In order to provide additional information to road-users, the Dutch Ministry of Transport (Rijkswaterstaat) is studying the possibility to equip its service vehicles with electronic devices that can display text messages. On basis of literature research and specifications provided by the manufacturers, the present report provides recommendations for using these electronic displays on motorways. For the characters used on electronic displays one may assume a legibility distance of 6.2 m per cm letter height. As for required luminance, the same levels as recommended for dynamic traffic management signs are suggested (between 3500 and 120,000 cd/m2). These levels may be somewhat lower because it is expected that the luminance of the environment of the electronic text devices is somewhat less. With respect to traffic safety, it is suggested that in busy motorway driving, a maximum of two words can be presented at the same time, assuming that in case of short following distances, as observed in busy traffic, not looking at the road for a maximum of 0.5 s can still be considered as safe. When the message is too long to be presented at once, one should use the so-called rapid serial presentation technique (RSVP) rather than the technique in which message are presented running from left to right. However, as long as no systematic research on the unsafety aspects of dynamic electronic message devices is conducted, the use of these devices on busy motorway should be strongly discouraged.
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