Effects of Variable Message Signs (VMS) were investigated in a field study. 2 VMS on motorways were used in the study. On three evenings, the VMS presented alternating no message, and a message which recommends to choose an alternative route because of a closed road section. Compliance with themessages was measured by registering which proportion of vehicles chose the recommended route. Driver behaviour was registered with speed-recordings and observation of braking behaviour. Results show a large proportion ofdrivers complying with the messages. It was estimated that about every 5th vehicle changed route choice according to the recommendation. Almost none drove as far as the closed road segment. Speed recordings showed large speed reductions when the VMS showed messages. A much higher proportion of vehicles braked when a message was shown on the VMS than when no message was shown. Many vehicles additionally changed driving lane or made others change lane. Braking also led to increased proportions of small and very small spaces between vehicles following each other. Braking, lane changes and short distances can lead to conflicts and accidents. The results lead tothe conclusion that VMS attract attention and can be effective in influencing route choice, but that VMS and driving impose conflicting attention demands on the drivers. Explanations can be located at the different levelsof the driving task. On the operational level, driving can be impaired byincreased visual load, e.g. due to short reading distance. The tactical level of the driving task can be affected by the attempt of the driver to gain time by reducing speed, which additionally can make lane change necessary. The strategic level of the driving task, which includes navigation, is the target level of the traffic information used in this study. Depending on formal and content aspects of the messages, the information can additionally affect driving on the operational and tactical level, by reducing available information processing capacity. Consequently, traffic information should make it possible for drivers to establish an immediate link between the message and the driving task on the target level, without detracting from driving on other levels. This can be achieved by increasing reading distance, by an appropriate design of display elements and by use of symbols corresponding to drivers mental representations of the driving task, possibly in combination with speed limits. The research has been conductedat the Institute of Transport Economics in Oslo, funded by the Norwegian Public Roads Administration. For the covering abstract see ITRD E135582.
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