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Publication Number:  FHWA-HRT-16-058    Date:  December 2016
Publication Number: FHWA-HRT-16-058
Date: December 2016


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



Cooperative adaptive cruise control (CACC) combines the following three driver assist systems: (1) conventional cruise control, which automatically maintains the speed a driver has set, (2) adaptive cruise control (ACC), which uses radar or light detection and ranging sensors to automatically maintain a gap the driver has selected between the driver’s vehicle and a slower moving vehicle ahead, and (3) dedicated short-range communications to transmit and receive data with surrounding vehicles so that the cruise control system can more quickly respond to changes and speed and location of other CACC vehicles, including vehicles that the driver cannot see.(1)

When using CACC, drivers share vehicle control with an automated system that includes vehicle-to-vehicle and vehicle-to-infrastructure communications. Communications between nearby CACC-equipped vehicles enables automated coordination and adjustment of longitudinal control through throttle and brake activations. Automated control should enable CACC-equipped vehicles to safely travel with smaller gaps between vehicles than drivers could safely manage on their own. Smaller gaps should subsequently increase the roadway capacity without increasing the amount of roadway.(2)

Although technically feasible from computational and communications perspectives, the ability of users to safely interact with CACC-equipped vehicles in the scenarios envisioned by engineers has yet to be demonstrated. The goal of the CACC human factors study, of which the present experiment was a part, was to investigate the effects of CACC on driver performance, workload, situational awareness, and distraction. The goal was not to address all human factors issues associated with CACC use but rather to suggest additional lines of research that may be required to model the influence of human drivers on overall CACC performance.

The present experiment was the third in the human factors study and is a follow-up to the first experiment. Experiment 1 included two conditions in which a crash occurred ahead of the platoon that the participant driver was in.(3) The participant could not see the crash and therefore could not anticipate that the vehicles ahead would brake hard with a maximum of 1-g sustained deceleration. This event was called a “crash avoidance event,” and the CACC group that experienced the event was labeled “CACC with crash avoidance.” The other group that experienced the crash avoidance event did not have any type of cruise control and was provided a multifunction display to assist in maintaining a 1.1-s gap. This other group was labeled “control.”

Out of the 11 participants in the control group, 5 collided with the vehicle ahead. Out of the 12 participants in the CACC with crash avoidance group, only 1 collided with the vehicle ahead. The difference in collision experience between the groups was large and statistically reliable. This finding indicates that CACC systems configured as in experiment 1 could be effective in reducing crashes precipitated by the rapid deceleration of vehicles not within the driver’s field of view. However, there were several factors that may have contributed to the difference between the CACC-equipped group and the control group. The CACC with crash avoidance vehicles (and all other platoon vehicles other than the lead vehicle) began 0.4-g deceleration 0.1 s after the platoon lead vehicle initiated 1-g deceleration. Simultaneous with the onset of 0.4-g deceleration, the CACC-equipped vehicles sounded an audio alarm. The first indication of a need to respond for control drivers was the looming of the vehicle ahead as it began to decelerate at 0.4 g. The brake lights of the vehicle ahead did not come on until 1.9 s later, when the vehicle ahead began decelerating at 1 g. The control group had no auditory alarm, and the control group vehicle did not decelerate at 0.4g, although the other vehicles in the platoon did. Thus, the only indication for the control group that there was a problem prior to the brake lights illuminating on the vehicle ahead was the looming (i.e., the increase in size and decrease in distance of the simulated vehicle ahead). In addition to looming, the CACC group had an audible alarm that came on at the same time the vehicle ahead and their own vehicle began to decelerate.

The original intent for experiment 1 was that the vehicle ahead would have its brake lights come on simultaneous with the 0.4-g deceleration and that the 1-g deceleration of all CACC-equipped vehicles (except the participant’s vehicle) would commence 1 s after the lead vehicle began its 1-g deceleration. If the original intent had been implemented, then the CACC group would have had 0.8 s less time to react than it did. Also, under the original intent, both the control and CACC groups would have had the cue of the brake lights of the vehicle ahead coming on at the same time the 0.4-g deceleration began.

In addition to the lack of an alarm and the lack of 0.4-g deceleration of their vehicle, the control drivers could have conceivably been distracted by the in-vehicle display that showed their following (gap) distance. After 31 min of driving, it was expected that the distraction factor from the in-vehicle display would be minimal. However, the finding that the control group looked at the forward roadway only 90 percent of the time compared with the CACC group, which looked at the forward roadway 95 percent of the time suggests that distraction could be an alternative explanation of why the control group was more likely to crash.

The advantage that the CACC-equipped vehicles had in avoiding collisions would diminish with shorter following distances. Experiment 1 examined only 1.1-s gaps because this was believed to be the shortest gap that manufacturers would allow until there is more experience with these systems. With gaps less than 1.1 s, CACC users might maintain greater alertness (e.g., hovering their feet over the brake pedal). Therefore, the point at which driver intervention would become irrelevant is not necessarily linearly related to gap size. Driver reaction times might be less with smaller gaps. In experiment 3, the 1.1-s gap was maintained in all four experimental conditions.

To address whether an auditory alarm and automated braking at 0.4 g were necessary and sufficient for the crash avoidance benefit observed in experiment 1, the braking and alarm features were factorially combined in CACC experiment 3, as shown in table1. The ACC control group did not have automated braking in the same sense as the CACC with automated braking but no auditory alarm (CACC-B) and CACC with automated braking and alarm when automated braking authority was exceeded (CACC-AB) groups. However, it did have engine braking with about 0.2 g of deceleration. The ACC group deceleration was delayed in onset by 0.3 s from when the vehicle ahead began braking at 0.4 g. The CACC with alarm when engine braking authority was exceeded (CACC-A) group had neither engine braking nor automated braking.

Table 1. Factorial design of experiment 3.
Factor 0.4-g Automated Braking
Auditory Alarm No Yes
No ACC Control CACC-B


In experiment 3, the original intent of experiment 1 was implemented (i.e., the brake lights of the vehicle ahead came on at the same time that the 0.4-g deceleration commenced, and the vehicle ahead began braking with 1-g deceleration 1 s after the onset of the 0.4-g deceleration).



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