An evaluation was performed of human reaction time to threatening robot movements. The cohort consisted of ten male volunteers, 20 to 30 years old. Subjects observed an industrial robot whose arm was moving at speeds of 150 to 450 millimeters per second (mm/sec) towards or away from a target position situated 1.22 meters (m) from the base of the robot's resting position and 1.52m above the floor. Ambient lighting intensity was 560 lux. The robot was programmed to randomly overshoot the target 10% of the time. The subjects were instructed to press an emergency stop button whenever they detected the robot arm overshooting the target. The effects of illumination were assessed by attaching one of three colored sleeves to the robot's arm that altered the degree of contrast between the robot and the background. Overshoot distance, reaction time, and decision cost, a parameter related to the robot down time resulting from misses and false alarms, were determined. The number of misses increased with robot speed and decreasing distance of overshoot. Mean reaction time decreased with increasing robot speed and with decreasing decision cost. Luminance contrast had little effect on reaction time or overshoot distance. The authors conclude that interactions between decision cost and robot arm speed suggests that at low robot arm speeds reactions to a robot moving toward a worker indicate a decision making component is involved, whereas at higher robot speeds reactions are more reflexive in nature.