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REPORT
This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-16-057    Date:  December 2016
Publication Number: FHWA-HRT-16-057
Date: December 2016

 

Cooperative Adaptive Cruise Control Human Factors Study: Experiment 2—Merging Behavior

 

CHAPTER 2. METHOD

In this chapter, an overview of the approach to assessing workload, arousal, and merging behavior is described before providing more extensive details on the experimental design and procedures.

APPROACH

Three groups drove the same stretch of simulated limited access roadway. This was the same simulated roadway used in experiments 1 and 3. All groups were asked to exit and enter the roadway four times. The first group completed this task with no cruise control. The second group used CACC while travelling in the main roadway stream but was required to adjust the vehicle speed when entering and exiting the CACC stream. The third group was provided with CACC that controlled speed both while in the travel lane and while using the entrance and exit ramps.

Workload Assessment

Driver workload was assessed by administration of the National Aeronautics and Space Administration Task Load Index (NASA-TLX) and was measured four times.(3) The first assessment was during a practice drive and was intended to accustom participants to providing verbal response to the NASA-TLX protocol. The second workload administration was approximately 9.33 km (5.8 mi) into the drive, immediately after the first complete exit and reentrance to the main travel lane. The second NASA-TLX administration was intended to assess the workload imposed by a merge into traffic. The third assessment was 32.19 km (20 mi) into the drive and was intended to assess the workload associated with driving in a stable, unchanging state (i.e., a baseline index). At this point, drivers were between merging events and were likely to feel comfortable with the driving task in general. The fourth and final NASA-TLX was administered immediately after the final merging event, approximately 37.02 km (23 mi) into the drive. The final workload assessment was intended to assess the workload change that may occur over time with practice merging.

Physiological Arousal

Physiological arousal was assessed by measuring eyelid closure, pupil diameter, and skin conductance. These measures were assessed at eight different 15-s periods during the drive. Four of these periods were immediately before exiting the roadway (cruise periods), and four were in the last portion of the merge events.

Merging Behavior

At exits 4, 8, 12, and 16, participants were asked to leave the CACC platoon by using a left exit ramp and then reenter traffic using a left on-ramp. Exits were approximately 2.33 km (1.45mi) apart. Traffic was continuous and did not stop, which forced participants to enter mid-stream and not at the beginning or end of a platoon. The two groups with CACC were required to join a platoon of vehicles with a constant 1.1-s gap. The group with CACC merge assistance was not required to adjust speed in any way. The system longitudinally controlled the entire drive without failure. However, if the participant pressed the brake, the system did disengage. The group without CACC merge assistance was required to manage and adjust their own vehicle speed to appropriately enter the platoon gap. The third group, which drove without CACC, maintained longitudinal and lateral control of the vehicle. This group was provided with a variety of gap sizes to merge into, which was hoped to help determine whether participants generally prefer a shorter or longer gap distances or whether no preference is given (i.e., drivers will accept the gap presented).

Equipment and Materials

The Driving Simulator

The experiment was conducted in the Federal Highway Administration (FHWA) Highway Driving Simulator. The simulator’s screen consists of a 200-degree portion of a cylinder with a radius of 2.7 m (8.9 ft). Directly in front of the driver, the design eye point of the simulator is 3 m (9.5 ft) from the screen. The stimuli were projected onto the screen by 3 projectors with resolutions of 2,048 horizontal by 1,536 vertical pixels. Participants sat in a compact sedan. The simulator’s 6-degrees of freedom motion base was enabled. The typical motion for roll, pitch, and yaw fell within ±4degrees.

The simulated vehicle was equipped with a hidden intercom system that enabled communications between the participant and a researcher who ran the experiment from a control room. The researcher in the control room could also view the face video from the eye-tracking system and monitor the participants’ well-being.

Eye-Tracking System

The same eye tracking system used in the first in this series of four experiments was used in the present experiment. Gaze direction accuracy varied by participant. For the left eye, the mean accuracy of gaze position across all participants was 1.4 degrees (radius) with a 0.69-degree standard deviation. For the right eye, the mean accuracy was 1.2 degrees with a 0.94-degree standard deviation. In this study, the eye-tracking system was primarily used to determine whether participant glance behavior varied systematically based on experimental condition. The following display locations were tracked:

In addition to tracking the direction of gaze, the eye-tracking system computed eyelid opening and pupil diameter. These measures were also recorded at 120 Hz.

Multifunction Display

Similar to the other three experiments in this series, a 17.78-cm (7-inch) liquid crystal display touchscreen display was mounted on the center console above the radio. For the two conditions that used the touchscreen, it displayed the set speed (always set to 112.65 km/h (70 mi/h)), the set following distance (always set to near), and the status of the CACC system (engaged or not engaged). The engage button on the right side of the display could be used by the participants to engage CACC. When the system was engaged, the text and icons appeared green; when the system was not engaged, the text and icons appeared red.

For the control group, the multifunction display was turned off. The control condition was not given any specific directions in terms of following distance other than to drive as they normally would.

Skin Conductance Sensor

As in experiment 1, galvanic skin response (GSR) was measured with silver-chloride salt electrodes placed on the palmar-side base of two fingers on the participant’s left hand. The electrodes were connected to a small sensor with a Bluetooth® transmitter strapped to the
left wrist.

The Simulation Scenarios

Participants drove in a dedicated center lane on a simulated eight-lane interstate highway (fourlanes in each direction). The roadway was the same used in experiment 1 with a few minor variations. The entrance to the center dedicated lane was accessed from the left side of the roadway from a ramp. The simulation began with the participant’s vehicle as the third in the CACC platoon queue. Once the participant was ready to begin, the two vehicles in front of the participant accelerated and merged into the CACC lane and cruised at 112.65 km/h (70 mi/h). The two groups that drove with CACC engaged the system, and the participant’s vehicle maintained a 1.1-s gap between it and the vehicle in front of it. The control group participants could follow at any distance they chose.

In total, there were 18 exit ramps, each placed approximately 2.33 km (1.45 mi) apart. Participants were asked to use exits 4, 8, 12, and 16. Exit ramps to the left of the main travel path that were not used by the participants were blocked by traffic barrels. This was intended to serve as a reminder to participants as to which exit ramps to use. Additionally, no traffic to the right of the barrier was present.

Calibration of CACC Vehicle Size

In experiment 1, the vehicle size was scaled down so participants could accurately perceive the correct following distance. This same vehicle scaling was used in experiment 2.

Procedure

Upon arrival at the research center, participants were asked to review and sign an informed consent statement. This was followed by the health screening to ensure that the participants were not at an increased risk of simulator sickness as a result of illness or lack of sleep. Participants were asked to show a valid driver’s license. A Bailey-Lovie eye chart was used to verify a minimum of 6/12 (20/40) visual acuity with correction if necessary. A slideshow presentation was shown to all participants. The presentation provided an overview of the experimental instructions and familiarized participants with the NASA-TLX questions. The participants assigned to the CACC conditions were also shown videos that explained the CACC concept.

Participants in all three experimental conditions were told the following:

“I am going to ask you to exit and reenter the freeway every fourth exit. I will give you verbal reminders to exit the freeway. There will be orange construction barrels blocking the other exit ramps. There will be other traffic on the freeway. The traffic is continuous and will not stop.”

Each condition was given additional instructions.

The CACC with merge assist instructions were as follows:

  1. Set the gap to Near.

  2. Set the speed to 70 mi/h (112.65 km/h).

  3. You will control steering—follow the car in front.

  4. The system will accelerate and brake.

    • You do not need to use your brake on the exit/entrance ramps.

    • You can take over control by pressing the accelerator or brake.

    • Pressing the brake disengages the CACC system.

    • If you need to take control, press ENGAGE as soon as possible.

The CACC without merge assist instructions were as follows:

  1. Set the gap to “Near.”

  2. Set the speed to 70 mi/h (112.65 km/h).

  3. You will control steering—follow the car in front.

  4. The system will accelerate and brake in the CACC lane.

    • The system will turn off once you leave the CACC lane (i.e., take the exit ramp).

    • The CACC system will NOT control your speed while merging.

    • After you reenter traffic, you will need to engage the CACC system again.

    • You can take over control by pressing the accelerator or brake.

    • Pressing the brake disengages the CACC system.

    • If you need to take control, press ENGAGE as soon as possible.

The control condition instructions were as follows:

  1. The speed limit is 70 mi/h (112.65 km/h).

  2. Drive as you normally would.

Following the slideshow presentation, participants were fitted with the GSR sensor and seated in the simulator cab where the controls and displays were reviewed, and the instructions were repeated. While seated in the cab, participants were asked to complete the simulator sickness questionnaire (SSQ) to provide a symptoms baseline. Finally, the eye-tracking system was calibrated to the participants.

Next, participants were asked to complete a brief practice drive. During the practice drive, participants were asked to merge into the dedicated CACC lane. They were asked to accelerate and gently brake. This was followed by more aggressive accelerating and braking, which was designed to help participants understand the limits of the vehicle dynamics. Drivers also moved between the travel lane and the breakdown lane (to the left) to familiarize themselves with the steering system. Once comfortable with the speed and steering dynamics of the simulated vehicle, participants practiced exiting and entering the main dedicated travel lane with a left exit and entrance ramp.

If assigned to a CACC condition, participants were then asked to engage the CACC system. With no vehicles ahead, the CACC system accelerated to 120.70 km/h (75 mi/h) until it closed on a platoon of CACC vehicles traveling at 88.51 km/h (55 mi/h). The platoon traveled at 88.51km/h (55 mi/h) for 2 min and then accelerated to 112.65 km/h (70 mi/h). This highlighted the ability of the cruise control system to follow at a specified distance, even when traffic slowed.

Participants in the control condition were asked to drive as they normally would. The other vehicles in the simulation performed similarly as they did in the CACC conditions; they drove at 88.51 km/h (55 mi/h) for 2 min and then accelerated to 112.65 km/h (70 mi/h). The platoon behaved in the same manner as for the CACC conditions.

After traveling in the platoon at 112.65 km/h (70 mi/h) for 2 min, the NASA-TLX was administered verbally to all participants. This administration was intended to further familiarize participants with the workload assessment tool. With the conclusion of the workload assessment, participants were asked to take the next available off-ramp and come to a complete stop.

After completion of the practice drive, participants were asked to exit the vehicle and complete the SSQ.

The experimental scenario began with the participant seated in the third vehicle of a platoon of four vehicles. Once the participant was ready to begin, the two vehicles in front of the participant accelerated and merged into the CACC lane and cruised at 112.65 km/h (70 mi/h). The two groups that drove with CACC engaged the system, and the participant’s vehicle maintained a 1.1‑s gap between it and the vehicle in front of it. Participants in the control condition were asked to drive as they normally would, with no specific instructions given about following distance.

Participants were verbally reminded to exit the travel lane and then reenter traffic at the appropriate ramps. As soon as the participants successfully merged into traffic in the dedicated lanes after the first (exit 4) and fourth (exit 16) ramps, the NASA-TLX was administered to assess workload during the merge event (“during the preceding minute or so”). The NASA-TLX was also administered as soon as exit 14 was passed. (Participants did not use this exit.) This administration was intended to assess workload during uneventful cruising in a CACC platoon (also described as during the last minute or so).

After exiting the simulator, participants were asked to complete a final SSQ, debriefed, and paid for their time.

Experimental Design

The primary between-group independent variable was the level of cruise control automation used throughout the scenario.

The three distinct participant groups were as follows:

In addition to workload, there was one additional within-subjects variable—driving period—with eight levels that were intended to distinguish the effects of CACC on driver behavior. The eight periods are described in table 1.

Table 1. Driving period descriptions.
Period Description
1 15-s period ending 45 s prior to exit for first merge event.
2 15-s period beginning 45 s prior to completing the first merge.
3 15-s period ending 45 s prior to exit for second merge event.
4 15-s period beginning 45 s prior to completing the second merge.
5 15-s period ending 45 s prior to exit for third merge event.
6 15-s period beginning 45 s prior to completing the third merge.
7 15-s period ending 45 s prior to exit for fourth merge event.
8 15-s period beginning 55 s prior to completing the fourth merge.

Participants

Participants were 60 licensed drivers recruited from the Washington, DC, metropolitan area. In total, data from 12 participants were not used due to poor data quality or simulator failures; data from 48 participants were used in analysis. Participants were required to be at least 18 years old and were screened for susceptibility to motion and simulator sickness. Table 2 shows the age group and gender counts by treatment group for the participants who provided useable data. The mean age of the younger participants was 33.4 years (range 19.4 to 44.5 years). The mean age of the older participants was 56.6 years (range 46.5 to 77.9 years).

Table 2. Demographic breakdown of participants in experiment 2 by treatment group.
Condition Younger Females Younger Males Older Females Older Males Total
Control 4 4 5 4 17
CACC without merge assist 4 4 4 4 16
CACC with merge assist 4 4 3 4 15
Total 12 12 12 12 48

Participants were paid $80 for their participation, which lasted between 1.5 and 2 h.

 

 

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