The objectives of this study are to analyze the effect of a short nap on the level of alertness and on the drivers’ ability to detect the motorcyclists on the road according to the size of his/her useful visual field (UVF). Nineteen participants ( m  = 21 years old) carried out a driving task with a simulator, after taking a short nap (30 min) in the vehicle or not. They had to distinguish the colour change of a signal on a vehicle they were following and to detect a motorcycle which briefly appeared on the road at different eccentricities. Drivers with a limited UVF are those who detect the fewest motorcycles. Taking a short nap before the driving does not have any significant effect on data related to vision (motorcycles and signals detected), driving and alertness state (( α + θ )/ β ). There is a linear relationship between the size of the UVF and the ability to detect the motorcycle appearing in the left outside mirror. The implications of the results concerning the short nap as a countermeasure to a decrease in alertness and the perception of motorcycles according to extent of the useful visual field of the driver are discussed in terms of road safety.