Steer-by-wire technology could be used to realize such decoupling/recoupling strategies. In this paper an interaction concept for driver de-/ recoupling in automatic evasive manoeuvres using steer-by-wire is introduced and results of a real vehicle evaluation study are reported. Beside the investigation of automatic steering manoeuvre effectiveness special attention was given to the detailed analysis of the driver’s capability to control the steering intervention in case of false activation. The study revealed some insight into how to improve the controllability of false steering activation.

2. Theoretical Background Assistance for evasive collision avoidance manoeuvres concern vehicle manufacturers, suppliers as well as research institutes for several years now (e.g. [11], [12]). The variety of concepts ranges from systems designs to increase driver-performed evasive manoeuvres for example by giving short haptic steering impulses (e.g. [13]) or visual cues (e.g. [14]) to active assistance by system intervention. Here two types can be distinguished. Systems that try to actively support driver performed evasive manoeuvres, in order to prevent exaggerated steering (e.g. [15]), and systems that execute the 3

complete automatic evasive manoeuvre (e.g. [16], [17]). Most of the publications dealing with the latter systems focus on the technical aspects of these systems (e.g. [18], [19], [20]) or the evaluation of collision avoidance effectiveness or driver acceptance, but not on the controllability of unjustified system activation. As far as we know, no study focusses on the manoeuvre effectivenesscontrollability dilemma in automatic steering for collision avoidance from an interaction design perspective. When the driver remains in the control loop he can be the source of noise by unintentionally disturbing the execution of the automatic evasive manoeuvres. In studies conducted by VTEC and DLR it turned out, that drivers often disturb the automatic evasive manoeuvres by holding the steering wheel ([6] & [8]). There are two possible and not mutually exclusive explanations for that. Regarding the first explanation several studies in the area of psychophysiology and neurologic science show a relationship between workload, arousal and higher muscle tension in different domains (e.g. [21], [22], [23], [24]). Collision avoidance situations are in most cases very surprising for the driver and often related to some kind of psychological shock state. Shock reactions are also related to a sudden increase in muscle tension like higher grip force [25]. An increase in grip force was also observed in collision avoidance studies [26]. An approach attempts to make use of the human tendency to show a shock related grip reflex, by using the resulting grip force at the steering wheel as an indicator to justify emergency braking manoeuvres [27]. The second explanation is related to the activation of familiar reaction patterns. Studies on human collision avoidance behaviour frequently report, that most of the drivers tend to brake where evasive manoeuvres would have led to more successful collision avoidance (e.g. [28], [13], [4]). This is not surprising because braking is a well-known manoeuvre to most drivers and in driving schools students are even encouraged not to try evasive manoeuvres for example in avoiding collisions with animals. Additionally, in the face of an imminent collision respectively the related full application of 4

the brakes; drivers tend to hold the steering wheel in order to prepare for the impact and deceleration forces. Drivers are quite well supported to the rear and to the sides by the seat. But in the frontal dimension the only option to get some hold is the steering wheel or the floor room with the pedals [29]. So there are several aspects in imminent collision avoidance situations that all increase the driver’s tendency to firmly hold the steering wheel and to brake while neglecting the option to steer. By this, drivers often unintentionally work against automatic steering manoeuvres. Another option arises with the development of steer-by-wire systems. Here the steering column is interrupted, so that there is no direct physical connection between the steering wheel and the tires. The steering commands are detected by a sensor and sent to a servo motor which adjusts the wheels. Steering commands can also be given by the vehicle automation. Steering feedback can be given via active steering wheels, which are also equipped with a servo motor (e.g. [29]). The direct physical connection between steering wheel and vehicle tires can be restored by closing a clutch. Steer-by-wire systems are designed fail safe, so that in cases of an energy breakdown or a system failure the clutch closes automatically, and the driver immediately regains direct control (e.g. [31]). Regarding automatic evasive manoeuvres, steer-by-wire systems allow to decouple the driver from direct steering control for a defined time. By this, the negative interference of the driver with the automatic steering manoeuvre execution could be minimized. The challenge in the interaction design for such a decoupling strategy is to maintain controllability in situations in which the driver needs to intervene due to unjustified and critical system activation. One strategy to design the decoupling of the driver from steering is a time-dependent manner, so that the driver is only decoupled for short periods of time when his tendency to hold the steering wheel is highest. The idea behind a time-dependent decoupling is that the possible initial shock related increase of the grip force does only last for a short period but can already disturb the overall performance of automatic steering collision avoidance manoeuvres. If the driver is decoupled from 5

steering in this short period this interference could be minimized. At the same time the changing vehicle orientation in the moment of decoupling could increase the driver’s tendency not only to brake alone but to perform an evasive steering manoeuvre thus no longer counteracting the automatic steering intervention. Therefore it could be sufficient to totally decouple the driver just for a split second. Alternative or additional options could be the detection of driver override intention for example in form of counter-steering, change in steering angle, angular velocity etc. A combination of time-based and intention based recoupling of the driver is possible. Other options to detect drivers override intention seem not to be suited. Recognizing the driver’s intention to override the system intervention via the accelerator pedal position is not adequate, because especially in vehicles equipped with ACC (Adaptive Cruise Control) the drivers do not always have the foot on the accelerator pedal. Another reason is that in systems with collision avoidance/mitigation by automatic braking the driver often unintentionally pushes the accelerator pedal by “falling” into it (e.g. [32]). This would also be a constraint in collision avoidance systems with combined braking and steering. A different approach than recognizing driver intention is to better prepare him. In this context, advance warnings and response priming are imaginable (e.g. [33]). Here haptic impulses on the steering wheel or visual elements which are presented shortly in advance to the actual evasive manoeuvre can prepare the driver for the upcoming manoeuvre (e.g. [13], [14]).

3. Method Participants A total of 47 participants attended the present experiment. Due to technical problems two participants were excluded from the analysis. The remaining 45 participants (27 male and 18 female) were between 23 and 50 years old with an average age of 30.4 years (SD = 9.9). They had a valid 6

driver license for a minimum of one year. Participants were assigned in a balanced group-design (9 male, 6 female) to the three groups. Each of the three experimental groups (Manual Driving, Driver Always Coupled, Driver Decoupled) consisted of 15 participants. All participants were recruited through the participant pool from the German Aerospace Centre (DLR). For the participation in the experiment the test persons were paid 10€ per hour.

Independent Variables: Interaction strategies and control group For this study two interaction designs were developed and tested against a control group Manual Driving without automatic steering. These designs were: a) automatic steering intervention with driver de- and recoupling using steer-by-wire; b) automatic steering intervention with Driver Always Coupled. The automatic intervention in both the Driver Decoupled and the Driver Always Coupled condition was based on the same automatic steering functionality. The two interaction design conditions are explained in the following part.

Driver Decoupled condition In this condition the automatic steering command was directly applied to the steering actuator and the corresponding steering wheel position offset was commanded to the steering wheel actuator to give the driver feedback of the automatic steering intervention. To overcome the negatively interfering driver input a time-dependent driver decoupling strategy was used. That means that the driver was decoupled completely from steering for 0.5 seconds without the ability to override the automatic steering. During this time the driver got feedback on the steering manoeuvre in form of torques on the steering wheel, but he/she had no influence on the vehicle guidance and holding the steering wheel or counter steering was ignored by the system. The time of 0.5 seconds was chosen by expert ratings in test trials during development. Aim of the expert ratings was to define a decoupling time which was kept below a value with which the vehicle would have reached a too 7

large lateral deviation by the automatic steering. More precisely within the non-overrideable decoupling time the vehicle should stay within the lane markings of its own lane so that the driver maintains a chance to compensate for unjustified automatic steering after the 0.5 seconds decoupling time-window. For recoupling of the driver an override recognition based recoupling was used. After the first non-overrideable 0.5 seconds the driver could recouple himself through counter steering above a certain threshold in a time window of 0.3 seconds. The trigger for recoupling through counter steering was a steering angle to the right higher than the steering angle of the steering wheel middle position. If no intention of counter steering by the driver was recognized, the driver was fully recoupled and back in control of the vehicle after 0.8 seconds from the start of the automatic steering intervention (Figure 1).

Beginning of automatic steering

Obstacle

Manual Driving

Manual Driving System ignores steering inputs 0.3 sec.

0.5 sec.

Re-coupling through counter steering

Figure 1: Design of the condition Driver Decoupled

The feedback given to the driver was a torque on the steering wheel. The gradient of the torque had a linear characteristic and was dependent on the absolute angle deviation (Figure 2). The torque of the steering impulse was limited to 8 Nm and the relationship between steering torque and absolute angle deviation was dependent on the driving speed. 8

Figure 2: Torque dependence on difference between actual and commanded steering wheel position

Driver Always Coupled condition

In the Driver Always Coupled condition the driver had full control over the steering (no decoupling by steer-by-wire). For the automatic steering intervention the steering angle was not directly commanded to the tire actuator but the corresponding steering wheel position was commanded to the steering wheel actuator. Thus, in case there was no disturbing influence of the driver on the steering wheel the automatic steering manoeuvre was the same as for the Driver Decoupled condition. The deviation from the commanded steering wheel position offset was signalled to the driver by a torque on the steering wheel. The relationship between the torque on the steering wheel and the difference between steering wheel position and the tire position was the same as in the Driver Decoupled condition. The relationship between steering torque and difference between commanded and actual steering wheel position is depicted in Figure 2.

Control Group: Manual Driving In the control group Manual Driving there was neither an automatic steering intervention nor a force feedback on the steering wheel.

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Design of the study The interaction design for the driver decoupling strategy was tested in two different scenarios; both are explained in detail later in this part. The first driving scenario was a collision avoidance scenario in which a collision with a suddenly appearing obstacle should be avoided; here the effectiveness of the automatic steering manoeuvre was tested. The second driving scenario was a driving scenario with an unjustified activation of the automatic steering while no obstacle was present for testing controllability. Each driver was randomly assigned to either one of two interaction strategy conditions or the control group. In the first driving scenario, the collision avoidance scenario, the driver decoupling strategy was tested against the Driver Always Coupled condition and the control group Manual Driving without automatic steering intervention. In the false alarm scenario the driver decoupling strategy was tested only against the Driver Always Coupled strategy. The Manual Driving/no automatic steering condition was not tested because Manual Driving does not include false system intervention. The two scenarios are not to be seen as a factor but as a measurement procedure, thus each test scenario resulted in a different experimental design. In the collision avoidance scenario the factor ”Interaction Strategy” had three levels, the Driver Decoupled variant, the Driver Always Coupled variant, and the Manual Driving variant (Table 1).

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Table 1: Experimental design for the collision scenario

Design

Variant

Collision Scenario

Driver Decoupled

N = 15

Driver Always Coupled

N = 15

Manual Driving

N = 15

Interaction Strategy Between groups

In the false alarm scenario the factor “Interaction Strategy” had two levels, the Driver Decoupled variant and the Driver Always Coupled variant (Table 2) Table 2: Experimental design for the false alarm scenario

Design

Interaction Strategy Between groups

Variant

False Alarm Scenario

Driver Decoupled

N = 15

Driver Always Coupled

N = 15

Test Vehicle To realize the different steering interventions in the present study, a specially instrumented car, the FAS-Car II from the German Aerospace Centre (DLR) was used [31]. The FAS-Car II is based on a VW Passat that is equipped with a steer-by-wire system. The steer-by-wire system makes it possible to decouple the driver from steering. The FASCar II is also instrumented with a wide variety of sensors and a capable vehicle automation which enabled automatic collision avoidance steering manoeuvres.

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Test track The test was conducted on a 2 kilometre long round course which is located on the area of a former military barracks in Brunswick, Germany. The test track contained two test situations, a collision avoidance scenario and a so called false alarm scenario. It also contained several other driving tasks like slalom driving, sharp curves, turning areas, narrow points etc. These were distractor tasks to support a cover story in order to minimize the participants’ expectancy of the original purpose of the experiment. Participants were made to believe, that they should test the parameterization of a steerby-wire system. Because of the ostentatious gadgetry of the obstacle two dummy obstacles were installed on the test track. This was done to make participants more familiar with passing the obstacle in order to minimize special attention for this kind of situation.

Obstacle In the present study a 1.5m x 2.5m net functioned as an obstacle. It was held by two thin iron cables. One iron cable was strained flat across the street, while the other was strained in a height of 2.5m. In folded state the net was stored invisibly behind a wooden cover. The appearance of the obstacle was trigger by a light barrier. A rope made of rubber was tensioned to pull the net out of its hideout. The obstacle appeared within 0.8 seconds from the right side of the lane and covered nearly half of the lane (1.35m). It was important, that if a collision occurred neither the participants nor the FAS-Car II could have been damaged and therefore the net had predetermined breaking points. By this, the lateral deviation from the centre of the lane, which was needed to pass the obstacle without a collision, was rather small. No complete lane change was necessary to avoid a collision; thus the time to perform a successful collision avoidance manoeuvre was minimized. This way the distance between light barrier and obstacle could be limited to 22.2 meters, a distance which did not allow successful collision avoidance by braking only at the defined velocity of 50 km/h. 12

Scenario 1: Collision scenario The collision scenario was a part of a round course and consisted of an approximately 500 meters long and 3.2 meters wide straight lane marked by traffic cones. The obstacle was positioned at the very end of the 500 meters lane. At the beginning of the 500 meters lane a cruise control system was activated which accelerated the vehicle to a constant speed of 50 km/h. This was done to hold the vehicle velocity constant at the moment of sudden appearance of the obstacle. This way each participant had the same criticality and time left for reaction. 22.2 meters before the obstacle a light barrier was positioned. Driving through this light barrier triggered the appearance of the obstacle. At a given speed of 50 km/h and the distance of 22.2 meters to the obstacle (TTC of 1.6 seconds) human driven collision avoidance by braking only was not possible. Driving speed and distance to the obstacle were chosen in a way that the evasive manoeuvre was the only option to successfully avoid a collision. In the condition Manual Driving no system support was given and the driver had to avoid the collision manually. In the condition Driver Decoupled and Driver Always Coupled the system intervention started at the same time the obstacle started to unfold. This was at a TTC with the obstacle of 1.6 seconds. The Driver Decoupled and the Driver Always Coupled strategies both were based on the same automatic steering functionality. The only difference was that in condition always coupled drivers were able to override and cancel the evasive manoeuvre and in the Driver Decoupled condition their steering inputs had no effects for a certain time. If drivers followed the automation and steered in the same direction, the car drove an evasive trajectory to avoid the upcoming collision. In both conditions the Driver Always Coupled as well as the Driver Decoupled condition the drivers were given the same force feedback on the steering wheel. The steering wheel actuator tried to 13

follow the wheel position, a torque was applied depending on the discrepancy between automation commanded wheel position and the current steering wheel position (Figure 2). This means that drivers in both conditions experienced a torque to the left on the steering wheel. In the Driver Always Coupled condition drivers would have overridden the steering intervention, if they steered against the system (to the right). In the condition Driver Decoupled the steering input of the drivers was ignored for 0.5 seconds. Due to steer by wire it was possible that drivers in condition decoupled counter steer the steering intervention (steer to the right) but the wheels follow the evade trajectory to the left.

Vehicle width 1.82 m

Speed control 50 Zone Light barrier Obstacle 1.35 m

3.20 m 22.20 m

1.6 sec TTC at 50 km/h

Figure 3: Design of the evade manoeuvre

Scenario 2: False alarm scenario with unjustified automatic intervention In comparison to the collision scenario the false alarm scenario was placed at a different location in the round course and was driven at 30 km/h instead of 50 km/h. This was due to safety reasons because at this part of the test track there was only limited space to compensate unplanned but possible automation behaviour. As a consequence of the lower vehicle velocity the scenario consisted of an only 150 meter long and 3.2 meter wide straight lane. The lane was also marked by traffic cones. As in the obstacle scenario a cruise control system was activated at the beginning of the lane which accelerated the vehicle to 30 km/h and held it constant. Compared to the obstacle situation in the false alarm test situation there was no obstacle appearing. After 70 meters there was a 30 meter gap in the row of traffic cones on the left side of the lane (Figure 4). At this point the 14

automation showed false system activation in form of an intense steering manoeuvre to the left side as if there had been an obstacle on the lane. The participants had to override this unjustified steering manoeuvre to avoid a lateral deviation to the adjacent lane. A cardboard box was placed on the adjacent lane in the direction of the steering manoeuvre (see Figure 4). This cardboard box should emphasise the aversive and unjustified character of the automatic steering manoeuvre motivating the participants to override it.

Vehicle width 1.82 m 3.2 m

35.57 m 20 m

1.85 m

Cardboard 2.36 m

Reference lane

False alert Reference lane

Figure 4: Design of the false alarm scenario

Procedure Each participant completed the experiment in approximately 1 hour on a single day. At first, all participants filled out a consent form. Then they were instructed that the purpose of the study was to test and rate the parameterization of a steer by wire system in different driving tasks on a test track. After that all participants got two laps on the test track to familiarize themselves with the vehicle and the different driving tasks of the test track. In addition to that a manual emergency brake and an emergency brake plus steering were practiced, so that participants on the one hand lost fear of damaging the experimental car in dangerous situations and on the other hand that participants were able to react properly on real unexpected failures of the automation. This was done to increase 15

overall safety of the participants. After completing these test trials, the participants started with the experimental trails. In condition Driver Always Coupled and Driver Decoupled participants drove five rounds. The collision scenario with the appearing obstacle took place in the fourth round. In the previous rounds this part of the test track was repeatedly passed without activation of the obstacle, but same as in the obstacle scenario at this part of the track the cruise control function was used. The false alarm scenario was driven in the fifth lap. Same as in the collision scenario this part of the test track was repeatedly passed in the previous rounds without false system activation but with cruise control activation. So every participant experienced the collision avoidance scenario before the false alarm scenario with the unjustified system intervention. In the Manual Driving condition the drive ended after the obstacle appearance in the fourth round, because in Manual Driving there was no false system activation. After the last round the original purpose of the study was explained to the participants and they received their payment.

Dependent variables The dependent variables of this study can be differentiated in subjective and objective measures. The subjective measures reported in this paper are the perceived manoeuvre effectiveness as well as how the participants rated the steering torque in the collision avoidance scenario and the perceived controllability of unjustified system intervention in the false alarm scenario. The objective measures are the frequency of braking and steering manoeuvres, the frequency of the collision with the obstacle in the collision scenario, the lateral deviation from the own lane due to the system intervention, intensity and time to initiate steering against the system intervention (counter steering) and the development of the steering angle of the participants over time. The frequency of braking and counter steering was collected to analyse how the participants reacted to the system intervention. Based on the reaction times it is possible to understand the order of reaction. The number of crashes with the obstacle is a consequence of the lateral deviation. If the lateral deviation 16

is high enough, the collision could be avoided. The lateral deviation is defined as the difference of the position of the car right before the system intervention starts and the maximum deviation sideways produced by the system intervention. In the collision avoidance scenario the lateral deviation describes how effective the system intervention works. In the false alarm scenario the lateral deviation reflects the controllability of the system intervention. A low lateral deviation implies a good controllability of the unjustified system intervention. Time to counter steering is defined as the time from the beginning of the evade manoeuvre until the participant steers against the commands of the system intervention. The intensity of the counter steering shows how strongly participants acted against the commanded steering wheel position.

4. Results The focus of the analysis lies on the driver behaviour as a result of his/her interaction with the system variants. The analysis in this chapter is organized according to the two different scenarios, first the collision avoidance scenario and second the false alarm scenario.

Results Collision avoidance scenario Figure 5 illustrates the mean value of the lateral deviation at the obstacle and the frequency of collisions for the three different conditions. All drivers of the Manual Driving condition and condition Driver Always Coupled crashed into the obstacle. In contrast to that, there was only one collision in condition Driver Decoupled. The automatic steering manoeuvre in the condition Driver Decoupled helped to avoid more than 93% of all crashes when the driver was decoupled. This clear finding shows that there are no significant differences between the Manual Driving condition and the condition Driver Always Coupled, but significant differences between the condition Driver Decoupled and the other two (fishers Test: x² = 25.38, p< .000**). By means of the parameter lateral deviation it is possible to illustrate whether the automation helped the participants to start an evade 17

manoeuvre. The condition Driver Decoupled shows the highest lateral deviation with a mean of 1.31m. The condition Driver Always Coupled (0.45m) and the Manual Driving condition (0.36m) both yielded much lower mean lateral deviations. An ANOVA reveals significant differences in the lateral deviation between the conditions (F(2,41)= 275.97, p< .000**). A host-hoc test with Bonferroni correction shows that drivers in the variant Driver Decoupled reached a significant higher lateral deviation than the other two (p< .000**). The variants Driver Always Coupled and Manual Driving do not differ significantly from each other (p= .196).

Figure 5: Mean lateral deviation at obstacle and frequency of collision

Almost every driver hit the brake pedal in reaction to the automatic steering manoeuvre. There was only one participant in the condition Driver Always Coupled and one in the condition Driver Decoupled who did not brake (Table 3). The brake reaction times were very similar in the three groups. An ANOVA shows no significant difference between the conditions (F(2; 39)= .285; p= .754).

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Table 3: frequency of braking reaction at obstacle

No

Yes

Mean braking reaction time (seconds)

Manual Driving

0

15

,95

Driver Always Coupled

1

14

,98

Driver Decoupled

1

14

1,03

Total

2

43

Braking action

The time to counter steering shows how long the participant followed the collision avoidance system (Figure 6). A t-Test shows, that the differences between the two groups are not significant (t(28)=1,503; p=.114). However, in condition Driver Decoupled the system ignored these initial steering inputs and this counter steering had no effects. Regarding the steering magnitude in terms of maximum steering angle, drivers in the condition Driver Decoupled exhibited stronger counter steering. Maybe this was a result of the fact, that the driver was not able to control the vehicle for 0.5 seconds. A t-Test shows that the differences are significant (t(18)=-3,228,728; p=.05).

Figure 6: Mean values of time to counter steering (light bars - Time) and mean values of magnitude of counter steering (dark bars - Angle) in the evade scenario

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To rate the effectiveness of the manoeuvre the participants were asked how well the automatic steering manoeuvre was suited to avoid the collision with the obstacle. Participants who experienced the Driver Always Coupled condition rated the steering manoeuvre as “rather bad” while participants in the variant Driver Decoupled rated the steering manoeuvre as “rather good” up to “pretty good” (Figure 7). A t-Test shows that the difference is statistically significant (T28=-3.02; p=.005) and participants rated the automatic steering manoeuvre in the Driver Decoupled condition better than drivers in the Driver Always Coupled condition.

Driver Always Coupled

Driver Decoupled

Figure 7: Results of question „how good could the automatic steering manoeuvre avoid the collision with the obstacle” The mean values of the driver responses are shown.

The participants were also asked how they would rate the perceived torque at the steering wheel in the collision avoidance scenario. The torque on the steering wheel was judged as “too weak” in condition Driver Always Coupled and “rather weak” up to “just right” in condition Driver Decoupled. A t-Test shows that the difference is statistically significant (T27=-3.27; p=.003).

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Results false alarm scenario Table 4 shows how the drivers tried to control the vehicle in the false alarm situation. All 30 drivers steered against the evade manoeuvre to get the control back. There were five participants in the condition Driver Always Coupled and 9 participants in the condition Driver Decoupled who hit the brake to regain control of the vehicle. Based on the reaction times it is possible to understand the order of reaction. In both conditions counter steering is the first action drivers perform. If participants braked, they did it after their counter steering manoeuvre. A t-Test shows no significant differences in the braking reaction times (t(17)=-,17 ; p=.867) nor in the counter steering reaction times (t(28)=1,868 ; p=.072). Table 4 also shows large differences in magnitude of counter steering between the groups. Drivers in the condition Driver Decoupled braked harder (104.48 bar) and reach a greater mean steering wheel angle (163 deg.) than participants in the condition Driver Always Coupled (34.74bar, 21.77 deg.). A t-test proves, that the braking intensity (t(17)=-4.75 ; p