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


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

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This report presents human factors research to examine the effects of cooperative adaptive cruise control (CACC) on driver performance in a variety of situations. It summarizes driving simulator experiments in which the driver was required to drive in a stream of vehicles. Participants experienced a vehicle merge in front of them as well as an emergency event that required driver intervention. The participants’ preferred following time gap did not significantly affect collision avoidance. However, those participants following at shorter distances were more likely to intervene more rapidly that those following at a far distance.

These findings support the idea that performance depends more on overall CACC following distance settings than with drivers’ personal preferences. This will allow CACC systems to implement a single following distance gap (or set of gaps based on vehicle physics). The results show that it is critical that drivers receive clear alerts when it is necessary to take over control of the vehicle. Without such measures, it is possible that CACC implementation may not result in improved roadway safety. This report should be useful to transportation professionals, State transportation departments, and researchers interested in the effects of automation on driver behavior and performance.

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 4—Preferred Following Distance and Performance in an Emergency Event

5. Report Date

April 2017

6. Performing Organization Code
7. Author(s)

Stacy A. Balk, 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. (TRAIS)

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: December 2013–July 2016

14. Sponsoring Agency Code


15. Supplementary Notes

The Contracting Officer’s Representative was David Yang (HRDS-30), and the Government Task Manager was Brian Philips (HRDS-30).

16. Abstract

This study is the fourth in a series of four experiments exploring human factors issues associated with the introduction of cooperative adaptive cruise control (CACC). Specifically, the goals of this experiment were as follows:

  •  Assess drivers’ workloads under two different CACC following gaps (near and far).
  •  Assess drivers’ reactions to a vehicle merging in front of them under different following gaps.
  •  Assess drivers’ reactions to an emergency event that requires driver intervention to avoid collision.
  •  Determine whether preferred time gap following distance affects the first three goals.

As measured by the National Aeronautics and Space Administration Task Load Index, drivers’ perceived workloads did not vary between the cruise period and after the vehicle merge. However, workloads were significantly greater after the emergency crash event. Workloads varied based on neither assigned following distance nor preferred following distance.

Those participants in driving in the assigned near distance were more likely to hover their foot over the brake during the merging event and to react faster to the emergency event. Preferred following distance did not affect performance. Throughout the study, participants’ following distance preferences did not affect performance. In other words, one’s abilities may not necessarily reflect his or her following preferences. This finding is promising for widespread implementation of CACC.

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 Classification
(of this report)


20. Security Classification
(of this page)


21. No. of Pages


22. Price


Form DOT F 1700.7 (8-72) Reproduction of completed page authorized




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