Visual Demand of Bilingual Message Signs Displaying Alternating Text Messages
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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.