# Drivers' perception of simulated loss of adherence in bends

Thomas Denoual, Franck Mars, Jean-François Petiot, Gilles Reymond, Andras Kemeny

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Thomas Denoual, Franck Mars, Jean-François Petiot, Gilles Reymond, Andras Kemeny. Drivers' perception of simulated loss of adherence in bends. Driving Simulation Conference Europe 2010, Sep 2010, Paris, France. pp.43-53. hal-05406855

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# Drivers' perception of simulated loss of adherence in bends

Thomas Denoual, Franck Mars, Jean-François Petiot, Gilles Reymond, Andras Kemeny

IRCCYN (Institut de Recherche en Communications et Cybernétique de Nantes)

Renault, Technical Center for Simulation

1 avenue du golf – 78288 Guyancourt, France

thomas.denoual@renault.com, franck.mars@irccyn.ec-nantes.fr,

jean-francois.petiot@irccyn.ec-nantes.fr, Gilles.reymond@renault.com,

andras.kemeny@renault.com

**Abstract** - Loss of adherence (LOA) in bends, due to excessive speed or alteration of road grip, can lead to loss of vehicle control, which is a cause for many accidents. This paper presents preliminary results of an experiment studying how drivers perceive and react to vehicle skidding in a fixed-base simulator. Situations of LOA inducing a significant modification on the vehicle trajectory without involving a brutal loss of control or a road departure were chosen. The intensity and the duration of the LOA were manipulated. Naive participants repeated short drives on a track made of a straight road followed by a bend and were asked to answer a questionnaire after each track. To describe the LOA, two types of indicators were used: objective indicators of steering control and subjective indicators based on verbal descriptors scaled after each track. Preliminary results showed that drivers were able to discriminate the different conditions of LOA. The intensity of the perturbation was well perceived, with minimal influence of duration and not apparent relation to the magnitude of the steering correction. By contrast, a distortion of subjective time was observed when the duration of the LOA was assessed. Further analyses will be conducted to determine to what extent objective and subjective indicators were related. This study is the first step to develop an evaluation method that could be applied to the evaluation of ESC system intervention in high performance simulators.

**Résumé** - Les pertes d'adhérence (PA) en virage, dues à une vitesse excessive ou une altération de la tenue de route, peuvent entraîner des pertes de contrôle qui sont la cause de nombreux accidents. Cet article présente les premiers résultats d'une expérience qui étudie comment les conducteurs perçoivent et réagissent face à un dérapage du véhicule. Les situations de PA choisies induisent une modification perceptible de la trajectoire du véhicule sans provoquer une brutale perte de contrôle ou une sortie de route. L'intensité et la durée de la PA ont été manipulées. Des participants naïfs ont répété plusieurs conduites sur parcours simple composé d'une route droite suivie d'un virage.
Pour décrire les PA, deux types d'indicateurs ont été utilisés: des indicateurs objectifs du contrôle de la trajectoire et des indicateurs subjectifs basés sur des descripteurs verbaux cotés après chaque passage. Les premiers résultats ont montré que les conducteurs sont capables de distinguer les différentes conditions de PA. L'intensité de la perturbation a bien été perçue, avec une influence minime de la durée, sans relation apparente avec l'amplitude de la correction au volant. En revanche, une distorsion du temps perçu a été observée. Des analyses plus poussées seront menées pour déterminer dans quelle mesure les indicateurs objectifs et subjectifs sont liés. Cette étude est une première étape dans le but de développer une méthode d'évaluation qui pourrait être appliquée à l'évaluation des interventions d'un ESP dans un simulateur haute performance.

Introduction

Loss of adherence (LOA) can lead to loss of vehicle control, which causes
many accidents. Electronic stability control (ESC) can limit the consequences by
correcting the vehicle trajectory according to the driver's intentions and dynamics
of lateral acceleration, yaw speed or drift of the vehicle (Liebemann, 2004; Erke,
2008). The calibration and validation processes are time consuming and require
physical prototypes and experts drivers on specific grounds, especially for very
low adherence situations. Consequently, driving simulators are being used to
study LOA episodes and ESC performance (Papelis et al., 2010). Driving
simulators are useful tools in vehicle design and perception studies. They allow to
safely explore critical situations with naive drivers without environmental bias
(Kemeny, 2009). The present study is the first step of a research program aiming
at understanding how drivers perceive and react to trajectory perturbations and,
further on, to the intervention of an ESC system. This could be useful for the
engineering specifications of ESC using driving simulators and to evaluate how
actual drivers perceive different system configurations.

During LOA episodes inducing sudden changes in the vehicle trajectory, the
driver must perform an appropriate steering response to maintain the vehicle into
the lane and avoid road departure. Numerous sensorimotor models have been
proposed to explain how drivers use visual, vestibular and haptic information to
steer a vehicle in normal conditions (Donges 1978, Reymond et al., 2001, Toffin
et al., 2007). However, little is known about sensory cues that are used by the
driver to detect LOA episodes and how steering responses are carried out.
Besides, hierarchical model of cognitive control applied to driving postulate that
steering mainly relies on sensorimotor loops which operate below the level of
consciousness (Hollnagel 2004, Michon 1985). Typically, emergence to
consciousness arises when external disturbances occur (Hoc and Amalberti
2007). Assessing at the same time steering responses to LOA and the associated
subjective experience may be a way to investigate how sensorimotor cues
determine the conscious evaluation of driving incidents.

This paper presents a driving simulator experiment in which episodes of LOA
were triggered to produce significant modification of the vehicle trajectory without
loss of control and road departure. Intensity and duration of the LOA were
manipulated. The first objective was to develop an evaluation method to describe LOA episodes by means of subjective indicators using a non-structured-scaled questionnaire (Strigler, 1998). Objective indicators of the vehicle's dynamic and driver behaviour were also analysed. Another objective was to determine to what extent objective and subjective indicators were related (Mellert, 2007).

# Method

## Participants

Four female and sixteen male drivers between 20 and 24 years old (mean age of 21.4) participated in the experiment. They had driving licence for 3.4 years on average and drove between 1000 and 25000 km per year (mean = 6325). All of them had normal or corrected-to-normal vision. Fourteen participants declared that they had already faced to a loss of adherence situation on the road, two of them during a specific driving lesson. Two participants had already used a simulator.

## Apparatus

The experiment was conducted on a fixed-base simulator at IRCCyN laboratory (Nantes, France). It consists of a compact size passenger car with actual instrument panel, clutch, brake and accelerator pedals, handbrake, ignition key and an adjustable seat with seat-belt. Transmission is done by an automatic gear box. Vibrators are installed at the bottom of driver seat and upper position of the steering column to render engine noise and vibrations. Active steering force feedback is rendered by a TRW steering wheel. The audio system renders the audio environment for an interactive vehicle. It contains an amplifier, 4 speakers and sub woofer.

<figure><img src="image_2.png" /><figcaption>Figure 1. (a) IRCCyN driving simulator. (b) Layout of the country track</figcaption></figure>

The SCANeR®II<fnref n="2" /> software package was used with CALLAS® dynamic vehicle model (Lechner et al., 1997).

<footnote n="2">² http://www.scanersimulation.com/</footnote>
The visual environment was displayed on three 32 inch LCD monitors with a resolution of 1280 x 720 each, one in front of the driver and two laterals inclined of 45° from the front one, viewed from a distance of about 1 meter and covering 115° of visual angle (Fig. 1a). The graphics database reproduced an open countryside environment. The experiment was performed on a short part of the environment which consisted in a straight line followed by a bend (total distance: 700 m, mean radius in the bend: 111 m) without traffic (Figure 1b).

A simple generic speed regulator was used, consisting of a PID corrector with a nominal speed of 75 km/h, using the automatic gearbox mode in order to reject inter subject velocity bias. This condition also allowed the subject to be only focused on the steering task.

Two type of LOA were simulated in the bend by modifying the adherence under the wheels when the vehicle reached a defined point. The intensity (adherence coefficient) and duration of the simulated LOA in the bend were manipulated as independent variables (IV). An adherence coefficient decrease corresponds to an increase of the intensity of LOA. These values of intensity and duration values were chosen to induce perceptible but controllable LOA. LOA was simulated either on the four wheels (LOA1) or on the rear wheels (LOA2). The LOA1 situation induced a skidding to the outside of the bend comparable to an actual situation of driving on a patch of black ice, a puddle or a pool of oil depending on the independent variables values. The LOA2 induced situations similar to over-steering. The results of the LOA2 are not presented in this paper. After the LOA, coefficient of adherence was set again to 1. The environment was not giving clues about a potential LOA (snow, rain or mark on the road) (Fig. 2b).

<figure><img src="image_3.png" /><figcaption>Figure 2. (a) Input and output data. (b) Visual environment in the bend</figcaption></figure>

## Procedure

The subjects settled themselves in the simulator while preliminary instructions were given. In particular, they were asked to keep their lane without cutting the bend even if there was no oncoming traffic. Then, they were invited to start the simulator and drive a 10 minutes session for training. Next, they drove around the test bend with automatic gearbox and speed regulator on with the repeated instruction to stay in their lane and focus on the trajectory. Four trials without any LOA were performed in order to familiarize the subjects with the bend.
For each type of LOA, a control condition (no LOA) was inserted in the experimental design. The order of presentation of LOA1 and LOA2 was counterbalanced across the subjects. The same Williams Latin Squares design (Williams, 1949) was adopted for each type of LOA to avoid rank and carry-over effects. 20 trials divided into two sequences of each type of LOA preceded by 4 preliminaries trials representing mild and strong LOA episodes were performed. Those 8 preliminaries trials were conducted in order to familiarize the participants to the range of steering perturbations they would encounter during the experiment. They were not analyzed. The two types of LOA induced very different modifications in the vehicle behaviour and situation's perception. Therefore we have chosen this block design to avoid heterogeneous scaling on subjective indicators. Moreover, the experimental design was different for each type of LOA to keep perceptible but controllable situations. A 3*3 factorial design was used for LOA1 (Intensity: 0.1, 0.3 & 0.5; Duration: 250ms, 500ms & 750ms) (Table 2). A constrained design was used for LOA2 (Intensity: 0.1 to 0.6; Duration: 100 to 500ms). After each trial, a questionnaire was displayed with 13 questions about subjects' perception of the event (Table 1). Answers to the questions were given by the mean of continuous horizontal scroll bars representing two ends of a continuous scale (0: totally disagree to 10: totally agree) (Fig. 3) excepted for 4th question (Yes / No). Only LOA1 results are presented in this paper.

Behavioural measures (lateral position, steering angle, lateral acceleration, etc.) were recorded all along the trials at 20 Hz.

<table><caption>Table 1. Summary of the items corresponding to each question</caption><thead><tr><td>Item</td><td>Question</td></tr></thead><tbody><tr><td>Danger</td><td>"I perceived a danger during the bend"</td></tr><tr><td>Fear feeling</td><td>"I was afraid during the bend"</td></tr><tr><td>Feeling of control</td><td>"I easily kept my vehicle in the lane"</td></tr><tr><td>Perturbation perception</td><td>"Did you feel a perturbation in the bend?"</td></tr><tr><td>Intensity</td><td>"The LOA appeared to be weak/strong"</td></tr><tr><td>Duration</td><td>"The LOA appeared to be short/long"</td></tr><tr><td>Visual cue</td><td>"I visually perceived the LOA"</td></tr><tr><td>Haptic cue</td><td>"I perceived the LOA through the steering wheel"</td></tr><tr><td>Physical move</td><td>"I had the impression of physically moving"</td></tr><tr><td>Skid direction</td><td>"I felt the vehicle was skidding from the front/rear"</td></tr><tr><td>surprise</td><td>"I was surprised by the vehicle response"</td></tr><tr><td>realism</td><td>"Driving the simulator was unrealistic/realistic"</td></tr><tr><td>comfort</td><td>"I was at ease during the trial"</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>Figure 3. Visual answer interface of a question</figcaption></figure>
# Data analysis

For each condition, the mean and the standard deviation of the subjective answers were computed. When the fourth question was ticked “no”, the following answers were settled to a “default value” depending on the meaning of the question.

For each run, a time to stability (TTS) corresponding to the time taken by the driver after the onset of LOA to bring the vehicle drift speed back into a stability envelope was computed. This envelope is defined as the standard deviation of the mean drift speed and was measured in the control condition. Drift speed was

calculated using the following formula: $\varphi_{drift} = \frac{d \arctan(\frac{V_y}{V_x})}{dt}$ with $\varphi_{drift}$ the angular drift speed, $V_x$ the longitudinal speed and $V_y$ the lateral speed. The following objective data were observed in TTS interval: lateral deviation, steering wheel angle, slip angle, yaw speed and lateral acceleration.

Repeated measures analyses of variance (ANOVA, $\alpha = 0.05$) with the intensity and the duration of the LOA as independent variables (IV) were performed on the data. Scheffé tests were performed for post-hoc analyses. A principal component analysis was also performed on the subjective indicators in order to determine if they could be summarized by one or several underlying factors.

# Results

## Subjective data

The principal component analysis of the subjective data showed that all indicators can be represented by a single factor, which means that all variables were highly correlated. The simulation was globally judged as realistic (mean score = 7.64) with no significant effect of intensity and duration.

There was a significant effect of intensity and duration of LOA and a significant interaction between both IV on the duration and danger perception, fear and feeling of control. The effect of intensity and the interaction between both IV on perceived intensity was significant, but the effect of duration was not (Fig. 5 & Tab. 3). Post-hoc tests confirmed that the effect of intensity on duration, danger and intensity perception, fear and feeling of control was significantly higher for longer LOA.

There was no significant effect of the IV on the perceived direction of skidding. Participants could not tell if the vehicle was skidding from the front or the rear side (mean value: 5.11, SD: 2.7).

All the LOA situations were clearly perceived through the steering wheel (mean value: 8.13, SD: 2.35) and there was no significantly effect of the IV. Conversely, only the strongest LOA were perceived visually, as shown by the significant effect of the intensity (F(2,38) = 62.53, $p < .05$) and duration (F(2,38) = 4.79, $p < .05$)
## Objective data

Intensity (F(2,38) = 200.97, p < .05), duration (F(2,38) = 57.65, p < .05) and interaction between both IV (F(4,76) = 13.14, p < .05) had significant effects on the TTS. Post-hoc test confirmed that the effect of intensity on TTS was significantly higher for long duration and that there was no significant effect of the duration for lower level of intensity.

<table><caption>Table 2. Maximum and mean TTS for each condition</caption><thead><tr><th>Conditions</th><th>C1</th><th>C2</th><th>C3</th><th>C4</th><th>C5</th><th>C6</th><th>C7</th><th>C8</th><th>C9</th></tr></thead><tbody><tr><td>Adherence coefficient</td><td>0.1</td><td>0.1</td><td>0.1</td><td>0.3</td><td>0.3</td><td>0.3</td><td>0.5</td><td>0.5</td><td>0.5</td></tr><tr><td>Duration (ms)</td><td>250</td><td>500</td><td>750</td><td>250</td><td>500</td><td>750</td><td>250</td><td>500</td><td>750</td></tr><tr><td>TTS max (s)</td><td>5.15</td><td>5.2</td><td>6.15</td><td>5.35</td><td>5.15</td><td>6.15</td><td>6.15</td><td>2.65</td><td>3.55</td></tr><tr><td>TTS mean (s)</td><td>3.02</td><td>4.13</td><td>5.55</td><td>2.18</td><td>3.2</td><td>3.93</td><td>1.14</td><td>1.07</td><td>1.44</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Figure 4. (a) ANOVA of the maximum steering wheel angle. (b) ANOVA of the maximum lateral deviation from the centre of the road</figcaption></figure>

The ANOVA performed on the maximum steering wheel angle (Fig. 4a) showed a significant effect of intensity (F(2,38) = 136.7, p < .05) and duration (F(2,38) = 47.21, p < .05), with a significant interaction between both IV (F(4,76) = 23.08, p < .05). Similar results were observed on the maximum lateral deviation (Fig. 4b; intensity: F(2,38) = 125.48, p < .05, duration: F(2,38) = 97.08, p < .05; interaction: F(4,76) = 30.08, p < .05).

<figure><img src="image_6.png" /><figcaption>Figure 5a. Intensity perception</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 5b. Duration perception</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 5c. Danger</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 5d. Fear feeling</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 5e. Control feeling</figcaption></figure>

<table><caption>Table 3. Summary of the statistical analyses performed on the effect of intensity and duration on the selected subjective variables</caption><thead><tr><th>Subjective Items</th><th>IV</th><th>F</th><th>LoS</th></tr></thead><tbody><tr><td rowspan="3">Intensity</td><td>Intensity</td><td>(2,38) = 108.47</td><td>p &lt; 0.05</td></tr><tr><td>Duration</td><td>(2,38) = 1.97</td><td>n.s.</td></tr><tr><td>Intensity*Duration</td><td>(4,76) = 5.35</td><td>p &lt; 0.05</td></tr><tr><td rowspan="3">Duration</td><td>Intensity</td><td>(2,38) = 34.78</td><td>p &lt; 0.05</td></tr><tr><td>Duration</td><td>(2,38) = 21</td><td>p &lt; 0.05</td></tr><tr><td>Intensity*Duration</td><td>(4,76) = 4.47</td><td>p &lt; 0.05</td></tr><tr><td rowspan="3">Control feeling</td><td>Intensity</td><td>(2,38) = 89.58</td><td>p &lt; 0.05</td></tr><tr><td>Duration</td><td>(2,38) = 11.36</td><td>p &lt; 0.05</td></tr><tr><td>Intensity*Duration</td><td>(4,76) = 8.2</td><td>p &lt; 0.05</td></tr><tr><td rowspan="3">Danger</td><td>Intensity</td><td>(2,38) = 63.04</td><td>p &lt; 0.05</td></tr><tr><td>Duration</td><td>(2,38) = 3.86</td><td>p &lt; 0.05</td></tr><tr><td>Intensity*Duration</td><td>(4,76) = 7.08</td><td>p &lt; 0.05</td></tr><tr><td rowspan="3">Fear feeling</td><td>Intensity</td><td>(2,38) = 50.1</td><td>p &lt; 0.05</td></tr><tr><td>Duration</td><td>(2,38) = 6.67</td><td>p &lt; 0.05</td></tr><tr><td>Intensity*Duration</td><td>(4,76) = 7.3</td><td>p &lt; 0.05</td></tr></tbody></table>
Discussion

From a general point of view, all subjective answers were correlated and can
be described along one dimension, opposed to the adherence coefficient, as
revealed by the principal component analysis. This suggests that all subjective
ratings were coherent and determined by the intensity of the trajectory
perturbation. The question remains now to determine if the participant were able
to discriminate the magnitude and duration of the manipulated LOA.

The intensity of the LOA was perceived correctly with only minimal influence of the duration for the higher intensity of LOA. Interestingly, the perceived intensity was neither related to the maximum steering angle nor to the maximum lateral deviation. Since the maximum steering angle can be considered as a good indicator of the intensity of the steering correction, this suggests that subjects were able to evaluate how much adherence the vehicle lost, independently of how long it lasted and how much steering correction was needed.

By contrast, the duration of the LOA was poorly perceived. There was a strong
interaction with the intensity of the LOA, revealing that the stronger it was, the
longer it was perceived. It could be argued that the participant confused the
duration of the LOA with the time needed to stabilize their vehicle, but the clear
instructions given prior to the experiment make this assumption hardly believable.

A more plausible explanation is that LOA of high intensity were more stressful
than milder one, as showed by the fear and danger ratings. Distortions of time
have been observed under stress conditions, especially under life threatening
conditions (Hancock, 2005) or during specific critical tasks by paramedics
(Jurkovich, 1987). The underlying processes may be the attention. Indeed, Tse et
al. (2004) proposed that novel or important events run in "slow motion" so that the
information may be processed in greater depth per unit of objective time than are
casual events.

Further experimentation with more experimented drivers on a high
performances simulator with dynamic motion rendering should lead to more
consistent results. Indeed, Kemeny et al. (2003) highlighted the importance of
vestibular cues rendering in speed perception and steering in a simulator.

Conclusion

This study demonstrated that drivers are able to discriminate different
conditions of LOA on a fixed-based simulator. Whereas the intensity of LOA
episodes could be assessed by the driver with only minor distortions, a subjective
expansion of their duration was observed. Further analyses will attempt to more
clearly link subjective ratings and steering responses, and also to match the
observed data with ESC triggering criteria.
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