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Publication Number:  FHWA-HRT-16-056     Date:  December 2016
Publication Number: FHWA-HRT-16-056
Date: December 2016

 

Cooperative Adaptive Cruise Control Human Factors Study: Experiment 1—Workload, Distraction, Arousal, and Trust

 

CHAPTER 4. DISCUSSION

This study set out to examine the following diverse number of questions regarding CACC use:

As assessed by the NASA-TLX, the CACC system did reduce perceived driver workload in this experiment.

The CACC group was more likely to listen to the radio or engage in other observable diversionary activities than the control group. It remains to be determined whether this tendency was the result of the CACC system relieving the drivers from the responsibility to continually manage gap or because the control group had the added diversion of monitoring the gap indication of the multipurpose display. The original intent of the gap display was to ensure that the gap maintained by the control group would be similar to that maintained by the CACC groups. It was assumed that control participants would not need to refer to the gap display for more than a few minutes of driving because they would learn what the desired gap looked like on the roadway. Contrary to this assumption, the control group looked at the multipurpose display as frequently at the end of the drive as they did at the beginning. This finding could provide an alternate explanation for the workload difference between CACC and control groups as well as account for the difference in diversionary activities. As a result of this finding, a second experiment has been proposed in which the control group is equipped with ACC rather than CACC so that a gap display would not be required.

The attempts to assess the effect of CACC on physiological arousal were largely unsuccessful. The GSR measurements were noisy and inconclusive. The eyelid opening data were also inconclusive, and the eyelid opening quality readings output by the eye-tracking software suggest that the readings should not be relied on. The pupil diameter readings were fairly reliable, assuming the eye-tracking software quality ratings are to be believed. The finding that pupil diameter decreased in the second half of the drive suggests all groups were somewhat less aroused during the second half of the drive. There was no indication that arousal differed between groups, but this could be the result of the aforementioned tendency of participants to engage in diversionary activities to keep their arousal at comfortable levels.

The results of the crash avoidance event that the control and CACC with crash avoidance groups were exposed to suggest that CACC does provide a substantial safety benefit. Half the control group crashed into the car ahead with substantial force, as indicated by negative TTC scores. By contrast, only one CACC participant crashed, and that participant’s response was questionable because he never attempted to brake and proceeded to drive through three of the vehicles ahead.

Because the control group’s brake reaction time and time to reach maximum braking were not significantly different from the CACC group in the crash avoidance scenario, the most likely explanation of the crash avoidance benefit from the CACC system is the 0.4-g braking that the system engaged soon after the car ahead began braking. This moderate braking enabled the CACC-equipped drivers to brake slightly later and with slightly less force than control drivers (recall these slight differences were not statistically significant) yet be much less likely to have
a collision.

The CACC-equipped drivers showed considerable trust in the system. Only 1 of 36 CACC drivers braked when a CACC vehicle merged into the platoon, and only 1 of 36 CACC drivers used the accelerator to close the gap at the end of the merge event when the system slightly overshot the 1.1-s target while slowing to reestablish the set gap. Furthermore, none of the CACC drivers in the CACC with cut-in group braked during the period 5 cut-in event.

Although CACC-equipped drivers showed considerable trust in the system, they did not appear to over trust the system; all but one CACC driver responded appropriately to the crash avoidance critical event.

 

 

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