Truck accidents are often as a single accident. Drowsiness is one of the causes of a single accident. Drowsiness usually caused by working in long time, monotonous work, poor health conditions, or lack of sleep. This study aims to assess the drowsiness level of industrial truck driver and analyze drowsiness patterns seen from the relationship between decreasing of driver alertness to road density and time intervals from the start of the trip to the time of measurement. Drowsiness level assessed using Karolinska Sleepness Scale (KSS) questionaire. The level of driver drowsiness during the trip rises along the travel time. In the morning trip, decreasing of driver alertness have occurred after 120 minutes of travelling and in the afternoon trip the alertness decreasing occurred after 60 minutes travelling. The driver's drowsiness level at two trips shows significant differences using paired sample t-tests. In the morning trip the mean drowsiness level was 2.91 while in the afternoon trip, the mean drowsiness level was 3.56, p value on two tail was 0.0002. There is a low correlation between drowsiness and traffic density that assessed by truck speed, with R = 0.18. Traffic density hasn't affected on driver drowsiness for day trip.