Document Type

Technical Report

Publication Date





Adaptive cruise control (ACC) represents a rapidly emerging in-vehicle technology that can enhance or degrade driving safety. A critical factor governing the safety benefit of ACC concerns the ability of the driver to assume control of the vehicle in situations that exceed the capability of ACC. This study examined the effectiveness of various warning modalities in reengaging distracted drivers during severe braking situations that exceed the capability of ACC. The study compared warnings that paired a visual icon with an auditory cue, seat vibration, brake pulse, or a combination of all three cues. A total of sixty participants drove for 35- minutes in the National Advanced Driving Simulator (NADS). Drivers experienced two severe, four moderate, and eight mild braking situations. The ACC could accommodate all but the two severe situations without driver intervention. ACC provided a substantial benefit during mild braking lead vehicle events, enabling drivers to maintain a larger and more consistent minimum time-to-collision. Contrary to previous studies (Stanton, Young, & McCaulder, 1997), ACC did not produce a safety decrement during the severe braking situations. The combination of visual, auditory, seat vibration, and brake pulse led to slower brake reaction time in severe braking situations, but all four warning strategies led to a similar minimum time-to-collision and maximum braking. In contrast to several previous studies, these results suggest that drivers can effectively assume control when they receive a warning that the braking authority of ACC has been exceeded. Further research is needed to identify the boundary conditions that specify when drivers can successfully intervene and retake control and whether a multi-modal combination of cues can be crafted to speed rather than slow drivers’ response.

Total Pages

37 pages


Copyright © The University of Iowa, 2006