Influence of Warning Message Content on
Message Understandability and When Drivers Respond
(UMTRI-97-22)

Daniel Damouth and Paul Green

June, 1997

The purpose of this project is to (1) evaluate the understandability of warnings and messages being considered for future Volvo products, and (2) determine the characteristics that lead to understandable (and complied with) warnings. Fifty U.S. drivers operated a driving simulator while 29 warning messages were presented on a center console CRT (e.g., "Throttle fault, service ASAP") one at a time. In response to each message, drivers said what they thought was wrong and what they would do about it (e.g., slow down, stop at the next service station, call on cellular phone for help, look in the manual). Subsequently, drivers completed several questionnaires concerning factors such as the drivability and risk of accident associated with the problems mentioned in the messages. Finally, drivers were asked to rank order a set of action phrases (e.g. service immediately, service soon) by how long they would wait before following it.

Messages that were not well understood by drivers included "fan fault," "airdistr fault," "rear backrest unlocked," "filler door ajar," "a/c switched off-engine temp high," "throttle fault," "wheel spin," "skid control temp disabled," and "road grip exceeded." Often, messages were not understood because drivers were unfamiliar with the function described. (What is the throttle? What is the filler door?). Recommended alternative messages were provided in almost every case, even where messages were reasonably well understood, to improve consistency and the timeliness of driver responses.

In addition, an equation was developed that predicted when drivers would respond to a warning. Key factors in the equation where the drivability of the vehicle after the warning, the risk of a crash if the warning is ignored, the perceived urgency of the action phrase, and if drivers could fix the indicated malfunction themselves.

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