Flashing Emergency Lights: Influence of Intensity, Flash Rate and Synchronization on Driver Visibility, Comfort and Confidence

Flashing emergency and warning lights are critical elements of public safety and traffic control during roadway incidents. These lights should not only alert drivers to their presence, but also should inform them of who and what is present on the scene, and should help to manage the responses of drivers as they navigate past the incident. First responder and driver safety depend upon all three of these functions, yet standards focus almost entirely on alerting drivers. A full-scale outdoor field study was carried out during daytime, during nighttime on dry pavement and during nighttime on wet pavement, using a mock-up roadside scene containing three police vehicles. The lights on the vehicles were adjusted to produce different levels of intensity, flash rate, and synchronization of lights across all three vehicles. In some cases, sequentially flashing lights were present. Upon viewing different combination of these factors, observers in stationary vehicles were asked to identify the location of a silhouette of a police officer randomly placed within the scene (to measure the lights’ ability to inform drivers) and to rate the attention-getting properties of the lights (to measure their ability to alert drivers), their perceptions of visual comfort and how confidently they would be able to drive around or past the lights (glare and confidence are measures of the lights’ ability to help manage drivers). The results build on previous research indicating that high intensities during the daytime but reduced intensities at night can alert drivers. They also suggest that slower, synchronized flashing, and sequential flashing lights can be used appropriately to inform and manage drivers navigating past roadside incident scenes.