Eye Gaze and Movement Behaviour in the Operation of Adaptive

In-Car Touchscreens

Katharina Bachfischer, Christoph Waeller, Sandra Troesterer, Annette Tatzel and Fernando Puente Leon

Abstract— The usability of in-car touchscreens with smalldisplay areas can be significantly improved if buttons are onlydisplayed or enlarged once the user’s hand approaches thedisplay. This survey examines the influence of such adaptivebuttons on eye and hand movements in the vehicle. Theresults show that movement behaviour in the use of bothstatic and adaptive interfaces comprises a number of movementphases whose interplay can be described with the OptimizedInitial Impulse Model (OIIM). The adaptive expansion ofbuttons leads to significantly lower error rates compared withnon-adaptive buttons, given comparable movement times. Aparticular observation was the influence of indicators (visibleinformation on the relevant target prior to start of movement)during the use of adaptive interfaces. It emerges that thesecontribute to a reduction of the movement times, as they makemotion planning easier and thereby significantly increase theprecision of the initial movement phase. In addition, this meansa clear reduction in eye gaze duration.

I. INTRODUCTION

Touchscreens feature flexible options for displaying in-

formation and functions that are making them increasingly

popular as combined display and input interfaces for in-

car infotainment systems. As interaction with a touchscreen

during a journey can only be realised as a secondary task

(after driving the vehicle), ensuring that the information

displayed can be read quickly and that functions can be

executed efficiently and error-free are especially important.

The performance parameters of an input interface include

the required movement time (MT ) for activating a button,

the error rate (ER) during multiple use, and the eyes-off-

the-road time. A significant factor of influence here is the

diameter of the button displayed. DIN EN ISO 9241-110 [1]

recommends a standard diagonal diameter of at least 2 cm for

all buttons, irrespective of specific operating situations. The

mathematical model devised by Fitts in 1954 (“Fitts’ law”),

on the other hand, provided a more complex description of

the influence of the diameter of a button and of the amplitude

(length) of the movement on the resulting movement time:

MT = a + b log2

2A

W+ 1 (1)

K. Bachfischer and C. Waeller are with Volkswagen AG, Group Re-search, Electronics, Interactive HMI, Letter Box 1776, 38436 Wolfsburg,Germany (phone: +49-5361-9-73884; fax: +49-5361-9-72837; e-mail: katha-rina.bachfischer@volkswagen.de, christoph.waeller@volkswagen.de)

A. Tatzel is with IAV Ingenieurgesellschaft Auto und Verkehr, Berlin,Germany (email: annette.tatzel@iav.de)

Sandra Troesterer is with Technische Universitaet Berlin, Zen-trum/Fachgebiet Mensch-Maschine-Systeme, Berlin, Germany (email:sandra.troesterer@mms.tu-berlin.de)

F. Puente Leon is with the Department of Electrical Engineering andInformation Technology, Technische Universitaet Muenchen, Germany (e-mail: f.puente@tum.de)

MT corresponds to the mean duration of movement, A to

the amplitude of the movement (distance between the starting

point of movement and the centre of the target object), and

W to the diameter of the target object in the direction of

movement. The empirical constants a and b are determined

using linear regression. The logarithm ID = log2(2A/W +

1) describes the index of difficulty of the task in question.

As the distance between steering wheel and display in the

vehicle is defined by fixed mounting positions, a reduction

of the movement times can be achieved above all through an

increase in W . In practice, however, the design screen con-

tents for smaller touchscreens involve an ergonomic conflict

between the need for maximum interface/buttons size and at

the same time a rapidly decipherable and correspondingly

large display of information.

This conflict can first be resolved by switching from a

mode optimised for showing information (i. e. display mode)

to a mode optimised for operation (i. e. operating mode) as

the hand approaches the screen. Depending on the design,

the display area available can – at the time of use – be used

completely for displaying operating interfaces or during non-

operation be used solely for displaying information. Fig. 1

illustrates this principle.

3. Activation of the

expanded button by

touching.

2. Expansion of button

as the hand approaches

the screen.

1. Only small button

visible in display

mode.

Fig. 1. Basic operational principle for the adaptive touchscreen.

In order to support motion planning and hand-eye co-

ordination before or during the operating movement, so-

called indicators can be employed that in display mode also

provide information on operating interfaces. A difference

can generally be made here between semantic indication

(meaning and function of the interfaces) and geometric

indication (final position and/or dimensions of the interfaces).

According to Meyer et al. [5] and Meyer, Abrams &

Wright [6], an intentional operating movement always com-

prises one, two or n partial movements and can be described

using the OIIM (Optimized Initial Impulse Model). Accord-

ing to the latter, a guided movement always begins with

a ballistic partial movement whose development is planned

prior to the start of movement and with which approx. 90 %

of the distance to target can be covered (see Fig. 2). During

2008 IEEE Intelligent Vehicles SymposiumEindhoven University of TechnologyEindhoven, The Netherlands, June 4-6, 2008

978-1-4244-2569-3/08/$20.00 ©2008 IEEE. 1027

this movement, visually perceivable changes in the target

area cannot influence the motor planning.

Accuracy and speed of movement behave indirectly pro-

portional to each other during this phase. The residual

deviation between target and current position is picked up by

a person’s sensory processing and determines whether further

partial movements are necessary for correction. These correc-

tive movements are performed with closed visual feedback

[9], [5].

Given an efficient balance of speed and accuracy, most

target movements require two partial movements. The correc-

tive movements occurring after the ballistic movement phase

are henceforth referred to collectively as the selection phase.

distance

velo

city

0A

W

target region

Fig. 2. Schematic view of sequences of partial movements leading to targetin acc. with the OIIM [5].

Within this context, McGuffin & Balakrishnan [4] exam-

ined the effect of expanding targets on the duration of motion

where buttons are activated with a mouse-driven cursor. The

results show that the increase in diameter of the buttons leads

to a significant reduction of the movement time, even when

this occurs only after 90 % of the distance to target. The

final diameter of the target object therefore has a far greater

influence on MT than the diameter at the start of the target

movement.

Regarding the adaptation of touchscreen interfaces, it can

thus be concluded that an enlargement of the target object

leads – in accordance with Fitt’s law – to a reduction in

the duration of the operating movement, even if this does

not occur until the beginning of the selection phase. As

visual feedback has no influence on the ongoing procedure

of the ballistic movement phase [5], it can be assumed that

interaction targets do not need to be displayed in their final

form that moment.

It should be remembered here, however, that the work of

McGuffin & Balakrishnan [4] examines changes only in the

dimension of the target object and not in the information

on its position or function. Another crucial factor regarding

the usability of adaptive buttons on a touchscreen is the

information available for motion planning about the target,

as the latter determines the quality of the ballistic initial

movement and the duration of further corrective movements.

II. FOCUS AND OBJECTIVES OF THE SURVEY

This study incorporates the investigation and description

of the influence that adaptive buttons have in conjunction

with the operation of a touchscreen in the vehicle on both

the movement behaviour and visual behaviour of the user. Of

particular interest here are the effects of an abstract display

of buttons in the display mode and also a comparison of the

adaptive system with a non-adaptive one.

A further aim is to examine whether adaptive buttons lead

to significantly less errors in comparison with a non-adaptive

variant (given similar movement times), i. e. due to the larger

button area.

The results are to serve an evaluation of alternative op-

erating concepts. This leads to the posting of the following

hypotheses:

H1. The operating movements involved in using an in-car

touchscreen can be described for adaptive and non-adaptive

interface versions with the OIIM.

H2. There is no eye gaze during the ballistic phase,

whereas the selection phase is performed with closed visual

feedback.

H3. An adaptation of the target areas during the operating

movement leads to lower error rates in comparison with a

static interface, given comparable movement times.

H4. The employment of indicators leads in connection

with the operation of adaptive targets to shorter movement

times.

H5. The employment of indicators leads in connection

with the operation of adaptive targets to a shorter selection

phase and thereby to a shorter eyes-off time from the primary

task.

III. TEST DESIGN

A. Test environment

The survey involved the use of a prototypical version of

an adaptive touchscreens (7” resistive touchscreen, 16:9) in

a 1:1 scale abstract cockpit environment. Only a reduced

section of the 7” display was used. The measurement of

movement times from the steering wheel to the activation of

a button was started through the release of a Tiptronic lever

(TT switch) behind the steering wheel and ended on the first

subsequent contact with the touchscreen (also if the target

was missed). During the operating movement, intermediate

times were measured upon triggering of the adaptation and

on conclusion of the adaptation phase. A Lane Change Test

(LCT) 1 was used to simulate a primary task (i. e. driving).

The hand/eye movement measurements had to be carried

out in two successive tests, as the measuring systems for

handtracking interfered with that for eyetracking.

The movement measurement involved a 3 D cave in which

the position of a marker object located on the operating finger

of the test participant was measured. The measuring system

was at the start calibrated to the relevant buttons on the

touchscreen. The direct distance A of the operating finger

to the target (air line) was measured, i. e. not the absolute

position in the coordinate space (in Fig. 3 the movement

is illustrated with the example of partial movements TB1and TB2). The eye movement measurement was performed

1The Lane Change Test (LCT) is an instrument for evaluating driverdistraction through secondary tasks and is specifically suited for dual-taskexperiments. It is currently undergoing the ISO standardisation process.

1028

TB1 TB2

Asteering wheel

calibration point

Fig. 3. Measurement of the distance A of the operating finger to thetouchscreen.

using the iView X HED by SMI (Senso Motoric Instruments,

Head-mounted Eye-tracking Device) with a scan frequency

of 50Hz.

B. Variants of adaptation

The survey involved the implementation of one static and

two adaptive interface variants for a display area measuring

12 cm × 4 cm. The design of the static interface considered

an available display area use of 50 % for displaying infor-

mation and 50 % for function buttons. The adaptive interface

variants differ in the way in which the function buttons are

shown in display mode and the layout of the function buttons

(see Fig. 4).

display mode

operating mode

(a) variant 1: non-adaptive

display mode

operating mode

(b) variant 2: adaptive, without indication

display mode

operating mode

(c) variant 3: adaptive, with indication

Fig. 4. Interface variants - the rear images are of the display mode, the frontimages show the function interfaces that appear when the hand approaches.

C. Test procedure

All test subjects were first given an introduction to the

LCT and the functions of the different operational variants

and were able to test both systems extensively. They were

then told to press two set buttons (Play and TP) for each of

the three operational variants in separate actions as rapidly as

possible while performing the task of driving. The procedure

was performed for variants 2 and 3 - in each case in two

different system modes:

• Static: the function buttons were displayed in full from

the outset, i. e. display mode was deactivated.

• Wizard: the function mode was activated by a human

observer in accordance with apparent intention of oper-

ation.

Variant 1 could only be used in one mode (static), however

the number of tasks in this variant was equal to the one in

the adaptive variants. Results are presented separately even

though there was no adaptive wizard mode. Fig. 5 shows a

typical example of operation and the data recorded.

0.1560.2660.3280duration of phase (s)

0.7500.5940.3280time since start (s)

button_playmenu_completeentrystartstate

5. Touch on

touch-screen,

end of time

measurement.

4. Conclusion

of adaptation

phase.

3. Entry into

detection area,

start of

adaptation.

2. Releasing of

the TT-lever,

start of time

measurement.

1. Pulling of the

TT-lever.

Fig. 5. Data recording of a typical operation.

D. Sample under investigation

All test participants in the first survey (movement tracking)

were Volkswagen employees who are familiar with the

functions and menu layout of the adaptive touchscreen. There

were 15 participants: 11 men and 4 women. The average age

was 30.13 years (σ = 5.74).

The second survey (eye movement measurement) involved

22 employees and students of the Berlin Technical Univer-

sity. The survey produced satisfactory visual data for 14

participants, of which 9 were male and 5 female. The average

age was 28.86 years (σ = 3.44).

IV. MEASUREMENT VARIABLES AND DATA

PREPARATION

In total, 180 recordings of operational movements were

available for the evaluation of the hand movement and 168

for the evaluation of the eye movement, which each comprise

two button operations per test person for each of the three

interface variants in the wizard as well as in the static system

modes. Incorrectly recorded data were not included in the

analysis.

In both system modes (wizard & static), the movement

measurement data for each of the three display variants was

used to determine the mean movement time (MT ) from the

1029

start to the touching of the touchscreen by averaging across

all relevant individual movement times. The error rate (ER)

was calculated from the relation of the absolute number of

operation errors made by all users in one variant to the total

number of operational tasks performed within the relevant

variant.

Similarly, the average movement times for the separate

movement phases (MT1, MT2, MT3) were determined via

the averaging of all individual movement times for the corre-

sponding phase within the relevant variant. MT1 refers to the

phase from start to entry, MT2 from entry to menu complete

and MT3 from menu complete to button play (see Fig. 5).

The OIIM model was tested by using the movement data

recorded to generate path-time diagrams (see Fig. 6), which

in turn were used to calculate time-velocity diagrams (Fig. 6

bottom illustrates an idealised velocity pattern).

Fig. 6. Idealised movement model with selection and ballistic phase.

The first phase of movement from T0 to T0′ corresponds

to the ballistic phase of the OIIM and is designated by a

steep progression in the path-time diagram.

The eye movement was evaluated by comparing the

duration of eye movement towards the display with the

movement times. The generation of the eye movement data

involved the definition of two visual-attention areas (LCT

monitor, touchscreen) and the determination of the fixations

and saccades for each of the visual-attention areas using

an evaluation algorithm. Fixations reflect the visual intake

and processing of information; their duration and frequency

provide insights into the intensity and depth of processing.

V. RESULTS

A. Average movement time

The results show that the adaptive operating variants with

an average movement time of 0.786 s (σ = 0.168 s) and

0.816 s (σ = 0.147 s) in the static version were significantly

quicker (p > 0.05) to operate in comparison with static

variant 1 (see Fig. 7, top). According to Fitts, this can be

attributed to the larger buttons.

In wizard mode, the adaptive variant 3 with indicators is on

average quicker to operate than the static version 1, although

the difference is not significant (p < 0.05). Likewise, the

difference in the average movement time between wizard

and static modes is significant only with variant 2, although

not with variant 3. In wizard mode, we see a clear increase

in the operating time with the adaptive variant 2 without

indicators, however, this finding is not significant.

0.9

13

0.7

86

0.8

16

0.9

80

0.8

89

0.9

17

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

variant 1 variant 2 variant 3

se

co

nd

s

static

wizard

(a) average movement time MT for each variant in static andwizard mode

0.917

0.229 0.236

0.177 0.173

0.574 0.480

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

variant 1 variant 2 variant 3

se

co

nd

s MT3

MT2

MT1

(b) average movement time for each variant and for theseparate movement phases in wizard mode

Fig. 7. Average movement times MT.

B. Operating phases

A closer inspection of the individual operating phases

MT1, MT2 and MT3 reveals that the last phase MT3 is in

the adaptive variant without indicators considerably longer

than in the variant with indicators. Phases MT1 and MT2,

on the other hand, are practically identical.

The longer duration of MT3 can probably be attributed

to the lower accuracy of the ballistic phase due to missing

information on the motion planning. Correspondingly longer

and more frequent corrective movements are necessary.

C. Error rate

In variant 1, two incorrect button pushes were registered,

and none in the other variants. The error rate of variant 1

thus amounts to 7%.

1030

D. Movement diagrams

Fig. 8 (top) shows the measured movement pattern for

all test persons in variant 2 in static mode. In order to

improve comparability, the movement patterns were – unlike

the depiction in Fig. 6 – depicted on the negative time

axis, where t = 0 s corresponds to a button being pressed.

Fig. 8 below shows the velocity pattern calculated from the

movement pattern. Initially, all movements exhibit a rapid

target movement. The subsequent phase sees small corrective

movements in both negative and positive velocity patterns.

The point of time t = 0 s corresponds in this representation

to the start of the movement. The ballistic phase is completed

−1.5 −1.25 −1 −0.75 −0.5 −0.25 0−400

−300

−200

−100

0

50path of movement

time in seconds before touching

path

in m

m

0 0.25 0.5 0.75 1 1.25 1.51.5

0

500

1000

1500

2000velocity plot

time in seconds since start of movement

velo

city in m

m/s

Fig. 8. Path of movement and velocity plot of different persons of variant 2in static mode.

after approx. 50 % of the total operating time.

In comparison, in Fig. 9 the movement and velocity pattern

in variant 2 are displayed for wizard mode. Compared with

the static mode, we see a greater dispersion of values and

also an extension of the selection phase. There is likewise

an overall increase in the movement time – as illustrated

in Fig. 7. Nevertheless, a number of partial movements are

also evident in wizard mode. Similar movement and velocity

patterns were also witnessed in the variants 1 and 3.

E. Eye movement

Of particular interest here was at which point in time of

the movement the eyes shift to the display. Fig. 10 shows

the fixation times as percentages of the movement time.

In general, it emerges that a shifting of the gaze from the

primary task occurs for around 96 % of the movement time.

An initial fixation focus on the target area (display) takes

place after approx. 40 % of the movement time. At that time,

the adaptive menu was displayed almost in full. The start of

the final fixation of the target area is approx. 65 % of the

movement time. The average duration of eye gaze to the

display (for all variants and modes) is 56 %. Of note is that

the necessary eye gaze with variant 3 is in wizard mode

shorter than with the other variants.

−1.5 −1.25 −1 −0.75 −0.5 −0.25 0−400

−300

−200

−100

050

path of movement

time in seconds before touching

pa

th in

mm

0 0.25 0.5 0.75 1 1.25 1.5

0

500

1000

1500

2000velocity plot

time in seconds since start of the movement

ve

locity in

mm

/s

Fig. 9. Path of movement and velocity plot of different persons of variant 2in wizard mode.

At the same time, the average number of fixations in this

variant is the lowest (2.64 vs. 2.31 vs. 2.21 for variant 1,

2 and 3 in static mode; 3.19 vs. 3.00 vs. 2.64 in wizard

mode). The indicators employed in this variant evidently lead

to individuals finding orientation more easy and therefore to

correspondingly shorter gaze and information uptake times.

The smaller buttons in variant 1 lead especially in static mode

to a longer gaze duration and higher number of fixations

required.

Anzahl Fixationen Display bei Bedienung 2.64 2.50 1.002

Wann begann die 1. Fixation auf dem Display

(Bedienphase)457.36 424.50 156.852

Dauer 1. Fixation Display bei Bedienung 125.41 100.00 64.512

Wann begann die letzte Fixation auf dem Display

(Bedienphase)800.00 721.50 271.978

Dauer letzte Fixation Display bei Bedienung 411.05 310.50 281.384

Blickdauer Display bei Bedienung 753.68 640.00 419.835

Bedienzeit_diff 19.4091 18.5000 9.59493

% Bedienzeit 1. Fixation Display

39.6385 38.7973 17.72271

% Bedienzeit letzte Fixation Display

65.5925 65.7078 14.53117

% Gesamtzeit Abwendung LCT

41.5501 40.5039 9.97433

% Bedienzeit Abwendung LCT93.7018 98.2318 7.99036

% Bedienzeit Blickdauer Display56.8543 58.1960 18.66641

0

20

40

60

80

100

variant 1

static

variant 2

static

variant 3

static

variant 1

wizard

variant 2

wizard

variant 3

wizard

perc

en

t

movement time shifting of gaze from LCT duration of eye gaze

Fig. 10. Movement times and eye gaze to the display in comparison.

The results also show that the required gaze duration is

shorter (53 %) in static mode than in wizard mode (59 %).

An additional orientation is evidently necessary if the infor-

mation is not per se provided. This can be attributed to a

possible premature peripheral perception of information in

static mode.

VI. INTERPRETATION

The results of the study confirm that changing the user

interfaces in accordance with user intentions offers the po-

tential advantage of significantly reducing operating errors

where movement times are identical or even somewhat

1031

shorter. For a more precise assessment of the results, we

proceed to an assessment of the underlying hypotheses:

H1. The operating movements when in using an in-car

touchscreen can be described for adaptive and non-adaptive

interface versions with the OIIM.

This hypothesis is confirmed on the basis of the re-

sults. The recorded movement patterns in both the static

and adaptive interface versions indicate a family of curves

corresponding to the OIIM. The ballistic movement phase

sees approx. 80–90 % of the path to the target covered, and

it is on average completed after approx. 50 % of the total

movement duration. This is followed by the selection phase,

which comprises varying numbers of partial movements.

H2. There is no eye gaze during the ballistic phase,

whereas the selection phase is subject to visual feedback.

The first fixation takes place at 40 % of the movement

time, with the last beginning after approx. 65 % of the

movement time. As the selection phase starts at approx. 50 %

of the movement time, it can be assumed that the majority of

the selection phase takes place with closed visual feedback.

H3. An adaptation of the target areas during the operating

movement leads to lower error rates in comparison with a

static interface, given comparable movement times.

This hypothesis can be confirmed. For the operation

of the adaptive user interfaces, the error rate was 0 % –

but for the non-adaptive variant 7 %. Referring to the low

overall amount of errors, numbers of this study are not

very meaningful. However, the basic tendency could be

reconfirmed in other studies. In practice, error rates of up

to 5 % are acceptable, although in the non-adaptive variant

this threshold was already far exceeded in the simulator used

for this study. An investigation into operability in a moving

vehicle would as such probably lead to still higher error rates

for all operating modes.

While the mean movement times with the adaptive in-

terface versions in wizard mode deviate marginally from

the movement times with the static operating variant, this

difference is not statistically significant. We can therefore

assume comparable movement times.

H4. The employment of indicators leads in connection with

the operation of adaptive targets to shorter movement times.

This hypothesis can only partly be confirmed. The move-

ment times when using variant 3 were shorter than the

movement times for using variant 2, yet not significantly.

Nevertheless we may assume that indicators enable a better

planning and thereby also a more precise performance of

the ballistic movement phase, which in turn requires corre-

spondingly fewer corrective movements. This is supported

by the fact that the employment of indicators likewise sees

in comparison with the adaptive variant without indication a

smaller residual distance to the target. This is probably due

to the reduced information for motion planning prior to the

start of movement. In addition, the gaze duration is shorter.

H5. The employment of indicators leads in connection with

the operation of adaptive targets to a shorter selection phase

and therefore to a shorter eyes-off-time from the primary

task.

Where indicators are employed, the gaze duration is far

shorter than in the other modes. At the same time, the average

number of fixations is lowest in this variant. The results

did not show a shorter selection phase, although the overall

movement time was shortest in this variant. As part of the

investigation at hand, the position and function of the final

interface were determined via indicators using an interface

that was reduced in terms of dimensions and contrast in

display mode. Some users stated that they were unable to

fully use the indicators due to the latter’s correspondingly

reduced visual presence.

VII. SUMMARY

The results of the investigation affirm the concept of adap-

tive interfaces for improving the operability of small in-car

touchscreens. Adapting the display can help to significantly

reduce the error rate in comparison with a non-adaptive

operating mode, given comparable operating times. When

using adaptive interfaces, the employment of indicators has

a positive influence on the duration of the movement times.

It was also shown that operating movements for using

static and adaptive buttons on an in-car touchscreen can

generally be described with the OIIM.

The results confirmed that the ballistic phase of movement

begins without closed visual feedback, while the selection

phase occurs almost completely with closed visual feedback.

Tests for the adaptive variants were all performed with

a wizard. The implementation of a corresponding sensory

mechanism, however, represents a major challenge within the

context of avoiding false triggers through hand movements

in the cockpit.

ACKNOWLEDGEMENTS

We would like to thank the volunteers who took part

in the survey for their patience and cooperation, and also

Volkswagen R&D’s Virtual Reality Laboratory for its expert

and unbureaucratic support.

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