In drag racing, the amount of time between the appearance of the last amber light and the moment when the driver first crosses the start line. A pro tree has a "perfect reaction time of 0.4 seconds, meaning that the time between the amber light and the green light is 0.4 seconds (if the driver leaves before the green light, he red lights and loses the race). A handicap tree has a perfect reaction time of 0.5 seconds. An experienced drag racer will usually have a reaction time no more than 0.67 or so.