We investigated whether collision avoidance systems (CASs) should present individual crash alerts in a multiple-conflict scenario or present only one alert in response to the first conflict.

Secondary alerts may startle, confuse, or interfere with drivers’ execution of an emergency maneuver.

Fifty-one participants followed a pickup truck around a test track. Once the participant was visually distracted, a trailing sedan repositioned itself into the participant’s blind spot while a box was dropped from the truck. Participants received a forward collision warning (FCW) alert as the box landed. Twenty-six drivers swerved left in response to the box, encountering a lateral conflict with the adjacent sedan. Half of these 26 drivers received a lane-change merge (LCM) alert.

Drivers who received both the FCW and LCM alerts were significantly faster at steering away from the lateral crash threat than the drivers who received only the FCW alert (1.70 s vs. 2.76 s, respectively). Drivers liked receiving the LCM alert, rated it to be useful, found it easy to understand (despite being presented after the FCW alert), and did not find it to be startling.

Drivers who are familiar with CASs benefit from, and feel it is appropriate to generate, multiple alerts in a multiple-conflict scenario.

The results may inform the design of CASs for connected and automated vehicles.