An empirical exploration of the impact of transition of control on situation awareness for potential hazards

An experiment about the hazard perception capabilities of drivers after interruption in a video-based scanning task


Vlakveld, W.; Vissers, L.; Hulleman, K.; Nes, N. van




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The Directorate-General of Highways, Waterways, and Water Systems (Rijkswaterstaat in Dutch) of the Ministry of Infrastructure and the Environment (Ministerie van Infrastructuur en Milieu (IenM in Dutch) has commissioned SWOV to conduct an experiment about transition of control in highly and fully automated vehicles from the perspective of the driver. Transition of control is the switch from (fully) automated driving to manual driving while in traffic. When drivers are driven in a (fully) automated vehicle and the system fails in the execution of the driving task, makes a mistake, or when drivers choose to drive the vehicle manually, drivers have to switch from automatic to manual mode. The study was intended to demonstrate the impact in highly controlled laboratory settings. The rationale for conducting a laboratory experiment was to control for as many confounding factors as possible without exposing participants to dangers in real traffic. As such, the results are an indication of the possible effects of transition of control on situation awareness in real traffic. However, the laboratory settings were too remote from real-life situations so that nothing can be concluded about the degree of impairment of situation awareness and its duration in real traffic on the basis of the results of this study.

This study has investigated whether the detection of latent hazards diminished after completion of a short, secondary task and, if so, how long this diminished situation awareness lasted. Three groups of experienced drivers watched thirteen animated video clips filmed from a driver perspective while their gaze directions were recorded. Each video clip lasted approximately 40 s. A high-priority latent hazard was present in each video clip. Latent hazards are traffic situations with a high likelihood to develop into a situation in which a crash is likely to occur should the latent hazard materialize. In none of the video clips did the latent hazard actually materialize. For half of the participants in a group, 50% of the videos were interrupted for 5 s by a screen with words. The interrupted videos appeared randomly. For the other half of the participants in a group, the other 50% of the videos were interrupted. Participants had to read aloud as many words as they could. The video continued after this task, yet having skipped 5 s of the scene. Each latent hazard had a time frame in which the latent hazard could have materialized. In group 1 the video reappeared 2 s before the time frame of the latent hazard started, in group 2 the video reappeared 4 s before the start of a time frame of a latent hazard, and in group 3 the video reappeared 6 s before the onset of the time frame of the latent hazard.

There were three dependent variables: eye fixations on the latent hazard - recorded by means of an eye tracker device - ,keys pressed when latent hazards were present (the so-called marked hazards), and the recall of the latent hazards immediately after each video clip. The results show that participants in group1 (2 second before hazard) fixated on fewer latent hazards in the interrupted videos than they did in the uninterrupted videos. This was not the case for group 2 (4 seconds before the hazard) and group 3 (6 seconds before the hazards). More or less the same pattern emerged from the key pressing data. Participants in group 1 marked fewer latent hazards by pressing a key during the interrupted videos than in the uninterrupted videos. This did not occur with participants in groups 2 and 3. Finally, there were no statistically significant differences between recall of hazards in the interrupted videos and the uninterrupted videos in groups 1, 2 and 3. Memory effects could have influenced the results of the recalled hazards. The results indicate that situation awareness for latent hazards is briefly diminished after completion of a short, secondary task.

As this study was a laboratory experiment, the results do not legitimate any measure in relation to automated vehicles, the roads on which they drive and the ‘drivers’ of these automated vehicles. For evidence-based measures regarding transition of control, a follow-up study is required. This follow-up study could be a simulator study in which the (simulator) vehicle drives fully automated on large road sections. In this recommended experiment, participants should wear a head-mounted eye tracker. They would have to resume manual driving at the end of these road sections. After having switched to manual driving a latent hazard would occur in the scenario: the planned moments of transition of control. Unexpected moments of transition of control would also occur, as a result of supposed equipment failure, for instance. Differences in the human-machine interface that prepare driver for the driving task directly before resumption of control might constitute an independent factor in this recommended simulator study.

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SWOV, The Hague