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Publication Number:  FHWA-HRT-16-056     Date:  December 2016
Publication Number: FHWA-HRT-16-056
Date: December 2016


Cooperative Adaptive Cruise Control Human Factors Study: Experiment 1—Workload, Distraction, Arousal, and Trust

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This report presents human factors experimental results from an examination of the effects of cooperative adaptive cruise control (CACC) on driver performance in a variety of situations. The experiment was conducted in a driving simulator using a scenario in which the subject driver was embedded in a platoon of CACC-equipped vehicles. CACC is envisioned as an automated vehicle application that complements the capabilities of the vehicle operator without degrading the vehicle operator’s alertness and attention.

The CACC system was effective in helping drivers avoid collisions when the vehicle at the head of the platoon decelerated with maximum force. No differences in driver alertness or arousal levels were found when comparing CACC with manual gap control. Drivers reported significantly less workload with CACC.

This report informs the discussion among transportation professionals about how automated vehicle applications will be embraced by everyday drivers. The experiment results should be useful to researchers and transportation professionals interested in the effects of automation on driver behavior.

Monique R. Evans, P.E.
Director, Office of Safety
Research and Development


This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

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Technical Report Documentation Page

1. Report No.
2. Government Accession No. 3. Recipient’s Catalog No.
4. Title and Subtitle
Cooperative Adaptive Cruise Control Human Factors Study: Experiment 1—Workload, Distraction, Arousal, and Trust
5. Report Date
December 2016
6. Performing Organization Code:
7. Author(s)
Vaughan W. Inman, Steven Jackson, and Brian H. Philips
8. Performing Organization Report No.
9. Performing Organization Name and Address
Leidos, Inc.
6300 Georgetown Pike
McLean, VA 22101-2296
10. Work Unit No.
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
Office of Safety Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296
13. Type of Report and Period Covered
Final Report, 10/1/2013–12/1/2015
14. Sponsoring Agency Code
15. Supplementary Notes
The Contraction Officer’s Representative was David Yang, and the Government’s Task Manager was Brian Philips.

16. Abstract
This study set out to examine the following diverse questions regarding cooperative adaptive cruise control (CACC) use:

  • Does CACC reduce driver workload relative to manual gap control?
  • Does CACC increase the probability of driver distraction relative to manual gap control?
  • Does CACC result in reduced driver arousal relative to manual gap control?
  • Does CACC result increase the ability to avoid a crash when exposed to an extreme breaking event?
  • Will drivers trust the CACC system?

These questions were addressed in an experiment conducted in the Federal Highway Administration Highway Driving Simulator. A total of 49 licensed drivers were tested, with 12 or 13 participants in each of 4 groups. All of the groups drove in the third position in a five-vehicle platoon in which all of the other vehicles were equipped with simulated CACC. The groups differed as to whether the participant vehicle was equipped with CACC and the type of event at the end of the drive that disturbed the longitudinal spacing of the platoon.

As assessed by the National Aeronautics and Space Administration Task Load Index, the CACC system did reduce perceived driver workload relative to driving without cruise control. CACC users appeared slightly more likely to engage in diversionary activities (e.g., listening to the car radio) than control group drivers. CACC yielded a substantial and statistically reliable reduction in the probability of a crash. No evidence was obtained to suggest that use of CACC leads to lower levels of driver arousal than manual gap control. Participants showed a great deal of trust in the CACC system. In a situation where all of the control participants used the brake to maintain a comfortable gap, only 2 of 36 CACC users overrode the system with the brake or accelerator.

17. Key Words
Cooperative adaptive cruise control, CACC, human factors, driving simulation, attention, distraction
18. Distribution Statement
No restrictions. This document is available through the National Technical Information Service, Springfield, VA 22161.
19. Security Classif. (of this report)
20. Security Classif. (of this page)
21. No. of Pages
22. Price
Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors









Figure 1. Screen capture. Appearance of control group multifunction display
Figure 2. Screen capture. Typical section of the simulated roadway
Figure 3. Screen capture. Entrance ramp meter
Figure 4. Graph. Results of field and simulator gap maintenance testing
Figure 5. Screen capture. Reduced-size (75 percent) lead vehicle depicted with 1.1-s gap
Figure 6. Screen capture. The Ponzo illusion. (The vehicles in the picture are all the same size.)
Figure 7. Graph. NASA-TLX scores as a function of treatment group and location in the scenario
Figure 8. Graph. Standardized pupil diameter as a function of condition and period
Figure 9. Graph. Estimated mean proportion of drivers engaged in non-driving-related diversions increased with time into drive
Figure 10. Graph. GEE estimated mean percent of time gazing at the multifunction display as a function of condition and period
Figure 11. Equation. Adjusted minimum TTC
Figure 12. Graph. Estimated adjusted mean TTC
Figure 13. Graph. Estimated mean brake onset reaction times for the two groups that had the crash avoidance final event
Figure 14. Graph. Estimated mean time to from beginning of brake pedal depression to full braking


Table 1. Demographic breakdown of participants by treatment group
Table 2. Number of participants engaging in observable non-driving-related activities as the experiment progressed
Table 3. Percent of gaze time to defined objects as a function of condition and period
Table 4. Crash counts for the two groups that were exposed to the crash avoidance event


ACC adaptive cruise control  
CACC cooperative adaptive cruise control  
DSRC dedicated short-range communications  
FHWA Federal Highway Administration  
GEE generalized estimating equation  
GLM generalized linear model  
GSR galvanic skin response  
NASA-TLX National Aeronautics and Space Administration Task Load Index  
SSQ simulator sickness questionnaire  
TTC time-to-collision  



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