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Publication Number:  FHWA-HRT-13-045    Date:  October 2013
Publication Number: FHWA-HRT-13-045
Date: October 2013

 

Cooperative Adaptive Cruise Control: Human Factors Analysis

RESEARCH SCENARIOS

Based on previous research and the potential impact of human-factors issues on CACC implementation, several areas of research are suggested for evaluation and presented in order of significance. For each suggested research question, the proposed independentvariable is provided in parentheses.

Willingness to Utilize CACC

Beyond the technical feasibility of a technology, primary concern pertains to acceptance by the intended audience. CACC provides the greatest benefit in dense traffic in highway environments, allowing more vehicles to travel in a given lane. However, there is a potential for drivers to be less willing to utilize the system in such conditions, preferring to maintain manual control and longer time gaps.

Research Questions

The following research questions relate to drivers' willingness to utilize CACC:

Potential Methodologies

The following methodologies are recommended for addressing the preceding research questions:

Workload, Situational Awareness, and Distraction

Use of CACC should reduce a driver's workload, enabling dedication of additional resources to driving-related tasks such as hazard identification and anticipation. However, a driver may apply these additional resources to secondary, unrelated tasks. These secondary tasks may have negative impacts on a driver's SA, limiting his ability to make emergency maneuvers or quickly retake manual control of the vehicle.

Research Questions

The following research questions relate to workload, SA, and distraction:

Potential Methodologies

The following methodologies are recommended for addressing the preceding research questions:

Platoon Entry/Exit

As indicated by van Arem et al., CACC platoons can make it difficult for vehicles in adjacent lanes to join an existing platoon.(11) In figure 1, the short gaps in the platoon of vehicles in the top lane hinders the vehicle in the center lane (noted with an X) from joining. Additionally, vehicles that want to exit a platoon may need to adjust their speed based on the adjacent lane before exiting, affecting the platoon stability. In figure 2, the middle vehicle (noted with an X) may disrupt the platoon of vehicles in the top lane by attempting to change lanes. Research is needed on how to aid these maneuvers and minimize their impact.

Three lanes of traffic are shown traveling horizontally across the image. In the top lane is a platoon of Cooperative Adaptive Cruise Control (CACC)-equipped vehicles that are traveling very close together. There is about a third of a car length between each car. In the middle and bottom lanes of the image, the vehicles are separated by much longer gaps, with more than a full car length between each vehicle. A vehicle marked with an X in the middle lane is unable to merge with the platoon, except by speeding up significantly to reach the gap at the front of the platoon or slowing down significantly to reach the gap at the back of the platoon.
Figure 1. Illustration. Merging into a CACC platoon.

Three lanes of traffic are shown traveling horizontally across the image. In the top lane is a platoon of Cooperative Adaptive Cruise Control (CACC)-equipped vehicles that are traveling very close together. There is about a third of a car length between each car. In the middle and bottom lanes of the image, the vehicles are separated by much longer gaps, with more than a full car length between each vehicle. A vehicle marked with an X in the center of the platoon in the top lane is unable to merge into the middle lane without adjusting speed.
Figure 2. Illustration. Exiting a CACC platoon.

Research Questions

The following research questions relate to platoon entry and exit:

Potential Methodologies

The following methodologies are recommended for addressing the preceding research questions:

Arterial Intersections

Microsimulations have indicated that with appropriate information, drivers could adjust (or I2V communication could directly adjust) approaching speeds to an intersection to reduce deceleration/ acceleration and the need to stop, saving time and fuel. Although the driver would be conscious of the speed change, following vehicles (if not CACC enabled) may not be aware of the upcoming red light or may simply not be willing to follow at what they consider a slower-than-necessary speed. In figure 3, the CACC-equipped lead vehicle initiates deceleration due to an upcoming red light. The non-equipped following vehicle may not be aware of the deceleration impetus, however. This incongruence may lead to passing events by following vehicles, which may not only affect the success of the CACC-induced speed adjustment (i.e., vehicles joining the driver's lane in front of the driver will likely require additional deceleration) but also increase the chance of accidents due to lane changes in intersection environments.

. At the left of the image, two vehicles are approaching a curve. Shortly beyond the curve, on the right side of the image, a traffic signal shows a red light. Trees obstruct the view of the traffic signal before the curve. The leading vehicle is Cooperative Adaptive Cruise Control (CACC)-equipped and receives information about the red light, causing it to decelerate. The following vehicle is not equipped with CACC and its driver is unaware of the reason for the deceleration. Arrows on the image indicate that the following vehicle may attempt to pass the leading vehicle.
Figure 3. Illustration. CACC-equipped vehicle followed by non-equipped vehicle.

Research Questions

The following research questions relate to arterial intersections:

Potential Methodologies

The following methodologies are recommended for addressing the preceding research questions:

Carryover Effects

CACC utilization permits a driver to follow a vehicle more closely than safely possible under manual control. After exposure to these shorter gaps, behavioral adaptation may result in drivers continuing to follow at shorter gaps during manual control, increasing risk.

Research Questions

The following research questions relate to carryover effects:

Potential Methodologies

The following methodologies are recommended for addressing the preceding research questions:

Following Vehicle Gap Comfort

Numerous studies have assessed drivers' comfort with following a lead vehicle at various gaps, regardless of whether they are driving manually or with some variety of automation, including CACC. However, an important aspect that has yet to be researched is how comfortable a driver would be having another vehicle closely following his car. The gaps utilized with CACC would likely be considered tailgating in normal driving conditions, which means drivers may be less accepting of others driving at close distances, especially if they are not certain the following vehicle is under automated control. In figure 4, the middle vehicle may feel comfortable following the lead vehicle but may not be comfortable with the rear vehicle being so close. Studies that show people typically overestimate their own skills and underestimate those of others hint at reduced comfort with succeeding vehicles. As drivers become more comfortable with the technology, their comfort levels for those behind them may increase.

Three Cooperative Adaptive Cruise Control (CACC)-equipped vehicles are shown traveling very close together in a single lane. An inset picture shows the view looking into the driver’s side mirror of the center car. This mirror reveals the following vehicle seemingly tailgating at a distance that may be uncomfortable to the middle driver.
Figure 4. Illustration. Closely following vehicles in CACC platoon.

Research Questions

The following research questions relate to following vehicle gap comfort:

If a positive effect is found for knowing the following vehicle is CACC-engaged, follow-up studies could include the best manner of notifying the lead driver. For example, CACC-equipped vehicles may have an indicator light on the front of the vehicle for when CACC is engaged, or drivers may have access to a display that indicates when CACC-engaged vehicles are immediately behind, in front, or to either side.

Potential Methodologies

The following methodologies are recommended for addressing the preceding research questions: