A study of 24 professional truck drivers investigated how drowsiness develops during real-road driving to test a DDAW system. At night, 17 of 24 drivers reached a KSS level of 8, with average time to reach KSS 8 being 56 minutes. Ten drivers chose to stop or were requested to stop in the sleep-deprived condition. A DDAW recording window of 600 s would fail to capture information relevant to the EDR, whereas time windows of up to one hour could provide important indications of increased driver fatigue.