Cooperative Adaptive Cruise Control Human Factors Study: Experiment 3—The Role of Automated Braking and Auditory Alert in Collision Avoidance Response

CHAPTER 4. DISCUSSION

Experiment 3 reinforces the main conclusion of experiment 1: the full CACC system (i.e., as configured in condition CACC-AB) has the potential to provide for a substantial safety benefit. The control condition in experiment 3 did not have an in-vehicle display or the requirement to frequently monitor the speedometer, which are two potential explanations for the high crash rate of the experiment 1 control condition. Nonetheless, the crash rate for the ACC control condition in experiment 3 was nearly identical to that in experiment 1. This suggests that it was CACC automated braking and an alarm that provided the apparent safety benefit in both experiments. Removing either the alarm or the automated braking from the CACC system appears to diminish or eliminate the safety benefit of the full system.

It is not clear from these results why the absence of an auditory alarm (ACC and CACC-B) condition resulted in an increased crash risk. The CACC-B group had the longest reaction times and the three incidences of no response. The ACC group also had no alarm, yet it reacted as quickly as the group with full CACC. Perhaps this is an example of overtrust in the system. The CACC-B braking force was twice that of the ACC braking (0.4 versus 0.2 g), so it is conceivable that the CACC-B group felt the system responding and trusted the automatic response until it was too late to recover. The mild braking in the ACC condition may been easier to perceive as inadequate than the more aggressive braking in the CACC-B condition.

The CACC-A group, which received an auditory alarm but had no automated braking, responded more quickly than any of the other groups but still had a high crash rate. The extra time the 0.4-g automated braking provided to the CACC-AB group appears to have been the key to enabling that group to respond more slowly while retaining an average of a 0.6-s cushion in the extra time available.

In conclusion, it appears that the CACC system alarm may mitigate overtrust, while the automated braking feature provides drivers with the extra time they need to respond to an emergency condition. Whether this combination of alarm and automated braking will be effective with other CACC implementations (e.g., with shorter gaps between vehicles or different automated braking deceleration rates) remains to be explored.