Driver impairment due to fatigue induced drowsiness is a significant cause of vehicle collisions. One countermeasure that is currently being implemented is Fatigue Warning Systems (FWS) to alert drivers that are drowsy. Behavioural adaptation of drivers to a FWS was evaluated in a closed track study. Thirty-two drivers completed two lengthy overnight drives, separated by one week, with half the drivers completing the second drive with an active FWS. During the drives, drivers voluntarily took breaks for as long as they liked. Behavioural results demonstrate that the FWS had no impact on objective and subjective driver fatigue, on driving time, on the number of breaks or on break duration. Results also demonstrate that 30 minute breaks are an ineffective drowsiness countermeasure These findings suggest that a FWS as currently conceived may not contribute to reduce fatigue induced collisions.