Flashing lights over intersections are used in conjunction with a sign are used to bring attention to unusual or unexpected conditions. As with warning signs, if flashing lights are overused they will lose whatever effectiveness they have. If after seeing flashing lights drivers do not consistently see a condition that justifies the extra attention they will tend to ignore flashing lights in the future.
Flashing lights over intersections, also called Intersection Control Beacons, are used to bring attention to an intersection when vehicles will need to stop at stop signs. Sometimes citizens request flashing lights over intersections where there is good visibility of the intersection and stop signs and no history of accidents caused by vehicles failing to stop. Flashing lights at an intersection like this would not be justified. Flashing lights are not a cure-all.
Arizona Department of Transportation - Flashing Lights - Do they Really Slow Traffic?
ODOT District One Planning & Engineering Web Site Manager: Charles Schreck