Visual Demand of Bilingual Message Signs Displaying Alternating Text Messages

Virpi Anttila, Juha Luoma and Pirkko Rämä 1

Visual demand of bilingual message signs displaying alternating text messages

Virpi Anttila*, Juha Luoma, Pirkko Rämä Technical Research Centre of Finland, P.O. Box 1902, FIN-02044 VTT, Finland *Corresponding author Tel.: 00 358 9 456 4517; fax: 00 358 9 464 850, e-mail: [email protected].

Abstract

This field study was designed to compare the visual demand of variable message signs (VMS). Specifically, three VMS types were evaluated: a sign displaying a message alternately in Finnish and Swedish (2.0 s in each language), a sign displaying the same messages simultaneously, and a sign displaying air and road surface temperatures in Finnish. The data was collected by recording the eye movements of 38 drivers during highway driving. The main results suggested that the sign displaying alternating bilingual messages was no more demanding than the VMS displaying the same messages simultaneously. However, this conclusion is limited to those specific signs and conditions, and more research is needed for related applications in various conditions. The results further suggested that VMSs involving effective technologies might be rather demanding in comparison to the fixed signs.

1. Introduction

In December 1997, the Finnish National Road Administration (Finnra) started testing bilingual variable message signs (VMS). The text message module displays text messages alternating between Finnish and Swedish. There is a clear economic advantage in using one sign showing different languages in turn, instead of constructing separate signs for each language when messages have more than 10 characters. It is possible, however, that alternating bilingual messages are more difficult to read and understand than the alternative of a separate message location dedicated to each language.

The first phase of the evaluation of a VMS displaying alternating bilingual messages focused on driver acceptance (Harjula, Luoma & Rämä, 1998). Totally, 350 drivers were interviewed downstream of the sign. The overall implication of the study was that most drivers accepted a VMS of this type. More specifically, 76% of the drivers recalled the sign correctly and 90% considered the time (2.0 s) they had for reading the message in their native language as being appropriate. However, older drivers experienced some difficulty in recalling the messages and they were more likely to report that the presentation duration was too short. It was concluded that more research is needed to investigate potential workload problems that might arise, especially among older drivers.

This field study was therefore designed to investigate driver visual behaviour while encountering a VMS displaying alternating bilingual messages, using VMSs displaying bilingual messages simultaneously and two monolingual temperature displays as controls. The primary comparison dealt with two bilingual VMSs, while the comparison between the bilingual and monolingual VMSs was secondary because of differences in message contents.

Field studies investigating the safety effects of VMSs have usually focused drivers’ speed behaviour and/or headways as well as driver perception, satisfaction and comprehension (for review see TROPIC, 1996). On the other hand, the visual demand of VMSs has been Virpi Anttila, Juha Luoma and Pirkko Rämä 2 examined by laboratory experiments or studies conducted in controlled field environments (e.g. Messer, Stockton, Mounce, Andersen & Turner, 1978; Padmos, van den Brink, Alferdinck & Folles, 1988; Alferdinck, Luoma, Rämä, van Dorresteijn & Harjula, 1998; MacLaverty & Buckle, 1998). In contrast to the long research tradition in the field of fixed traffic signs, the current literature includes only a few experimental evaluations of visual demand of VMSs conducted in dynamic in-traffic situations (e.g. Pauzie, 1998).

The data of this field study were collected by recording the drivers’ eye movements. The rationale of this approach is that traffic signing is supposed to be as simple and effective as possible to ensure easy and correct information to all drivers independently of driver or traffic conditions. However, the eye-movement technique has several limitations when used as an indicator of visual demand. The main limitations are as follows: First, the eye-movement technique allows us only to compare the visual demand of two similar messages presented with two different manners of presentation. No absolute assessment of the specific magnitude of eye fixations to the road and other road users required for a successful driving performance can be made, although we know that the driver in general needs to look at the road to conduct the primary driving task and may only occasionally look elsewhere (Wierwille, 1993). Second, the visual angle between the VMS on the roadside and the road is quite small for most of the time when the driver is approaching the VMS. Therefore, deviations between experimental and control situations need to be substantial in order to conclude that driver visual behaviour in one situation is more dangerous than in another.

The main hypothesis of the study was that the VMS displaying alternating bilingual messages is more demanding than the VMS displaying bilingual messages simultaneously. Furthermore, it was assumed that the VMSs are more demanding for older drivers than young drivers because of well-known age-related problems in divided attention tasks involving complex or demanding conditions.

2. Method 2.1 Signs

Three VMS types were evaluated (Fig. 1): (1) a sign displaying a message LOOSE GRAVEL alternately in Finnish and Swedish, (2) a sign displaying the same messages simultaneously and (3) a sign displaying air and road surface temperatures in Finnish. The VMS with alternating messages displayed each language for 2.0 seconds separated by an interval of 0.5 seconds (blank display). The VMSs consisted of two modules, a text message and a warning sign. The bilingual VMSs displayed a symbolic ‘general warning’ sign, while the temperature displays did not (i.e. the triangular sign module was black).

------Insert Figure 1 about here ------

The background of the displays was black, the triangle in the warning sign was red, the letters in the text message were yellow, and the pictogram in the warning sign was white. The height of the warning sign was 1,414 mm and width 1,633 mm. The warning sign used fibre-optic technology, which forms signs and letters using small lenses. Light is conducted to the lenses along optical fibres illuminated by a halogen lamp at the end of the fibre bundle. The text message module consisted of two lines of text of 10 characters each. The characters of the text Virpi Anttila, Juha Luoma and Pirkko Rämä 3 message module were 225 mm high and formed with 10 light-emitting diodes (LEDs) per pixel. The output level of the VMSs was automatically adjusted to ambient light.

Each sign was erected on the right side of the road, with a forested background as seen by the approaching driver. The visibility distance of the VMSs varied between 500 m and 600 m and the reading distance was approximately 250 m for a person with normal visual acuity.

2.2 Site

The experimental site was located in southern Finland on inter-urban road E18, with a speed limit of 80 km/h or 100 km/h and an average daily traffic of 9,900 vehicles. Drivers travelling on the highway had one lane in each direction.

2.3 Equipment

The subject vehicle was a 1992 Opel Astra with manual transmission. Eye movements were recorded by an eye movement recorder (EMR-NAC Model 5) based on the reflection of light rays from the cornea.

2.4 Subjects

In total, 44 licensed subjects participated in the study. However, six subjects were excluded because of technical problems with the recorder or occasional adverse weather conditions. Of the remaining 38 subjects, 19 were aged between 21 and 28 years (mean 23.2 years) and 19 were aged between 60 and 80 (mean 67.2 years). All subjects were males with Finnish as their mother tongue.

The subjects’ vehicle kilometrage during the previous year ranged from 1,500 km to 100,000 km (average 14,900 km). Most of the subjects (71%) had passed the test sites previously. These background variables did not differ statistically significantly between the two age groups. Fifty-eight percent of younger subjects understood written Swedish well or very well, compared with 21% of older subjects (χ2(1) = 5.40, p < 0.05).

2.5 Design

The independent variables included sign type (two bilingual VMSs displaying messages either alternately or simultaneously, and two temperature displays) and subject’s age (young vs. old). The dependent variables were number of eye fixations, total gaze duration and duration of the longest fixation. The rationale for the last variable was the nature of the VMS displaying alternating messages. One might expect this sign to result in long individual eye fixations because, for example, the message in a subject’s mother tongue could be invisible while approaching the reading distance.

In order to analyse the potential effects of other traffic in the vicinity of the sign on eye movements, the data was divided into two groups: with or without other traffic nearby. The cases with other traffic nearby included the following situations: another vehicle driving in front of the test vehicle (maximum headway 2 s) or another road user encountering the test vehicle as it approached the sign at the reading distance or closer, or another road user encountering the test vehicle within 3 seconds downstream of the sign. Virpi Anttila, Juha Luoma and Pirkko Rämä 4

2.6 Procedure

The subjects participated in the experiment individually. They were told that the purpose of the experiment was to compare visual behaviour of younger and older drivers during highway driving. Their task was to drive as normally as possible, following the experimenter’s directions. They were asked to familiarise themselves with all controls before the start. They drove a practice route of 8.7 km without the eye mark recorder to get used to the car. After the practice session the eye mark recorder was placed on the subject’s head and calibrated. The experimenter checked the calibration of the recorder on a monitor and recalibrated it if necessary. The length of the test route was approximately 25 km.

The order of the two bilingual signs was balanced by a subject group (i.e. 9 or 10 of the subjects in each age group first encountered the sign displaying the alternating messages, while 9 or 10 first encountered the sign displaying the messages simultaneously). The first temperature display was encountered before, and the other display after, the bilingual signs. Before encountering the first test sign the subject encountered two temperature displays. The data was collected in daytime, between 09:00h and 16:00h in good weather conditions, i.e. with no rain.

An eye fixation was defined when the eye mark of the recorder stopped on the sign. Several subsequent fixations on different parts of the sign were counted as one fixation. In order to minimise any bias caused by the limited resolution of the recorder, drivers’ visual behaviour before encountering the reading distance was excluded.

3. Results 3.1 Effects of sign and age on eye movements

The results showed no significant difference between the signs in percentages of subjects who fixated on the sign. The highest (100%) percentage was found for a bilingual sign displaying the messages simultaneously, followed by a bilingual sign displaying the messages alternately (95%), the first temperature display (92%), and the second temperature display (89%). Eighty-nine percent of the older subjects and 68% of the younger subjects fixated on each test sign. The following analyses concern these subjects.

Analyses of variance were performed on the number of fixations, gaze duration and the duration of the longest fixation. The analyses incorporated one within-subject variable (sign) and one between-subject variable (age). The mean number of fixations varied from 2.9 (second temperature display) to 3.3 (first temperature display). The mean number of fixations was 3.2 for older subjects and 3.1 for younger subjects. There was no significant difference between the signs or age groups.

The effect of the sign by age on gaze duration is shown in Fig. 2. The sign had a statistically significant effect on gaze duration (F(3,84)=10.9, p < 0.001), with the longest gaze duration for a bilingual VMS displaying messages alternately (4.70 s), followed by a bilingual VMS displaying the messages simultaneously (4.06 s), the first temperature display (2.52 s), and the second temperature display (2.24 s).

------Insert Figure 2 about here ------Virpi Anttila, Juha Luoma and Pirkko Rämä 5

The post hoc Tukey test (α = 0.05) showed that each difference between the bilingual signs and temperature displays was statistically significant, while differences between the two bilingual signs and between the two temperature displays were not significant. The effect of age or the interaction of the two variables was not statistically significant.

The effect of the sign by age on duration of the longest fixation is shown in Fig. 3. The sign had a statistically significant effect on the duration of longest fixation (F(3,84)=10.3, p < 0.001), with the longest fixations for the bilingual sign displaying messages alternately (2.93 s), followed by the bilingual sign displaying the messages simultaneously (2.52 s), the first temperature display (1.46 s) and the second temperature display (1.05 s).

------Insert Figure 3 about here ------

3.2 Effects of other road users on eye movements

The results according to the presence of other road users showed that the mean gaze duration for the bilingual message signs was 4.26 s if there were no other road users nearby, and 3.96 s if there were other road users. The corresponding results for the temperature displays were 2.32 s and 2.20 s, respectively. The effects of other road users were not statistically significant, although eye fixations tended to be shorter when there were other road users nearby. Similar patterns were found for the longest fixations.

4. Discussion

The study was designed to investigate whether a bilingual VMS displaying a message in Finnish and Swedish alternately is more demanding than a sign displaying the same messages simultaneously. In addition, these signs were contrasted with two VMSs displaying air and road temperatures in Finnish. The data was collected by recording the eye movements of 38 drivers.

The results showed that 89% to 100% of drivers fixated on the signs. The mean number of fixations varied from 2.9 to 3.3. The mean number of fixations was 3.2 for older subjects and 3.1 for younger ones.

The gaze duration results showed no statistically significant difference between the two bilingual signs, although there was a slight tendency towards longer gaze duration on the sign with alternating messages than on the sign with simultaneous display. The results of duration of the longest fixation showed a similar pattern. These results suggest that a VMS displaying bilingual messages alternately is no more demanding than a VMS displaying the same messages simultaneously. This is in agreement with previously obtained interview results (Harjula et al., 1998). However, this conclusion concerns specific signs (with simple Virpi Anttila, Juha Luoma and Pirkko Rämä 6 messages including only one word in two languages), display duration and site (inter-urban road in a relatively simple environment), and more research is needed for related applications.

Furthermore, the results showed that gaze duration and longest fixation duration were significantly longer for bilingual VMSs than for monolingual temperature displays. Although the bilingual VMSs and temperature displays were similar in terms of number of characters, this difference should be interpreted cautiously because the message contents were different. For example, it is possible that the drivers were able to obtain the information from the temperature displays very rapidly because they had seen two similar signs before the test section. The information on the displays was not new and thereby partly irrelevant. This was not the case for either of the bilingual VMSs.

Compared with the results on fixed signs obtained in earlier studies, the number of fixations found in the present study seemed to be high. Specifically, earlier studies have shown that the mean number of fixations on simple fixed signs (i.e. speed limit signs and symbolic warning signs with or without a supplementary sign) ranges from 1.7 to 3.3 and gaze duration from 0.5 s to 1.4 s (e.g. Zwahlen 1981, 1987; Luoma 1991, 1992, 1993). This is not to say that the property of being variable for a sign is more demanding than that of being fixed, but the VMSs might involve display technologies that are demanding. This finding is in agreement with the results of Rämä, Luoma and Harjula (1999) showing that drivers were less likely to recall the warning sign when it was in the vicinity of the fibre-optic speed limit sign than when it was close to the fixed speed limit sign. On the other hand, the improved visual properties of the VMS, conspicuity in particular, seem to result in relatively high recall rates of VMSs which is desirable (Rämä & Luoma, 1997; Harjula et al., 1998; Luoma & Rämä, 1998). Consequently, the visual demand of VMSs involving visually effective technologies and perhaps more complex messages than the VMSs used in this study should be addressed in the future.

The effect of age on gaze duration and duration of the longest fixation was not significant, although there was a fairly clear tendency for longer durations among older drivers than among younger drivers. However, the bilingual knowledge of the drivers was confounded, with younger drivers being more bilingual than older ones. Although the driver sample in this respect is assumed to represent Finnish drivers rather well, no general conclusion can be drawn from this finding.

The main conclusions of the study are as follows: First, the manner of presentation of the bilingual messages had no effects on drivers’ visual response that would suggest that this particular VMS with alternating messages is too demanding compared with that showing the same message simultaneously. However, this conclusion is limited to the circumstances of the conducted study (environment, sign, subjects etc.). Second, the results on visual demand of VMSs using effective display technologies supported earlier findings indicating that the demand can be high, and therefore this aspect should be addressed more thoroughly in future.

Acknowledgements

Appreciation is extended to the Finnish Ministry of Transport and Communications and the Finnish National Road Administration for their support of this research. Financial support for the study was also received from the European Union DGVII through the Tropic project aiming at a wide-scale and consistent application of VMSs in Europe. However, the content Virpi Anttila, Juha Luoma and Pirkko Rämä 7 of the paper is the sole responsibility of its publishers and in no way represents the views of the EC.

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FIGURE CAPTIONS:

Fig. 1. Test signs: (A) VMS displaying a pictogram “general warning” and a text message “loose gravel” in Finnish and Swedish alternately, (B) VMS displaying a pictogram “general warning” and a text message “loose gravel” in Finnish and Swedish simultaneously, and (C) VMS displaying the air and road surface temperatures in Finnish.

Fig. 2. Mean gaze duration by sign and age.

Fig. 3. Mean duration of the longest fixation by sign and age. Virpi Anttila, Juha Luoma and Pirkko Rämä 10

2.0 s 0.5 s 2.0 s

Fig. 1.

Virpi Anttila, Juha Luoma and Pirkko Rämä 11

5.19

4.33 4.05 3.71 Bilingual, alternating Bilingual, simultaneously 2.52 2.41 First temperature display 2.03 1.98 Second temperature display

Younger Older

Age Group

Fig. 2. Virpi Anttila, Juha Luoma and Pirkko Rämä 12

3.57

2.93

Bilingual, alternating 2.10 1.99 Bilingual, simultaneously First temperature display 1.45 1.26 Second temperature display 1.04 1.06 Duration of Longest Fixation (s)

Younger Older Age Group

Fig. 3.