Touchscreens that offer various functions to the driver have become an integral part of the user interface in vehicles. To enable the use of these functions while preventing driver distraction, there is a trend towards implementing driver monitoring systems (DMS). DMS detect distraction and warn drivers accordingly. However, there is limited empirical evidence on the effectiveness of DMS in preventing visual distraction. Furthermore, potential interactions with contextual factors like secondary task complexity and task experience are neglected so far. Therefore, the present study investigated the effectiveness of a DMS, based on the Euro NCAP protocol, that issued warnings when long distraction (>= 3 s) or visual attention time sharing (>= 10 s in a 30 s window) occurred while driving on a highway. Glance behavior of 57 participants was analyzed while performing secondary tasks on an in-vehicle display with varying levels of task complexity and task experience. The effectiveness of the DMS depended on the task complexity and task experience. For more complex tasks, the DMS significantly reduced the number and total duration of glances to the display, but only for inexperienced trials. For experienced trials, no effect of the DMS was found. For less complex tasks, the DMS significantly reduced the duration of single glances and the proportion of long glances at the display, regardless of task experience. The results indicate that the effectiveness of DMS in reducing distraction is not as straightforward as assumed, emphasizing the importance of evaluating DMS in the context of contextual factors to draw accurate conclusions.
