Flashing yellow warning lights are important for worker and driver safety in work zones. Current standards for these lights do not address whether and how they should be coordinated to provide directional information to drivers navigating through work zones. A field study was conducted to assess driver responses to warning lights. The luminous intensities and flash patterns of warning lights along a simulated work zone were varied during daytime and nighttime. During the daytime, driver responses were relatively insensitive to warning light characteristics, although drivers preferred sequential and synchronised flash patterns over random, uncoordinated flashing. At nighttime, the combination of a temporal peak luminous intensity of 25 cd and a sequential flash pattern was optimal for providing directional information. A single initial warning light having a higher luminous intensity may help drivers detect the work zone without creating unacceptable visual discomfort.