Conceptualization, J.M., E.L. and G.M.F.; methodology, J.M., E.L. and G.M.F.; investigation, J.M., E.L. and G.M.F.; formal analysis, J.M., E.L. and G.M.F.; data curation, J.M. and E.L.; writing—original draft preparation, J.M., E.L. and G.M.F.; writing—review & editing, J.M., E.L. and G.M.F.; visualizations, J.M. and G.M.F.; supervision, E.L.; project administration, J.M.; and funding acquisition, G.M.F. All authors have read and agreed to the published version of the manuscript.

Figure 1. Camera views captured by the DAS: Android AutoTM Interface Display (top left, upper panel), Driver Pedal (top left, bottom panel), Forward Roadway (top right), Over-The-Shoulder View (bottom left), and Driver Face view (bottom right). The driver’s face was blurred in this image to conceal their identity.

Figure 1. Camera views captured by the DAS: Android AutoTM Interface Display (top left, upper panel), Driver Pedal (top left, bottom panel), Forward Roadway (top right), Over-The-Shoulder View (bottom left), and Driver Face view (bottom right). The driver’s face was blurred in this image to conceal their identity.

Figure 2. Second study vehicle capable of launching a fake foam rubber muffler.

Figure 2. Second study vehicle capable of launching a fake foam rubber muffler.

Figure 3. Timeline of events for the surprise event trial. (a) Shows the taps made on the display, when the lead vehicle’s hazard lights activated, and when the surprise muffler was dropped. (b) Shows the taps made on the display, when the lead vehicle’s hazard lights activated, when the driver was asked to pause the task, when the driver resumed the task, and when the surprise muffler was dropped.

Figure 3. Timeline of events for the surprise event trial. (a) Shows the taps made on the display, when the lead vehicle’s hazard lights activated, and when the surprise muffler was dropped. (b) Shows the taps made on the display, when the lead vehicle’s hazard lights activated, when the driver was asked to pause the task, when the driver resumed the task, and when the surprise muffler was dropped.

Figure 4. Comparison of drivers’ hypothesized situation awareness between the “No Pause” and “Pause” conditions. (a) Plots the hypothesized situation awareness over time as the driver taps on the display in the “No Pause” condition. It was hypothesized that driver’s situation awareness drops to zero during the task. (b) Plots the hypothesized situation awareness over time as the driver taps on the display in the “Pause” condition. It was hypothesized that pausing mid-task to look back at the road might reduce the amount of time spent driving with low situation awareness and lead to a higher level of situation awareness at the end of the task.

Figure 4. Comparison of drivers’ hypothesized situation awareness between the “No Pause” and “Pause” conditions. (a) Plots the hypothesized situation awareness over time as the driver taps on the display in the “No Pause” condition. It was hypothesized that driver’s situation awareness drops to zero during the task. (b) Plots the hypothesized situation awareness over time as the driver taps on the display in the “Pause” condition. It was hypothesized that pausing mid-task to look back at the road might reduce the amount of time spent driving with low situation awareness and lead to a higher level of situation awareness at the end of the task.

Figure 5. Percentage of drivers, by experimental condition, who detected the lead vehicle’s hazard lights preceding the muffler drop hazard (self-report).

Figure 5. Percentage of drivers, by experimental condition, who detected the lead vehicle’s hazard lights preceding the muffler drop hazard (self-report).

Figure 6. Percentage of drivers observed to initiate precautionary behaviors in response to the lead vehicle hazard lights.

Figure 6. Percentage of drivers observed to initiate precautionary behaviors in response to the lead vehicle hazard lights.

Figure 7. Percentage of drivers who successfully avoided the muffler as a function of experimental condition (Pause, No Pause).

Figure 7. Percentage of drivers who successfully avoided the muffler as a function of experimental condition (Pause, No Pause).

Figure 8. Mean latency to driver’s first response, overall and by experimental condition (number of participants are indicated in the parenthesis for Overall, Pause, and No Pause).

Figure 8. Mean latency to driver’s first response, overall and by experimental condition (number of participants are indicated in the parenthesis for Overall, Pause, and No Pause).

Figure 9. Mean driver response magnitudes: peak brake and steering inputs. Peak brake (pedal percentage input) and steering (degrees) input.

Figure 9. Mean driver response magnitudes: peak brake and steering inputs. Peak brake (pedal percentage input) and steering (degrees) input.

Figure 10. Mean driver response magnitude: Peak lateral and longitudinal acceleration associated with the drivers’ avoidance response.

Figure 10. Mean driver response magnitude: Peak lateral and longitudinal acceleration associated with the drivers’ avoidance response.

Figure 11. Scatterplot of peak longitudinal acceleration by experimental condition.

Figure 11. Scatterplot of peak longitudinal acceleration by experimental condition.

Figure 12. Percentage of drivers estimated to have been surprised by the muffler drop based on observer video analysis.

Figure 12. Percentage of drivers estimated to have been surprised by the muffler drop based on observer video analysis.

Table 1. Comparison across Pause and No Pause conditions.

Table 1. Comparison across Pause and No Pause conditions.

MeasureNo PausePauseNoticed hazard lights 146%93%Lifted foot up prior to muffler23%29%Looked surprised 182%36%Avoided muffler54%50%Latency to first response (s)0.40.4Engaged brakes/steered100%79%Peak braking (g) 10.50.2Peak lateral acceleration (g) 10.20.1

Table 2. Comparison of surprise event scenarios.

Table 2. Comparison of surprise event scenarios.

Current StudyFitch et al. [39]Pre-surprise-event taskTask with pauseTask without pauseTask with audible alertSurprise event objectFoam mufflerFoam mufflerLarge boxSteered50%45%67%Braked28%55%27%No braking or steering21%0%Not documented, but <6%Avoided object50%54%94%