The Reverse Braking Assist (RBA) feature is designed to automatically activate full braking in a backing vehicle. When this feature activates, a backing vehicle is suddenly stopped or may slide to a stop. During this process, an understanding of the driver's behavior may be useful in the design of an appropriate human-machine-interface (HMI) for the RBA. Several experimental studies were done to examine driver behavior in response to an unexpected and automatic braking event while backing . Two of these studies are reported in this paper. A 7-passenger Crossover Utility Vehicle was fitted with a rear-view camera, a center-stack mounted LCD screen, and ancillary recording devices. In the first study, an object was suddenly placed in the path of a backing vehicle. The backing vehicle came to a sudden and complete stop. The visual image of the backing path on the LCD prominently showed that an obstacle was present in the backing path of the vehicle. Concurrently, a continuous auditory alarm sounded to indicate the presence of an obstacle in the rear. Further, a warning text message ‘CAUTION CHECK BEHIND’ was flashed across the top quarter of the LCD screen. The same test vehicle was used in the second study. All test conditions in the second study were identical to those in the first study with the exception that no camera view of the reversing path of the test vehicle was provided to the driver. The textual and auditory warnings were identical in both studies. Results of the two studies show that the application of full and sudden braking during the backing maneuver caused the driver to immediately place their foot on the brake within approximately 0.8sec of RBA activation. Almost immediately after, the driver began to explore his/her visual environment to look for ‘the threat’. In the absence of a visual image of the reversing path, and even though the auditory and textual warnings were continuously available to the driver, 25% of all participants backed over the obstacle in the rear. But when a visual image of the backing path was available to the backing driver, as in the first study, none of the test participants backed over the obstacle. These and additional results of eye gaze data are presented in more detail, and their implications for HMI design are discussed.