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Publication Number:  FHWA-HRT-17-024    Date:  June 2017
Publication Number: FHWA-HRT-17-024
Date: June 2017

 

Cooperative Adaptive Cruise Control Human Factors Study: Experiment 4—Preferred Following Distance and Performance in An Emergency Event

CHAPTER 2. PART 1

EQUIPMENT

The National Advanced Driving Simulator ¼ cab miniSim™ was used. The simulator uses three 182.88-cm (42-inch) 720p plasma screens to display the forward roadway and side and rearview mirrors. An additional 30.48-cm (12-inch)-wide screen was used to display dashboard information. The simulator operated without a motion base (i.e., it was stationary) but was equipped with a subwoofer underneath the driver’s seat that generated appropriate rumble road feel.

The researchers sat in an adjacent room separated by pocket doors. The participants and researchers were easily able to communicate if needed. The researchers were able to monitor the system and the participants’ well-being from this position.

SIMULATION SCENARIO

Participants drove in a dedicated center lane on a simulated eight-lane interstate highway (four lanes in each direction). Entrance to the dedicated center lane was accessed from the left side of the roadway from a ramp. The dedicated center lane was separated from other lanes with a jersey barrier. The environment was similar to suburban-rural interstate driving with a mix of trees and buildings along the roadway.

The simulation began with the participant vehicle as the only vehicle on the roadway. This time period was provided for participants to become accustomed to the feel of the driving simulator, including the steering, acceleration, and braking capabilities. After a few minutes, the participant came upon another vehicle. That vehicle acted as a lead vehicle and drove at 88.5, 104.6, 112.7, and 88.5 km/h (55, 65, 70, and 55 mi/h) for 3 min each. (The participant following task is described in more detail in the Procedure section.)

CALIBRATION OF LEAD VEHICLE SIZE

In the first experiment, the vehicle size was scaled in order for participants to accurately perceive the correct following distance.(6) This same vehicle scaling was used in the remainder of the studies. Given that the present experiment took place in a different simulator, the vehicle reduction scaling (to 75 percent of the original scale) was verified to ensure that the same following distance perception was attained.

PARTICIPANTS

The participants included 14 licensed drivers recruited from the Washington, DC, metropolitan area. Participants were required to be at least 18 years of age and were screened for susceptibility to motion and simulator sickness. Seven of the participants were male, and seven of the participants were female. Ages ranged from 22 to 72 years with a mean age of 46.7 years (median 50.5 years).

PROCEDURE

Upon arrival at the research center, participants were asked to review and sign an informed consent form. This was followed by a 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. An overview of the experiment was provided, and participants were given a chance to familiarize themselves with the driving simulator.

All participants completed two separate drives. The goal of the first drive was to assess the participants’ comfortable driving gap distance. In other words, under a normal low-traffic environment, how much space would the participants leave between themselves and the lead vehicle? Specifically participants were asked to “drive at what you consider a comfortable distance. In other words, follow that vehicle at a distance that you would normally follow another car in the real world.” Participants were reminded that the lead vehicle would change speed several times and that speed would need to be adjusted in order to maintain following distance. As previously mentioned, the lead vehicle drove at 88.5, 104.6, 112.7, and 88.5 km/h (55, 65, 70, and 55 mi/h) for 3 min each. The entire drive lasted 14–17 min.

After the completion of the first drive, participants completed a simulator sickness questionnaire (SSQ) and were provided with a break if necessary.(8) The goal of the second drive was to assess drivers’ perceived minimum safe following distance. The drive was identical to the first drive. The only component that varied was the instructions to participants. Participants were told the following:

“Instead of following at a comfortable distance, I want you to drive more closely. I’d like you to follow that vehicle at the minimum distance that you might ever follow another car on the roadway. For example, imagine that you are on a busy road and are trying to change lanes. Or even if you were simply in a hurry to get somewhere.”

Participants were given an opportunity to ask questions to ensure that the task was fully understood.

After the completion of the second drive, participants completed a second SSQ. Participants were provided time to ask any questions about the study, debriefed, thanked, and paid for their time. In total, participation lasted 45–60 min.

RESULTS

The goal of part 1 was to determine drivers’ perceived minimum safe following distance. This information was used to determine whether participants in part 2 would be labeled as near or far followers.

While the data from the first comfortable following task are not used to inform part 2, those data are presented here for descriptive purposes. To provide participants with sufficient time to adjust the following gap for each speed change (88.5, 104.6, 112.7, and 88.5 km/h (55, 65, 70, and 55 mi/h)), the first 30 s of vehicle following at each speed were excluded from analysis. Table 1 presents participants’ following time gap distributions by speed averaged across all 14 participants during the comfortable following distance drive.

Table 1. Participant following time gaps (s) by speed during comfortable following drive.

Speed
(km/h)
Minimum
(s)
Quartile 1
(s)
Quartile 2
(s)
Quartile 3
(s)
Maximum
(s)
Mean
(s)
88.5 1.26 2.04 2.50 2.71 7.93 2.68
104.6 1.18 2.25 2.64 2.83 6.08 2.76
112.7 1.08 2.08 2.48 3.88 11.84 3.36
88.5 0.92 1.80 2.56 2.79 7.36 2.79
1 km/h = 0.62 mi/h

 

Data from the second drive were initially explored to look for outlying following distances. Time gaps from a single participant were greater that 3 standard deviations away from the mean (2.74 s) and were subsequently excluded from further analyses. Once again, to provide participants with sufficient time to adjust following gap for each speed change (88.5, 104.6, 112.7, and 88.5 km/h (55, 65, 70, and 55 mi/h)), the first 30 s of vehicle following at each speed were excluded from analysis. Table 2 presents drivers’ following time gap distributions by speed averaged across 13 participants during the comfortable following distance drive.

Table 2. Participant following time gaps (s) by speed during close following drive.

Speed
(km/h)
Minimum
(s)
Quartile 1
(s)
Quartile 2
(s)
Quartile 3
(s)
Maximum
(s)
Mean
(s)
88.5 0.55 0.65 0.92 1.02 1.43 0.89
104.6 0.41 0.60 0.86 1.10 1.51 0.88
112.7 0.38 0.64 0.90 1.28 1.70 0.94
88.5 0.52 0.69 1.03 1.23 1.88 1.04
Mean 0.38 0.64 0.91 1.15 1.88 0.94
1 km/h = 0.62 mi/h

 

The median overall following distance time gap was 0.91 s. This value was used to assign participants as near or far followers.

 

 

 

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